What's causing incomplete combustion, yellow tipping of flame and white soot on exchanger?

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Dave1

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I have a Weil-McLain CGA-3-SPDN Series 1 Natural Gas boiler that was installed in 2006. To my knowledge is has not been serviced/cleaned since we purchased the home, about 7 years ago (really stupid on my part, no bashing comments necessary). We just had a technician come to clean/service, and he found white soot build-up on the heat exchanger. He asked me to meet with him and another person from their company the following day to look at it and discuss my options. In the meantime he recommended we shut it down and he turned the gas off. The following morning the technician (and his superior or a sales rep.) showed me the sooting and the analyzer reading inside the flue (+700 ppm of CO) , and what they said was excessive yellow tipping of the flames. They said cleaning the exchanger would be expensive (+/- $1,000) and they couldn't guarantee how clean they could get it, or how long it would be until it would need another such cleaning, therefore, they recommended a new boiler. Despesite asking, they were not able to tell me what is causing this white sooting or the yellow tipping of the flames. Can you anyone here tell me what would cause this incomplete combustion, yellow tipping and sooting? Or whether a good cleaning is all that's needed? Any information is appreciated.
IMG_20191030_190916resized.jpg IMG_20191030_190252856resized.jpg IMG_20191029_192234182resized.jpg
 

Dana

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Yellow tipping on the flames can either be dirty burners or (rarely) improper fuel pressure/mixture. That is the sole cause of your CO issue, not the crud.

Since it's still measuring 82.5% combustion efficiency I wouldn't sweat the boiler plate crud too much:

index.php


Spec on a brand-new current version CGa-3 is only 84%, and boilers do lose some raw efficiency over 13 years. Whether you clean off the white crud (which is NOT soot) you can clean the burners or fix any pressure issues to get nice blue flame front to lower the CO, the raw combustion efficiency is going to rise a bit too once there is clear-blue flame, no yellow. Unless that white stuff is hard-baked and hard to scrape off it'll probably disappear over a season's of use as long as the boiler is running at a reasonable duty cycle.

Usually white crud like that is simply a light ash from dust/crud being burned by a continuously burning standing pilot flame or dust-contaminated combustion air, not soot. Soot deposits are always black:

maxresdefault.jpg

If you clean up the basement/boiler room and air-seal the foundation walls to limit outdoor air infiltration (a common source of dust) there will be less dust to burn, and slower accretion of white ash. If the boiler is only used for space heating (not domestic hot water) it may make sense to turn off the pilot in summer too.

The ash-crud has absolutely zero effect on the CO levels- CO is all about the burner's cleanliness & air/fuel mixture that's creating the yellow tipped flames. The ash does have a modest effect on raw combustion efficiency. Cleaning it once every 10 years may make sense if it keeps showing up, but not every year or every 5 years. The difference between 84% combustion efficiency and 82.5% doesn't add up to much in terms of annual ccf or therms, an increase of only 84/82.5= ~2%. If you're burning 1000 therms/year that's 20 therms. You're probably burning up more than that with just the standing pilot ignition running 24/365.
 

Dave1

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Yellow tipping on the flames can either be dirty burners or (rarely) improper fuel pressure/mixture. That is the sole cause of your CO issue, not the crud.

Since it's still measuring 82.5% combustion efficiency I wouldn't sweat the boiler plate crud too much:

index.php


Spec on a brand-new current version CGa-3 is only 84%, and boilers do lose some raw efficiency over 13 years. Whether you clean off the white crud (which is NOT soot) you can clean the burners or fix any pressure issues to get nice blue flame front to lower the CO, the raw combustion efficiency is going to rise a bit too once there is clear-blue flame, no yellow. Unless that white stuff is hard-baked and hard to scrape off it'll probably disappear over a season's of use as long as the boiler is running at a reasonable duty cycle.

Usually white crud like that is simply a light ash from dust/crud being burned by a continuously burning standing pilot flame or dust-contaminated combustion air, not soot. Soot deposits are always black:

maxresdefault.jpg

If you clean up the basement/boiler room and air-seal the foundation walls to limit outdoor air infiltration (a common source of dust) there will be less dust to burn, and slower accretion of white ash. If the boiler is only used for space heating (not domestic hot water) it may make sense to turn off the pilot in summer too.

The ash-crud has absolutely zero effect on the CO levels- CO is all about the burner's cleanliness & air/fuel mixture that's creating the yellow tipped flames. The ash does have a modest effect on raw combustion efficiency. Cleaning it once every 10 years may make sense if it keeps showing up, but not every year or every 5 years. The difference between 84% combustion efficiency and 82.5% doesn't add up to much in terms of annual ccf or therms, an increase of only 84/82.5= ~2%. If you're burning 1000 therms/year that's 20 therms. You're probably burning up more than that with just the standing pilot ignition running 24/365.


Thanks for all the information Dana!

If the white stuff is hard baked-on, would that be cause for a new boiler? How would one know if it is baked-on? (I stuck my hand up in there and light touched it and that portion fell off in very fine white powder; ash, as you said.)

The boiler is in the middle of the basement (not enclosed) and about 15' from the dryer. Typically we try to keep dryer lint from entering the home, but in the winter months my wife occasionally diverts the dyer exhaust air into the basement to help with heat (insanely cold where we live) and low humidity. I imagine this would contribute to the crud buildup since it's during the time of year when the boiler is running.

When they showed me the yellow flames, this is what the burner looked like.
IMG_20191030_185458983_HDRresized.jpg


If CO is all about the burner cleanliness and air/fuel mixture, why wouldn't they recommend a fix as opposed to shutting it down?
 
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Dana

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How would one know if it is baked-on? (I stuck my hand up in there and light touched it and that portion fell off in very fine white powder; ash, as you said.)

The fact that it flakes off in a light powder means it's not a hard-fired deposit bonded onto the cast iron. If it's flaking off and falling onto the burners, that could cause some amount of yellow-tipped flame. Cleaning it up wouldn't be a terrible idea if that's happening.

...in the winter months my wife occasionally diverts the dyer exhaust air into the basement to help with heat (insanely cold where we live) and low humidity.

The lint is more than likely the source of the ash problem, as you surmised.

The "...insanely cold..." and "...low humidity..." in winter means your house is leaking tons of air, and (I'm guessing) your basement foundation walls, foundation sill & band joist are not insulated. In a basement of an averaged sized house that's reasonably tight, and insulated to current IRC code minimums the standby & distribution losses of the boiler are usually enough to keep the basement above 60F even at negative double-digits F, often above 65F.

For reference I live in a 2400' house + 1600' of insulated but not directly heated insulated to R17 continuous insulation (only slightly better than code), and it's just fine down there at -10F outdoors. (And I don't even have the standby losses of a cast iron boiler to help out.)

What is the ZIP code in this insanely cold location? The fact that you're successfully heating the place with a CGa-3 means it probably isn't a 4000 square foot house in Fairbanks AK.

If CO is all about the burner cleanliness and air/fuel mixture, why wouldn't they recommend a fix as opposed to shutting it down?

Maybe because they don't know how to fix it(?) The burner tubes look pretty clean on the exterior, but did the pull them and brush them, inside and out? You may have lint inside the tubes screwing things up a bit.

By "...shutting it down...", do you mean to say they don't recommend running it due to the 724 ppm CO?
 

Dave1

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How would one know if it is baked-on? (I stuck my hand up in there and light touched it and that portion fell off in very fine white powder; ash, as you said.)

The fact that it flakes off in a light powder means it's not a hard-fired deposit bonded onto the cast iron. If it's flaking off and falling onto the burners, that could cause some amount of yellow-tipped flame. Cleaning it up wouldn't be a terrible idea if that's happening.

...in the winter months my wife occasionally diverts the dyer exhaust air into the basement to help with heat (insanely cold where we live) and low humidity.

The lint is more than likely the source of the ash problem, as you surmised.

The "...insanely cold..." and "...low humidity..." in winter means your house is leaking tons of air, and (I'm guessing) your basement foundation walls, foundation sill & band joist are not insulated. In a basement of an averaged sized house that's reasonably tight, and insulated to current IRC code minimums the standby & distribution losses of the boiler are usually enough to keep the basement above 60F even at negative double-digits F, often above 65F.

For reference I live in a 2400' house + 1600' of insulated but not directly heated insulated to R17 continuous insulation (only slightly better than code), and it's just fine down there at -10F outdoors. (And I don't even have the standby losses of a cast iron boiler to help out.)

What is the ZIP code in this insanely cold location? The fact that you're successfully heating the place with a CGa-3 means it probably isn't a 4000 square foot house in Fairbanks AK.

If CO is all about the burner cleanliness and air/fuel mixture, why wouldn't they recommend a fix as opposed to shutting it down?

Maybe because they don't know how to fix it(?) The burner tubes look pretty clean on the exterior, but did the pull them and brush them, inside and out? You may have lint inside the tubes screwing things up a bit.

By "...shutting it down...", do you mean to say they don't recommend running it due to the 724 ppm CO?

When I touched the white stuff, it was ever so slightly that I didn't really feel if anything was hard-baked on the cast iron. Wasn't sure if was supposed to be sticking my hand up in there touching stuff (but everything was shut off)??? I can certainly give it a better feel if it's okay to do so?

To clarify, when I say it's insanely cold, I just mean outside during the heart of winter. Zip code is 54880. Basement temp is easily 60F even during the coldest time of the year. Dehumidifier is set at 50% and never runs in the winter. The thought was, why waste the dryer heat by diverting it outside if we can just send it into the basement. Probably bad idea. There won't be any more of that. However, there wasn't much dust/lint accumulating on flat surfaces. Didn't occur to me that the furnace is sucking in all that air/dust/lint during combustion. Foundation walls are not insulated, but everything else is insulated pretty nicely.

Guess I'm not sure if they brushed the inside of the tubes... I'm not confident enough to pull them out, inspect and replace them. Unless I had instructions to go by.

By "...shutting it down..." I mean; They told me I should not run it, they turned it off, and they turned the gas supply to the boiler off as well. They had me sign a waiver releasing them of liability if I were to run it.
 

Dana

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The 99th percentile temperature bin in ZIP 54880 is about -15F, and yes, I know it's colder than that (about 1% of all hours over 25 years of temperature data.) The Duluth Int'l Airport's 99% design temp is -16F.

You are in US climate zone 7A. The IRC calls out R15 continuous insulation for basement walls in zones 5 & higher, per TABLE N1102.1.2. The R-value of an 8" poured concrete foundation wall is about R1.35. When it's -15F outside and +60F inside (a 75F temperature difference) every square foot of above-grade exposed foundation wall is losing 75F/R1.35= ~55 BTU/hr. The ground temps aren't quite that cold, but cold enough- it's still pretty lossy below grade in your climate. It doesn't take very much foundation wall to add up to a significant fraction of the whole house heat load, and correspondingly sized fraction of the fuel use.

And that's just the conducted heat loss. Air leakage at the foundation sill and band joist can be huge when it's cold out due to the stack effect pressures. The 68-70F air leaking out of leaks near the top of the house depressurize the house, and the leaks near the bottom of the house experience the greatest amount of stack-pressure difference. That stack-effect driven infiltration draws ultra-dry outdoor air into the house, which is a large part of why it's so lip-chapping fingernail-splitting dry when temps drop below 15-20F.

Air sealing and insulating the basement to R15 continuous insulation will raise the basement temp by at least 5F, and reduce the 24/7 infiltration drives. Installing a better than average back draft preventer on the dryer vent is also a good idea in your climate, otherwise it'll suck air from stack effect drives all winter. The most important air leaks (by far) are those at the top of the house and the bottom of the house- the upper floor ceilings an all electrical & flue & plumbing stack penetrations of attic floor should be meticulously sealed, and sealing any plumbing/electrical/flue chases that run from the basement to the attic need to be sealed at both ends to limit stack effect drives.

It's time to start calling around to different boiler service companies to find somebody willing to deal with the CO problem. The high CO count isn't a disaster as long as the boiler isn't back drafting and the venting doesn't leak. Whether it's fixed or not, it's worth installing some fresh CO detectors, including one near the bedrooms, and one in the boiler room. If the chimney isn't on an exterior wall, and is instead running in a chase surrounded by warm conditioned-space air it's not very likely to back draft unless it's CRAZY oversized.

Your mean January temp is about +15F, about 10F -12F cooler than my location, which means your outdoor air will somewhat drier than mine, but not seriously drier. Since getting more serious about air sealing the house the indoor relative humidity stays above 30% RH @ +70F except during extended cold snaps that don't rise above +10F for a few days. YMMV.

Adding humidity to the indoor air should only be done with caution- at 40% RH there is a significant risk of moisture accumulation in the wall sheathing, and along any air leaks into the attic, creating high mold-spore counts indoors in the springtime, and potentially rot conditions in the sheathing. Public health recommendations usually deem 30% - 50% RH the "healthy zone" for humans, but given the number cold winter hours in your location keeping it at the low end of that range will limit the seasonal moisture burden on the structural wood in your house.

High summertime humidity in the basement in climate zone 7A is also from outdoor air leakage, since the outdoor dew points go well above the basement slab and wall temperatures. Air sealing and insulating the walls will reduce the duty cycle of your basement dehumidifier too.
 

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at 40% RH there is a significant risk of moisture accumulation in the wall sheathing,
Getting a little off topic here (I could start a new discussion?), but a few years ago while scrapping the siding in preparation to repaint we came across some significant amounts of rotten siding that needed to be replaced, but on the north side only. The rot in some spots was bad enough that you could stick your finger through the siding so we decided to replace those particularly bad sections. Here is the north was prior to repairing the siding, and a photo during the repair.

IMG_20160815_172109623_HDR.jpg

IMG_20160903_183220615.jpg


Upon removal and replacement of the rotted siding, just to the left of the window in the image above, behind where the shutter had been, it seemed like it may be rotting from the inside out because even the sheeting was significantly rotten (the sheathing was in worse condition than the outside of the siding). It hadn’t rained for several days but the back of the siding was still very damp. This was concerning. Most of the rotting was occurring behind the siding, which I suppose is normal if rain water had been infiltrating where the shutters were fasted to the wall, but as I said, to the best of my knowledge it hadn't rained in several days.
I didn't have time to tear into it then, but I and made notes for a time when I may want/need to tackle it. To help with the humidity in the house, I've since installed a VERY powerful bathroom exhaust fan, and never divert clothes dryer air into the basement. All this time I've been wondering what would cause the sheathing behind the siding to be rotting. I don't think this wall is very well insulated, because in the winter we get frost showing up in some areas on the inside of these north rooms such as down by the floor trim on the nail heads. Also, there was a time during a cold snap when my daughter left a huge pile of clothes in her closet (piled against that suspicious north wall), and when I removed the clothes there was a thick layer of ice built up on the wall! This last summer I removed the sheet rock from this inside-the-closet section of the north wall to assess the situation. Behind the sheetrock was about 3" of flimsy fiberglass insulation. Behind the insulation was tar paper... curved around and stapled to the studs. Behind the tarpaper was the interior side of the sheathing. It doesn't seem right that the tarpaper was on the inside of the sheathing.
This tarpaper had to have been added after the house was built, because to was stapled to the studs. Not sure if this is the case for the whole north wall or just this spot where the closet is.


Anyways, back to the furnace topic. I know it's been a while but I wanted to close the thread on this. I ended up having the boiler replaced because my relative was able to get me one (Weil-McLain Gold CGA-3 Series 2) at cost, and his friend installed it for practically nothing. Total came to less than half of what the original company wanted.... they quoted me $5,300. I don't remember if I had ever gotten a second opinion about the CO levels/yellow tipping etc. but I probably did. At any rate, problem has been solved by way of a new boiler. A high efficiency unit would have been preferred, but I'm satisfied.
 

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o2 & co2 should be 6-8
Co less than 50 like to see it at 0
The dirtier the flame the higher the efficiency but can't run a burner like that. Also it would of been nice if they wrote the gas pressure down. Still have the old boiler?
 

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Getting a little off topic here (I could start a new discussion?), but a few years ago while scrapping the siding in preparation to repaint we came across some significant amounts of rotten siding that needed to be replaced, but on the north side only. The rot in some spots was bad enough that you could stick your finger through the siding so we decided to replace those particularly bad sections. Here is the north was prior to repairing the siding, and a photo during the repair.

The north side is always at highest risk of moisture issues due to the lack of solar warming, which helps dry out both the siding & sheathing. The fact that there is nearly zero overhang on the rake of the roof guarantees chronic wetting of the siding with even the slightest amount of wind. (The overhang at the eaves looks adequate for protecting those sides of the house.) If/when re-roofing it's worth adding a foot or so of overhang to the rakes to better protect the wall assemblies from direct wetting (and possible bulk water incursions from mis-applied window flashing or mis-lapped housewrap/tarpaper.)

Upon removal and replacement of the rotted siding, just to the left of the window in the image above, behind where the shutter had been, it seemed like it may be rotting from the inside out because even the sheeting was significantly rotten (the sheathing was in worse condition than the outside of the siding). It hadn’t rained for several days but the back of the siding was still very damp. This was concerning. Most of the rotting was occurring behind the siding, which I suppose is normal if rain water had been infiltrating where the shutters were fasted to the wall, but as I said, to the best of my knowledge it hadn't rained in several days.
I didn't have time to tear into it then, but I and made notes for a time when I may want/need to tackle it. To help with the humidity in the house, I've since installed a VERY powerful bathroom exhaust fan, and never divert clothes dryer air into the basement. All this time I've been wondering what would cause the sheathing behind the siding to be rotting.

Your local climate zone in ZIP 54880 is climate zone 7A:

IECCmap_Revised.jpg


In climate zones 4C and zone 5 (A, B, or C) and higher air leakage into the wall from the interior (wallboard) side can put significant amounts of moisture into the sheathing over a winter, and if it can't dry easily to the exterior both the exterior wetting drive and interior water vapor and air airborne moisture wetting drives leads to rotting sheathing. In zone 7A it's cold enough long enough for even vapor diffusion through standard latex paint to deliver enough moisture burden to create rot on the north side.

The tarpaper on the interior side of the sheathing doesn't do much either way. When bone-dry tarpaper is a pretty good vapor retarder at about 0.5 US perms (which is still 5x more vapor open than a true vapor barrier like 4 mil polyethylene), but when wet it becomes far more vapor open more than 5 perms (the vapor retardency of standard interior latex paint). If there is/was a sheet polyethylene vapor barrier or foil faced batts it would also prevent drying toward the interior. (Kraft facers would still be OK, they have variable vapor retardency in much the same way that tarpaper is, and even more vapor-open than tarpaper.) In zone 7A the wall has to be both air tight and relatively vapor tight (<1 perm) to the interior during winter to protect against wintertime moisture build up. But for the assembly to allow drying toward the interior it's better to use a variable permeance membrane such as Intello Plus or Certainteed MemBrain, which are <1 perm when the air inside the cavity is dry (which it is, when the sheathing is cold, absorbing any available moisture), but become relatively vapor open when the air inside the insulation is high enough to create a mold & rot risk.


I don't think this wall is very well insulated, because in the winter we get frost showing up in some areas on the inside of these north rooms such as down by the floor trim on the nail heads. Also, there was a time during a cold snap when my daughter left a huge pile of clothes in her closet (piled against that suspicious north wall), and when I removed the clothes there was a thick layer of ice built up on the wall! This last summer I removed the sheet rock from this inside-the-closet section of the north wall to assess the situation. Behind the sheetrock was about 3" of flimsy fiberglass insulation. Behind the insulation was tar paper... curved around and stapled to the studs. Behind the tarpaper was the interior side of the sheathing. It doesn't seem right that the tarpaper was on the inside of the sheathing.
This tarpaper had to have been added after the house was built, because to was stapled to the studs. Not sure if this is the case for the whole north wall or just this spot where the closet is.

If the wallboard is getting cold enough to form ice during a cold snap it means there are large air leaks into the wall cavities from the exterior. That can only be fully rectified on either the exterior starting by pulling off the siding), or the interior (if you gut the wallboard.) Those sheathing side air leaks are big enough to bypass and defeat the low-density fiberglass insulation, but not reliable enough to provide adequate drying capacity. The person installing the tarpaper may have been trying to use it as a combination air barrier and smart vapor retarder, but it's nearly impossible to tarpaper sufficiently air tight to be effective.

If you are going to either completely re-side or gut the wallboard on that wall I can go into detail on how to air-seal the wall. If re-siding, along with air sealing details, adding even 1/4" of rainscreen gap can improve the drying capacity of both the sheathing and the siding.

Air leakage and vapor retardency aside, BULK WATER incursions and mis-lapped window flashing or exterior side tarpaper/housewrap are responsible for more rot problems than air leaks or vapor diffusion combined. If the rot issues are limited to just around and under the windows it's a clue.
 

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You should have your only vapor barrier on the inside side in WI. I don't know if tar paper is a vapor barrier, but it sure sounds like it would be. Tyvek house wrap etc will block wind but will pass vapor enough.

Vapor barrier inside can largely be done with paint, from what I understand.
 

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You should have your only vapor barrier on the inside side in WI. I don't know if tar paper is a vapor barrier, but it sure sounds like it would be. Tyvek house wrap etc will block wind but will pass vapor enough.

Vapor barrier inside can largely be done with paint, from what I understand.
Tar paper is not a vapor barrier- it's a vapor retarder. When bone dry it's vapor permenance is in the Class-II vapor retardency range (0.1-1.0 perms, or semi-impermeable), but when it's damp it's Class-III (1-10 perms) or higher, depending on how thick it is. To be a true vapor barrier requires a vapor retardency <0.1 perms. In Canadian building codes the definition of "vapour barrier" corresponds to Class-II or lower vapor redardency, which is good enough.

It's actually hard to make housewrap into a reliable air barrier, but it is usually Class-III or higher in vapor redardency, either semi-permeable (such as Typar at about 8-12 perms) or VERY permeable (such as Tyvek, at 25+ perms). With plywood sheathed homes detailing the plywood as the primary air barrier usually works well, but detailing wallboard as an air barrier is also very worthwhile, especially in colder climates. The amount of moisture transfered through a square inch of air leak is worth more than a whole wall's worth of vapor diffusion through latex paint on gypsum board.
 

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Tar paper is not a vapor barrier- it's a vapor retarder.
So tar paper would be just fine under the siding for a cold place, and not be a moisture trap/
 

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So tar paper would be just fine under the siding for a cold place, and not be a moisture trap/

That's right- #15 felt has a 100+ year history of use as siding underlayment in IECC zones 5 & higher. Behind stucco & brick more vapor retardent & heavier #30 felt (most commonly used under roofing) has been used as well, usually with good results.

In the middle of the last century asphalted fiberboard is even more vapor-open than #15 felt, and was often used behind masonry claddings as structural sheathing due to it's inherently higher moisture resilience- it tolerates wetting well, and is well north of 10 perms, allowing ample drying to the exterior when there is a vented cavity between the masonry & fiberboard. But a drawback with higher vapor permeance asphalted fiberboard shows up in air conditioned homes in humid climates as mold & rot on the interior finish walls of AIR CONDITIONED houses when the wallboard or plaster & lath dwell below the dew point of the outdoor air for weeks on end. Fiberboard sheathing combined with a vented rainscreen gap between the sheathing & siding works very well in cold climates. It's not as structural as plywood or OSB, and may need let-in bracing or shear panels etc. to handle the wind loading in some locations. The down side of plywood & OSB is their comparatively low vapor permeance (~ 1 perm when dry), which creates something of a moisture trap for insulated wall assemblies.

In Europe fiberboard sheathing is currently being marketed as moisture tolerant exterior insulation in lieu of rigid foam board for both walls & roof and it performs pretty well. Maine startup GO-Lab is opening up a mill in Maine for a similar range of higher-R insulating fiberboard (and wood-fiber batt insulation), slated to hit the market in 2022 (after being delayed by the pandemic.) At higher-R the thermal mass effects of fiberboard insulation can do a lot for lowering peak cooling loads when used on roofs as well.
 
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