subfloor in basement which is correct

[JVIPER]

I like the idea of keeping the floor as warm a possible cause this will probably be the place I spend the most time when home. Should I skip the insulation completely and go radiant? Also can radiant floor heating be laid over bare concrete and tiled over directly? This would barely cost me an inch of headroom. Will typical moisture from the concrete ruin radiant?

[mtcummins]

are you talking about electric radiant like you see in a bathroom remodel or hydronic radiant (using water lines), which is what i have in my house. electric is a lot more expensive to run, but hydronic requires more equipment to install.

unless you know that there is sufficient insulation under your slab already, you def want insulation between your heated floor and the slab, or most of your heat will go into the ground. I would do an inch of radiant, plywood, electric heat, and tile if that's the way you want to go. on a large scale like that, you'll want to consult an electrician for load requirements, etc.

if you want hydronic heat and the initial expense of the pumps and such required for that, you would do an inch of xps, then they make plywood with knobs on it made to click in the pex water lines, and then you can put down whatever floor you want (that is approved for radiant heat - some laminates/engineered woods are, some aren't) over that knobbed plywood. this will be about 5/8" thicker overall than the electric option.

either way, if you're heating the floor, you definitely want insulation. if the house was built to have a radiant slab, they should have put at least 2" foam under the slab to keep the slab's heat inside, rather than heatsinking into the dirt.

[cacher_chick]

I wouldn't worry too much about the thermal expansion/contraction here, but might consider doing 1" or 1.5" xps under your floor, and putting a 1/2" xps panel loosely between your duct and the wood panel if you're worried about it.

XPS is flammable. Most building codes require that when it is used inside a structure that is must be covered with a fire block. (drywall is common on walls and ceilings)

[mtcummins]

this is a good code point. its one i would probably ignore in this case, as while i understand the basic intent, I don't think this would really apply to the intent of the code. thats not to say that an inspector wouldn't potentially flag it though...

that said, the OP said the duct is insulated to some degree already, so you could just leave it alone and put the panel near the duct, but not touching it, and it will probably be fine. or put drywall over it... i have in past lives attached drywall directly to heat ducts rather than creating a bulkhead for it to run inside. saved a couple inches. i don't advocate that practice anymore, but i can say that nothing happened to the drywall from doing this. i don't think the level of heat through the duct is enough to really worry about it that much

[Dana]

I used to have a humidity issue with a small rain issue until I did the foundation grading, gutters and de humidifier inline with my hvac system. 2 years have past and no moisture issues but I like to be over prepared so...

Would 6mm poly and 2" xps be way over kill for the floor?

I you're putting down 2" of XPS, there's no point to the poly (assuming you meant 6 mils or 0.006" not 6mm.) The foam is fairly vapor-retardent, provides a superb capillary break, and it's keeping the subfloor at room temp, not subsoil temp- the subfloor won't be wick up groundwater OR taking on room air moisture via condensation. In the event of a bulk water intrusion the poly isn't going to save you.

2" of XPS is over code-min for new construction in MA, and may not have a reasonable 25 year present-value on the future energy savings at buck-a-therm natural gas in an 90% burner, but 1.5" (R7.5) would. (See: http://www.buildingscience.com/docum...-climate-zones you're in zone 5) If you're heating with oil, propane, or electricity by all means take it up to 2". Alternatively going with reclaimed 2-2.5" Type-I EPS (R8-10) would be dead-cheap, and payback is fast. In MA there are several vendors handling reclaimed rigid foam- The Insulation Depot in Framingham is the biggest, but if you searched craigslist materials for "rigid foam insulation" you'll find others. There's a guy in Winchendon advertising there that always seems neck-deep in EPS for cheap. Use only EPS or XPS, not iso.

If you go with radiant heat (any type) you want MORE insulation, not less, otherwise you'll spend quite a bit of money heating up the 48-50 suboil in Tyngsboro. R15 (3" XPS) would even be cost-effective if electric radiant is used, but that's quite a hit in headroom.

Half-inch OSB or plywood would be sufficient for meeting code on ignition barriers for foam, as would half-inch or thicker gypsum board.

[diyfun]

Poly sheeting is a cheap & effective Class-I vapor retarder ( ~0.05 perms at 6 mils), which takes the guesswork out of it. While vapor-sealing foundation walls could have negative consequences (rotted foundation sills, efflorescence & spalling on the exterior above grade), there's no such issue with vapor sealing below-grade slabs- indeed, it can even reduce radon levels.

Hi Dana, why vapor-sealing wall has such negative consequences? for a full basement, if we don't lay the vapor barrier up to the top and end the barrier at least 1 foot away from the top, does that still has negative affect on the wood? Any also how it will cause efflorescence and spalling on the exterior above grade? I just want to fully understand what you have said. Like many others, I really appreciate you answer my questions and help us choose a right way to make our home better.

[Dana]

Hi Dana, why vapor-sealing wall has such negative consequences? for a full basement, if we don't lay the vapor barrier up to the top and end the barrier at least 1 foot away from the top, does that still has negative affect on the wood? Any also how it will cause efflorescence and spalling on the exterior above grade? I just want to fully understand what you have said. Like many others, I really appreciate you answer my questions and help us choose a right way to make our home better.

Fully vapor sealing the wall keeps the moisture in the concrete, which raises the moisture content to where it meets the wooden foundation sill, and the sill can rot. (In some cases it can cause the exterior concrete to spall/crumble from freeze-thaw cycles too, but that's more common in places colder than MA.) Removing the vapor barrier at the top doesn't reduce the moisture in the concrete very much, and allows wintertime condensation of moisture from the interior air occur, since at the top the foundation is colder.

As long as surface drainage has been dealt with and the exterior of the foundation has been waterproofed, much of the moisture in the foundation is water wicking up the wall from the footing. If you leave a 1-foot gap in the insulation & vapor retarder at the BOTTOM of the wall closer to the source of the moisture it will be more protective than leaving a gap at the top. At the bottom the foundation will be much warmer in winter, so it won't condense water out of the room air there the way it will at the top.

[Pikazza]

I have a question for anyone that is able to help. My wife and I just bought a 1953 rambler. The basement had old, shag carpet that we removed. Underneath that was old tile, which we also removed. This left the black mastic glue, which I had hoped to remove so we could seal the concrete and keep out efflorescence (which we found a lot of when we removed the tile). The problem is that removing the glue has turned into an unsuccessful tar pit mess! The product I used, Bean-e-doo, has not lived up to it's claim of being easy. Since I plan to put a subfloor over the concrete, I'm wondering if I need to worry about getting the rest of the mastic glue up, or if I could just lay my subfloor over it. Obviously, laying over it would save a whole lot of elbow grease and added cost. I would appreciate any help that can be offered. Thanks!

[jadnashua]

That black goo may contain asbestos, so be careful of it! If you have moisture, the bigger issue is what can you apply that will not rot out or create lots of mold. I'm not sure...wait for others.