Burnham MPO-IQ versus Buderus G125BE Oil Boilers

[danboston]

I have decided to replace my 50-Year old American Standard oil boiler (tankless coil) with a new oil-fired unit. Four adults live in a 1,700 ft2 ranch with 1,500 ft2 basement. We have 145 feet total length of Al-fin baseboard heating, all on the ground floor. I will be adding baseboard heating to the basement in the future. A heat loss calculation indicates a 90,000 btu/hr rate for the house.

I am contemplating either the Burnham MPO-IQ115 with an ouput of 98,000 btu/hr or the Buderus G125BE/28 with an output of 97,000 btu/hr. Both of these are cast-iron boilers, with an AFUE rating of 86% for the Burnham MPO-IQ and 90% AFUE rating for the Buderus G125BE. The Burnham MPO-IQ apparently comes standard with an outdoor temperature control that adjusts the boiler water temperature based upon outdoor temperatures. Buderus supplies a similar control (Logamatic 2107) as an add-on for additional cost. The Buderus goes one step further and offers a condensing mode option as well (as a side note, I live about 10 miles from the Buderus headquarters in Londonderry, NH).

If I go with the Burnham, I also plan to get a Burnham Alliance 50-gallon stone-lined indirect water heater. The contractor told me that the water in my area is not so great and not good for a stainless steel water heater – therefore he recommended the stone-lined tank. If I go with the Buderus, I would likely get the Buderus Logalux LT-200, which is a 53 gallon indirect water heater. It’s not clear to me what Buderus uses – I believe it is stainless steel. But I also heard that stainless steel made in the US is not up to par with stainless steel made in Europe?

Anyone have any experience (good or bad) with either of these models? I heard that Burnham has had issues with the cast iron boilers cracking? Any recommendations regarding options with the Buderus (are they worth the additional cost)?

[Dana]

FWIW: Unless you live in the leakiest uninsulated house in Andover or have more glazed area than code allows, that 90K number is almost certainly 2x reality. (My measured heat load at 0F design temp on a circa 1923 ~2000' house + ~1500' of semi-conditioned basement in Worcester is ~30K, and that's with less than R20 in a significant fraction of cathedralized roof in the attic rooms, and a few known gaps in the wall insulation.) Getting rid of the cast iron beast and using a gas-fired tankless HW heater as a boiler + buffer tank I've been able to trim the output downward and modulate with load somewhat. With all zones calling for heat it doesn't put out more than ~42K (needed to bump up the temp for exit air comfort on a hydro-air zone, or I'd back it off even more), and it keeps up just fine, even cycles at -10F, so I'm pretty confident of the actual heat load. Prior to insulating the basement walls, foundation sill & band joist , pounding some cellulose into the 2x4 cavites where it was easy, and the heat load was closer to 43-45K, based on fuel-use history rated against heating degree-day data.

If you have accurate fuel use data on your existing system you can use that (and the boiler's specs) to get a fairly accurate heat load measurement using the FSA tool downloadable from a link on this page.

OTOH, the smallest oil burners are still likely to be 2x oversized for your actual load too, and if you're multi-zoning it with low-mass emitters such as baseboard, the individual zones will be a tiny fraction of the boiler output and you'll have to buffer it to keep it from short-cycling itself into lower efficiency and higher maintenance. If you can figure out what your actual design day water temperature needs are (I't take a WAG at ~140F, which is the min-temp of the return water for a cast-iron oil boiler) it may make sense to use a buffer-centric approach using a reverse-indirect rather than a standard indirect (Everhot EA-series, TurboMax, or Ergomax), with the boiler slaved to the aquastat on the boiler as it's only zone, with the zones sipping from the reverse-indirect/buffer, like this:



That way there's a minimum burn time determined by the mass of the buffer/indirect- it can't short-cycle, and the water temp or room temp doesn't need a huge hysteresis to keep the boiler from short-cycling. Under long hot water draws the boiler kicks and it'll deliver pretty much a full shower flow continuously, and has the mass of the tank to back it up a bit. If you set the tank's aquastat to about the design day heating water requirement, it'll probably be a reasonable temp for the DHW load as well, but you'll still have to install an thermostatic mixing valve at the DHW output should you ever need/want to crank the tank temp higher.

I've read bloggery in recent years that the fuel availble in the US isn't really up to snuff for running a condensing Buderus- I'd research that a lot closer before going that route. Their non-condensing models have a good rep.

Given that oil demand worldwide looks like demand will continue to increase faster than supply for at least a decade prices on heating oil are likely to continue to be very volatile. With the Allegheny shale gas now coming on line, the future for natural gas in New England are likely to be both more stable and continue to be much lower per BTU- if you're anywhere near the local gas grid, this might be the time to switch fuels, and you'd have quite an array of modulating/condensing options (in which case you might NOT want to use a buffer-centric system architecture.)

Last, not least, if it's not too late, DO insulate your basement walls before finishing it out, and do it in such a way that you don't create a mold/rot problem. See:


http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

I did mine at ~R20 using recycled 3" semi-permeable rigid iso, but 2-3" of EPS and unfaced batts in a studwall, no interior vapor barrier gets you there too. Recycled rigid board goods can be had on the cheap locally in Framingham at The Insulation Depot. Unless you know for certain that you have a capillary break at the foundation sill, don't use any foil-faced rigid-board, and if you use XPS (pink/blue) you can't go more than 2" before it cuts the drying capacity of the foundation too much. You can put up to 4" (~R16) of un-faced EPS though. With an all-foam-board approach you can hold it in place with furring through-screwed into the foundation with tap-cons and hang the gypsum on the furring. If you have a fieldstone foundation 2" of closed cell foam + unfaced batts in a studwall are probably the better approach. Set the studall up first, and make sure the foam seals up to the stud edges before adding the batts. Put 6mil poly under the floor plate of the studwall as a capillary break.

[danboston]

Dana:

I feel that the house is well insulated. We had treated cellulose blown into the wall cavities, as well as, 8" in the attic. We also replaced all the windows, including the basement with Andersen 400-Series windows. As I mentioned, it is a small ranch (the square footage of the ground floor is 1,700 ft2 and the basement area is 1,500 ft2). I am currently insulating the basement as well (the basement will have dri-core panels on the floor, R-13 foamboard on the walls, and R-30 fiberglass batts in the ceiling). We will add some additional baseboard heating elements to the basement as an additional zone in the next year.

The house is currently heated with a circa 1950's tankless American Standard oil boiler (Arcoliner 3B J3 series) and a forced hot water Mono Flo baseboard aluminum fin system on two zones. The total length of the baseboard emitters is 145 feet and they are all on the ground floor. There are currently no heating elements in the basement. The tankless coil boiler provides all the heat and hot water for the house. We currently run out of hot water quickly if two showers are taken back-to-back or if the forced hot water heating system kicks on when in the shower.

In 2010, we burned 720 gallons of oil. The total number of heating degree days in 2010 for our area totalled 5,778. If I did this correct, this yields a K-factor of 8 (5,778/720). I tried the NORA Fuel Savings Analysis (FSA) calculator and under Step 2 it gave me a design day heat load of about 30,000 btu/hr when I input Boston, MA as the location, design temperature of -2 F and a K-factor of 8.

I would like to switch to gas, unfortunately the gas company wants to charge me $6,200 to move the gas main in front of my house. That is a dilemma since I am convinced that oil prices will continue to rise at a much greater clip than natural gas. I may bite the bullet and just do it. But that leads to the question then of what type of gas-fired boiler is best for my low-mass baseboard system?

I had a contractor come by and he told me that since we have a Mono-Flo baseboard heating system with aluminum fins (not cast iron), then he does not recommend a gas-fired high-efficiency mod-con boiler. The main reason being that the return water ends up being too hot and therefore the boiler will never have a chance to enter into condensing mode. If we had cast-iron fins, or a radiant heating system, then he said that is a much better situation for a mod-con boiler since the return water ends up being much cooler. Therefore, he recommended either a conventional gas-fired or oil-fired boiler (both Burnhams).

I provided the contractor with information about room, door and windows sizes, insulation, etc. and he had a Heat Loss calculation done for me. That indicated a Heat Loss of about 90,000 btu/hr (I can send that to you as part of a PM if you would be willing to review it for me – that would be much appreciated). Obviously 30,000 but/hr is substantially different than 90,000 btu/hr. Something is not right there.

[Dana]

Dri-Core has no appreciable R value, and with ~50F subsoil temps the heat loss out the slab will be signficant- well worth putting down a couple inches (R8) of EPS under a floating tap-conned OSB or plywood subfloor. (Or at a minimum 1" of XPS sheathing for R5). The prescriptive conservation code for new construction in MA is for R10 sub-slab, but in retrofits you can run out of headroom. R5 is WAY better than R0.5 (the probable R value of Dri-Core), and would protect rugs from developing mold on the underside by keeping it well above the dew point of the interior air despite the R1-2 of the rug itself. See: http://energycode.pnl.gov/EnergyCode...=Massachusetts

You simply can't have a design day heat load that would indicate 2x the BTU content of the fuel you heated the house with, eh? ;-) I you used 720 x 139000=100.08MBTU in 5778 heating degree days, that's 17321BTU/HDD, or 722 BTU per heating degree-hour. Assuming a design temp of -2F (which is probably below the ASHRAE 99% binned hourly value) and an interior design temp of 68F, that's a 70 degree delta. 70 x 722= 50.5KBTU/hr of SOURCE fuel energy, with no discounting for boiler efficiency or hot water use! Assuming 20-25% of that is hot water use (likely, with an embedded coil- see: http://www.nora-oilheat.org/site20/u...ciencyTest.pdf ) only 38-40K of source-fuel was devoted to space heating (less than half the 90K number), and assuming 80% efficiency on the old boiler, that puts you again right at about 30-32K for a whole house design day heat load, not more. I'm buying the NORA-model calculated number much more than the contractor's calculated number. If your 60 year old boiler is running under 80% (which it might be), your design day heat load is even lower.

Mod con boilers can work just fine with fin tube baseboard, provided you don't oversize the boiler (the min-modulated output is more important than the max). But again, it's the amount of emitter in the smallest zone and the output of the boiler at minimum fire that determines whether it'll short-cyle without adding mass. But it's both cheap and EASY to add mass when needed by inserting a buffer tank (a small electric HW heater that isn't hooked up to power does nicely.) With fin-tube the min temp at which it has predictable output is ~110-120F, but that's plenty low enough to put the system in the mid-90s for efficiency.

Flat panel radiators or cast-iron baseboard can run predicably even below 90F, but it's sometimes a lot to pay for another 3% of efficiency (but there is an enhanced comfort-factor as well.)

Embedded hot water coils in boilers are notoriously low capacity when new, and drop significantly over time as they lime up. In my kludged up system I can run a shower literally all day long, but that's with a "free" kickback of 20-30K coming back from a drainwater heat exchanger on the cold-feed into the reverse-indirect. The boiler's output eventually hits ~50K on long showers (10 to 15+ minutes) as it draws the tank temp down to ~108F, causing the tankless water heater I'm using for a boiler to smoothly increase it's output. With a mod-con you'd get better results with a standard indirect sized for the number of users and the boiler's output, controlling the indirect as the "priority zone". At 30K of heat load you can use the smallest mod-cons out there, that modulated from ~15K to 50K, but with 4 showering adults and a 50K-max burner you'll want more than a 30 gallon indirect.

It's often cheaper/better to go with a gas-fired combi system based on a tank type hot water heater (condensing or otherwise) when your design day heat loads are under 25K, which they might be after you've insulated the basement. If your design-day heating water temp requirement based on the amount of fin-tube is 140F or less, it's nearly ideal, as long as it has enough burner. The nicer ones are like the HTP VERSA, which has a modulating burner to handle the much higher peak hot-water heating loads (2-3x your peak space heating load.), but there are cheaper versions like the B-W Combi-2. They're inherently high-mass, and literally can't short-cycle on zone calls no matter how finely you chop it up, and easier to design around than a mod-con. The Versa has a bigger burner than the Combi-2, and would have more margin for hot-water heating even if you went with the smallest version, but the Combi-2 plus a ~50% efficiency drainwater heat exchanger would still give you the hot-water heating output you need for multiple back-to-back showers with plenty of overhead for a 30K design space heating load.

[Dana]

If it only cost six grand to bring gas to the house, consider that part of the cost of the new heating system. It's a serious cost adder, but if you used only 720gallons of oil with a wheezing 1950s oil boiler, a right-sized cast iron or a low mass mod-con supporting those loads will use at most about 1000 therms/year. I'm not sure what your gas rates are (it's a different supplier than mine), but the most I've EVER paid was ~$1.50/therm, and in the past couple of years it's been in the ~$1-1.25 range. Even if it hit $1.50 again (probably will, if there's a rapid economic recovery) that's only ~$1500/year. With a better-efficiency oil boiler you might drop to 600 gallons/ year, but probably not 500, but even at 500 gallons/year, assuming it averages $3.50/gallon for the next decade rather than the current $4 that's still an annual savings of $250. A decade from now I'd expect gas to be a bit higher, but oil to be MUCH higher.

Predicting future prices in the short & medium term is a sucker game played out in the futures markets daily, and the decade-out guesses have many ways to fail, but unless people in India and China stop buying cars or the price of electric cars falls through the floor, the price-pressure on oil is going to be even more severe in 2020 than it is today. In 2010 more cars were sold in China than the US, and team USA's total share of the world oil consumption edged toward 20%, down from 30% 25 years ago. By 2020 we'll no longer be the consumer tail wagging the world oil price dog the way we have been, and the fraction of diesel (which competes for the same barrel-fraction as heating oil) is continuing on a rapid upswing that began in Europe in the 1990s. That $6K investment now might look pretty good.

[danboston]

Dana: Your heat load calcualtion makes perfect sense. Thanks!

I will scrap the dri-core idea and apply foil-faced DOW TUFF-R 2-inch polyisocyanurate foam boards to the floor and cover with floating 1/2" exposure 1 rated plywood. I'll use tapcon screws to secure to the floor. The headroom in the basement is not an issue.

Ok, I have decided to take the plunge and am going with gas. I am leaning towards the Burnham Alpine 80, which modulates between 16 K btu/hr and 80 K btu/hr. However, I am concerned that the return water may be too warm for the boiler to enter into condensing mode on a regular basis. What are my options to ensure that the return water is cool enough so the boiler burns efficiently? The smallest zone has 40 feet of total emitters. For a proposed gas-fired mod-con setup, would the small unplugged electric water heater acting as a buffer go between the end of the return line and the return inlet to the boiler? That seems like a waste of heat - any way to capture that without spending a lot?

Finally, if I go with a unit like the Alpine 80, do I still need a 50-gal indirect water heater? There are 4 adults. The house will have two showers available. Plus I am installing a 70-gall jacuzzi. Only two of these would be running at the same time (i.e., two showers or 1 shower and 1 jacuzzi filling up).

[danboston]

A Buderus contractor came by this morning to take a look. I told him that I wanted to go with gas and he recommended the Buderus wall hung model. When I told him my concern about the return water might be too warm for the boiler to enter into condensing mode on a regular basis, he looked a little stumped and said he would get back to me.

[Dana]

DON'T use foil-faced iso on the floor- it's hygroscopic, and would eventually saturate with water reducing it's R value by half. It's mechanical pressure loading is also lower than most grades of XPS or EPS. Both EPS and XPS have a better closed-cell structure than iso, and while they too could take on water over time if fully submerged, they won't wick it in and hang onto it the way iso does. Whatever you use for foam, lap the seams between the plywood vs. the rigid board by a foot to avoid getting compression-rockering at the edges.

The Burnham Alpine is a decent boiler, as are the Buderus condensing boilers, and there are many others (Triangle-Tube Solo 60, Peerless Pinnacle T50, etc, etc.) The design & installation competence of the contractor and local support from distributors is more important than the manufacturers. These boilers come with a control setup called "outdoor reset", whereby the output temp of the boiler varies responds to the outdoor air temp as a rough model of the heat load. When it's warmer out, the temperature of the heating water drops, guaranteeing more condensing time. These response curves are programmable, typically with settable min temps (useful for fin-tube, which is has a very non-linear output curve at lower temps compared to radiators or radiant floor), and have to be tweaked as part of commissioning of the boiler.

With 2 showers running you'll need a 50 gallon indirect. The max output of the -80 in water heating mode will be ~75K, which is enough to support one 2-2.5gpm flow, but not two. It's about right for filling the jacuzzi, but if the person stays in the shower for a long time or the fill rate of the Jacuzzi is too fast the person in the shower could end up with tepid water toward the end.

As long as the buffer tank is inside conditioned space, any "loss" is to the conditioned space, lowering the heat load. But since electric tanks are well insulated, the rate of loss is pretty slow, and won't much affect the temperature of the room it's in. What the buffer saves is wear & tear on the boiler, and preserves the efficiency of the boiler, since there is a fixed loss on every burn due to ignition cycles and flue purges. Ideally it would be set up with minimum burns in excess of 10minutes under all heat load conditions. The thermal mass of fin tube is low- about a gallon every 50', so you're looking at a whole-system of only 3-4 gallons, maybe 30-35lbs of water. Assuming a temperature hysterisis of even 10F, it takes at most 35 x 10= 350 BTUs to raise the temp that 20F, and at the minimum fire of the Alpine in condensing mode you're looking at ~15KBTU/hr it takes less than 1.5 minutes to achieve that rise, and that's with ALL ZONES RUNNING. With 30 gallons of buffer that's more like 15 minutes.

There are variations on how buffers can be configured in the system, and it depends somewhat on the zoning & pumping/valving schemes that will be necessary- it's a design issue that needs to be addressed by the competent contractor, since it may vary with how big the zones are, and the flow requirements of the boiler (which will differ with manufacturer and model. But with an indirect and a few zones, setting it up as the point of hydraulic seperation in a primary/secondary is a bulletproof approach:



which is similar to;



(but here the buffer tank has 4 connections rather than external Tees)

In simple systems it's sometimes easier to put the buffer in series with the boiler either the output or returns manifolds from which the zones are drawing, and put the indirect on it's own local loop. The indirect is self-buffering with plenty of thermal mass, the key is to get the mass of the buffer involved with every space-heating burn, independent of the size & mass of the zone that's calling for heat.

While this is something the contractor should be competent to design, correctly spelling "plumbing and heating" on the side of the truck is no guarantee of anything. Ask questions, get multiple proposals.

There should have been no confusion for the guy selling the contractor proposing the Budurus, who should have been able to tell you without looking it up or doing the math that you can run in condensing mode most of the time, even with fin-tube in a house with 145' of fin-tube and a 30K design day heat load. Literally 99% of the time the load is under 30K and the bulk of the fuel burned will be at heat loads under half that. Considering that 145' of fin tube can deliver about 40-45K at 140F, odds are good that it can be set up to be in condensing mode even during design conditions. It puts a question mark over the design competence.

Similarly, anybody who would hand you a heat-load calc so OBVIOUSLY wrong for the house without batting an eye isn't someone I'd trust the system design to.

But the guy in the truck isn't always the hydronic designer- find out who is, and work the details with THEM. One way to find the competent contractors serving your area is to get a recommendation from the boiler distributor- THEY know who's always bugging them with inanne support questions, and who is installing a lot of a particular line with few returns and minimal support. Hydronic design isn't rocket science, but you DO have to run the math- it's not just plumbing hook-up problem.

[danboston]

Thanks Dana.

Is it Ok to use the polyisocyanurate boards on the walls? The local Building inspector wants us to use a vapor barrier so I glued foil-faced polyisocyanurate boards on the walls instead of plastic sheeting on a wooden frame for fear of moisture buildup and eventual rot of the wood frame. I was going to leave a 1-inch gap between the polyiso boards and the wood frame to prevent condensation water that is likely to form on the polyiso boards in the warm months from contacting directly with the wood frame. I do not have a moisture block between the top of the foundation and the sill, but I think I am going to have to live with it. I live on a nice dry hill and have never had water in the basement - just moisture issues in the summer months that is controlled with a dehumidifier.

Good recommendation about talking with the designer. The person who did the heat calcs is the contractor's distributor. I asked the contractor if I should send along information about actual fuel use and the HDD numbers and he said that he would not need those. I do have the distributor's contact information, I will just have to figure out the best way to get that information to the distributor without torquing off the contractor...

[jadnashua]

The absolute best way to determine how much heat you need is to compare it to what you used to make the house comfortable (to you) and then assess it to the outside conditions during that time. Anything else has some assumptions, which are often wrong, oversizing the unit. It's okay to give it a little fudge factor. One thing to keep in mind is that the house won't magically become an icebox if it is a little small...it just will drop back a few degrees on those really cold days. Say, you design it for 0-degrees, and were 'perfect' in yous assessment. Then, it gets to -1. The house would gradually cool off that one degree from your preferred setting, and then only after it had been there for awhile (depending on the insulation levels). Then, when the sun comes back out, it will warm back up. Where you need, or probably want, some excess, is if say the house was vacant for a long weekend, and you then turned up the thermostat when you got home. Without some extra, it would be quite slow to recover. Now, some of this is a function of how much radiator capability you have, but obviously, some is a function of how much heat you can provide to them as well. You'll be far more comfortable with the boiler running at the right temp all the time, with the radiators just providing the necessary heat to maintain your desired set point rather than running at full hot for a few minutes, then cooling off and then starting all over. The system will be more efficient and last longer, too.

There are too many contractors that either can't or won't make the proper calculations, resulting in inefficient systems. It used to be the norm to have a system way oversized since energy was cheap. Many are taking the lazy way out and using 'rules of thumb' to determine what is needed (like 145' of radiators needs x sized boiler). This almost always means that the system is oversized, but that you won't come back to them and say the system can't keep the house warm (since it has way more capability than needed).

[Dana]

Thanks Dana.

Is it Ok to use the polyisocyanurate boards on the walls? The local Building inspector wants us to use a vapor barrier so I glued foil-faced polyisocyanurate boards on the walls instead of plastic sheeting on a wooden frame for fear of moisture buildup and eventual rot of the wood frame. I was going to leave a 1-inch gap between the polyiso boards and the wood frame to prevent condensation water that is likely to form on the polyiso boards in the warm months from contacting directly with the wood frame. I do not have a moisture block between the top of the foundation and the sill, but I think I am going to have to live with it. I live on a nice dry hill and have never had water in the basement - just moisture issues in the summer months that is controlled with a dehumidifier.

Good recommendation about talking with the designer. The person who did the heat calcs is the contractor's distributor. I asked the contractor if I should send along information about actual fuel use and the HDD numbers and he said that he would not need those. I do have the distributor's contact information, I will just have to figure out the best way to get that information to the distributor without torquing off the contractor...

Foil faced iso on the walls is somewhat risky, since it raises the moisture content of the concrete from ground moisture and is only able to dry toward the exterior on the above-grade portion. There are two issues that get created:

A. Efflorescence and spalling begins to occur on the above grade exterior portoin of the foundation a 100+ year problem from a structural degradation point of view, but it can be mitigated with a sacrafisial parge on the exterior.

...but more seriously...

B: It raises the moisture content of the foundation sill and possibly the band joist to a level where it rots out (sometimes a sub-decade strucutral degradation issue.)

The solution is to put only semi-permeable or semi-impermeable foam against the foundation, but make it thick enough that the temperature of the interior side of the foam where it meets the wood studs is always above the dew point of the room air, even in winter. The "perm rating" of the foam is ideally between 0.5 and 1 perms, which is acheivable with 1.5-2" of XPS (pink or blue) which delivers ~R7.5-R10, or 3-4" of unfaced EPS(bead board- usually white), delivering ~R12-R16. That way ground moisture doesn't wick up the foundation- it can dry toward the interior of the basement. Under NO conditions should you put a vapor barrier on the inteior of the studwall, not even kraft-facers on batts (which are about 0.4 perms).

A combination of R10 of foam and unfaced R11 or R13 batts in the studwall that it delivers~ R19-R20 whole-wall values (thermal briging of the studs & plates included), and has a stackup that will reliably keep condensation from building up in the wood in our climate zone. That's what all the hygric analysis stuff is about in the this document:

http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

The best/most-cost effective solution is essentially case 8, figure 41, p.57 in that document. Read the moisture control paragraph, and print out the relevant sections for your inspector(!). The simulations were for a Minneapolis climate, which has a significantly cooler winter, with much higher condensation potential than in MA.

Note, they also simulated a foil-faced iso solution (case 9), which protected the studwall well, but note carefully the locations of the capillary breaks in the diagrams- unless your house has a metal or 10mil polyethylene sill gasket, you're stuck with problem B that I outlined above. If anything, errring toward a MORE permeable foam than the lower limit, say 2.5-3" of unfaced EPS (~R10-R12) would have been the ideal retrofit. If you're not changing out the 2" iso, make sure that the exterior of the foundation never sees snow buildup and that the whole foundation is well drained. If it's as well-drained below the footing as you think it is, you're probably going to be just fine. No house is built perfectly, yet they're still standing, but foil or poly vapor barriers against the foundation without a high-performance break at the sill isn't good building practice, in general.

FWIW: That inspector needs an education on this subject- insisting on a vapor barrier on foundation insulation is just plain WRONG (even though it's enshrined in the Canadian national building codes.) A vapor RETARDER, yes (XPS or EPS qualifies, as does closed cell spray foam at 1.5-2" thickness), but his insistence has no basis, and WILL create problems in somebody's house, hopefully not yours.

Whether it's 2" of iso or something else, as long as it's ~ R8 or more, it's well-worth the additional expense of unfaced R11 or R13 in the studwall, cutting the heat loss literally in half. Don't forget to put a capillary break (maybe your inchor so of foam you're putting over the slab) under the bottom plate of the stud too. (Even 1/4" of XPS is a sufficient capillary break, but if you put in right on the slab, put some poly underneath.)

In summer there's zero chance of condensation forming on the interior facer of the iso- butt the studs directly up against it- better yet, seal each bay with a bead of caulk before adding batts to maximize the performance of the fiberglass (which is very susceptible to convection and infiltration losses wherever there is a gap to convect into.) To get condensation the surface has to be below the dew point of the interior room air. If you dehumidify/air condition the basement to 60% relative humidity or less (recommended by ASHRAE- health professionals say 50%), even if it's 75F in the basement, that would put the dew-point at 60F. With ~50F sub-soil at the bottom of the foundation wall, with half the R-value in the foam layer, the place where it's 60F is inside the foam, protected from contact with the air. If the basement is 70F/60% RH, the dew point of the room air is 55F, but the facer is at 60F, still no condensation- you'll always win the summertime condensation game in summer anyway.

It's the winter, where at the above grade section will experience condensation for a random assortment of hours (the pre-dawn hours of the coldest winter days, typically), but never long enough to cause a problem as long as you have a sufficient ratio of foam-R to fiber-R. Even at 40/60 foam to fiber you'd be more than good in this climate, and center-cavity with R13 batts and R10 foam you'd be at 43/57. With R7.5 foam/R11 batts you're still at 40/60, and if you count the concrete as R1 (valid), R7.5 becomes R8.5, and you're at 39.5/60.5, which still has real margin if you use XPS or EPS, since the R value of those rise more than 10% when it's 0F on the cold side. (The rated R is the 75F number.) Iso drops in R value with temp, but iso rated R13@75F is still good for R11.5@0F. (The typical derated K-value for sub-zero use on iso is R5.6-R5.8/inch, down from R6-6.5 @ 75F.)

Before putting up the studs it's also better to air-seal the rigid foam by either taping the seams with FSK tape (2" foil tape often used to seal ducts) if foil-faced, or housewrap tape if XPS/EPS, or paint on some fiber-reinforced duct-mastic spread to ~1" either side of the seam.

If your iso extends all the way to the slab it's worth cutting back the bottom inch or so to ensure that there's no way moisture that might condense on or wick up through the slab wicks into the inteior of the iso, since there's no exit path for the moisture once it's between two foil facers. It wicks slowly, but it'll be semi-permanent if it ever gets in there.

[danboston]

I used a combination of dry-lock paint and superthoroseal on the walls before I glued the polyiso boards to the walls and there is at least a 2-3" section of the floor extending outward from the walls that got inadvertently sealed. By shear luck, with application of the 2" polyiso boards onto the walls, the bottom portion should be in contact with the sealed portion of the floor.

Is it typically allowed to install electrical boxes in the wall with the foamboard behind it? That is another reason why I was considering a 1-inch air gap between the R-13 polyiso foamboard and the wood frame studs because that would give about 3.5" total between the foamboard and the 1/2' gypsum board on the interior wall. If not, then I might as well butt the wood frame right up against the polyiso boards. If the 2" polyiso foamboards give me R-13, you are saying I could/should add additional unfaced batting in the cavities to increase the R-value?

Finally, If I install pink/blue XPES foamboard on the floor, with 1/2" plywood on top, what is the best way to add tile on top of that for the basement bathroom? Should I use 1/4" plywood and then 1/4" cement board for that portion of the bathroom that will be tiled?

[jadnashua]

The minimium I'd want to consider tiling to is 1/2" ply over foam, with C or better faces (i.e., no D faces) (on a subfloor, it's normally 5/8"). You might get by with 3/8", but that will warp some when you screw it down. Then, you'd need some cbu. If you wanted to do the insulation and get ready to tile in all one step, consider Wediboard. Expensive, but easy. It is the only tileable foam board (that I know of) that is approved for direct thinset bonding to a slab. Comes in various thicknesses, so you can choose the level of insulation (and cost$$) you wish to make.

[danboston]

Wedi board looks pretty neat. Where can you get that stuff around here?

As far as mass for the heating system, the monoflo baseboard system is fed by 1.5-inch copper pipe. I calcuated about 13 gallons of water in the pipes at any one time (assuming 145 ft x 0.0918 gallon per foot).

[Dana]

I used a combination of dry-lock paint and superthoroseal on the walls before I glued the polyiso boards to the walls and there is at least a 2-3" section of the floor extending outward from the walls that got inadvertently sealed. By shear luck, with application of the 2" polyiso boards onto the walls, the bottom portion should be in contact with the sealed portion of the floor.

Is it typically allowed to install electrical boxes in the wall with the foamboard behind it? That is another reason why I was considering a 1-inch air gap between the R-13 polyiso foamboard and the wood frame studs because that would give about 3.5" total between the foamboard and the 1/2' gypsum board on the interior wall. If not, then I might as well butt the wood frame right up against the polyiso boards. If the 2" polyiso foamboards give me R-13, you are saying I could/should add additional unfaced batting in the cavities to increase the R-value?

Finally, If I install pink/blue XPES foamboard on the floor, with 1/2" plywood on top, what is the best way to add tile on top of that for the basement bathroom? Should I use 1/4" plywood and then 1/4" cement board for that portion of the bathroom that will be tiled?

I believe it's OK to install the electrical boxes even if in FULL CONTACT with the foam (or at least I've seen it done that way MANY times, including with spray foam) as long as you have gypsum between the foam and the room interior. If anything leaving a gap would increase, not decrease the rate of flame spread in the event of an actual fire, since the studs would then not act as a horizontal firestop. Butt the studs up to the foam board no matter what.

And yes, adding batting in the stud cavites is a cheap & effective way to roughly double the whole-wall R value. It's safe from a mold/rot/condensation aspect in your climate as long as the foam is at least ~ R10 @ 20F (roughly your January binned-hourly temperature average) as foam, which you do. R13 iso isn't really performing to R13 at 20F, but it's still more than R11 even at 0F. The difference between R13 and R20 is something you can actually FEEL in the depths of winter, and adding the batts is easily economic in this climate in a 15 year NPV analysis on fuel savings alone were you to stick with an 86% AFUE oil burner using reasonable discount rate & fuel price inflation numbers (it's more like 20-25 years with similar assumptions and condensing gas burner.) It also allows you to down-size the radiation in that room since it's cutting the heat load nearly in half (assuming you have an R10 floor and few basement windows), offsetting the cost slighly. A general rule of thumb of long term cost-effectiveness for new construction in this climate is R10/R20/R40/R60 for slab/foundation/above-grade-wall/roof, but the true economics will vary widely by heating system type and fuel price inflation. Obviously retrofitting the above grade walls is daunting and not likely to be something you'd pursue (but it's not impossible-see: https://www.powerofaction.com/media/..._Pictorial.pdf), and not likely to be cost-effective, but retrofitting the as-yet unfinished basement is, in your case.

The fact that your existing system is somewhat higher mass is a good thing(!), but keepin in mind it's the mass of the smallest zone that's key to keeping the efficiency up and not short-cycling the boiler into an early grave or higher-maintenance. A decent system designer will run the numbers, but it doesn't hurt to do the rough calc yourself.