Getting caught on the latest basement insulation recommendations
I have just done some reading on the site, getting caught up and saw a few articles specially discussing the issue of basement insulation etc.
This article hits on many of the key issues.
https://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall
My question is in relation to us “Northerner’s” who should be targeting R-15 per the article when adding insulation via the interior.
Currently with XPS and ISO your looking at apprx R -5 to 6.5 per inch
To get to a R -15 you’re looking at doubling up insulation boards (2 boards). My first question is in relation to the resulting perm rating that adding 2 layers would equate to. I don’t have the exact article in front of me, however do recall reading in one of Buildingscience.com bulletins not to exceed 2’’ as the perm rating would be too low.
However, I came across the following
“Lstiburek now says that the advice he gave in the Builder’s Guide was wrong. Lstiburek said, “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”
Lstiburek’s former recommendation to use vapor-permeable foam was due to a concern that imperfections in the foam installation might allow some warm interior air to contact the cold concrete, leading to condensation. However, experience has shown that most foundation walls, even those with small amounts of condensation, experience some drying activity at the top of the foundation wall”
If I am reading this correctly, doubling up rigid insulation may INDEED be ok?
Irrespective of the above answer, assuming the basement is not prone to moisture intrusion. It seems that rigid board, followed with unfaced fiberglass is still not completely endorsed per Martin’s comments here “a 2×4 wall on the interior side of the foam insulation and fill the stud bays with fiberglass batts. This approach is less risky than installing fiberglass directly against the concrete. However, I don’t think that fiberglass batts belong in a basement. My advice: if you want a higher R-value, just install thicker rigid foam, and leave the stud bays empty.
So can rigid board be doubled up and the concern over perm ratings be tempered? If not, and us “Yankee’s” still want to get to R – 15 can unfaced batts be added?
A few more questions to follow!
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Replies
I assume you have no insulation on the outside of the basement walls, and want to add insulation to the interior. You could use a single layer of 3" XPS board for R-15 on the interior. I would make sure to seal all the joints between the sheets to prevent vapor from migrating out of the concrete and into the living space. That joint sealing would also prevent condensation from vapor moving outward at some times and condensing on the concrete and wetting it, but that is not the purpose of the joint sealing. The concrete will be damp anyway from the ground contact on the exterior. The point of sealing the joints is to keep the concrete from drying to the interior.
Ron,
R-15 (or more) is good. Doubling up is, in fact, better than installing the foam in a single layer, because doubling up allows you to stagger the seams (making it more difficult for any interior air to sneak through the seams and contact the cold concrete).
For those who are still worried that thick foam might be problematic (it isn't): Think about ICF foundations. They have foam on both sides, and work just fine.
Below is a photo of a retrofit project that has two layers of rigid foam attached to the interior of a basement wall. The photo comes from this blog: The High Cost of Deep-Energy Retrofits.
MS,
I don't understand your noise issue. If it is a basement, it would seem like you would already have considerable noise blocking from the concrete and soil. How does adding a little fiberglass help?
Sounds like the concern of having more than 2'' of rigid foam and the resulting low perm ratings is not that big of a deal after all.
I know Martin in your basement insulation article you say
"“a 2x4 wall on the interior side of the foam insulation and fill the stud bays with fiberglass batts. This approach is less risky than installing fiberglass directly against the concrete. However, I don’t think that fiberglass batts belong in a basement. My advice: if you want a higher R-value, just install thicker rigid foam, and leave the stud bays empty."
However, Joe L. in his article here states "Another option is to
install one inch of XPS and then build a 2X3 or 2X4 frame wall just interior
to the XPS. Unfaced fiberglass batts can be installed in the frame wall for
additional insulation."
Assuming the basement is not prone to water intrusion, do any other concerns exist regarding adding unfaced batts exist. The building science article seems to indicate this would be perfectly acceptable?
Secondly, leaving the stud bays open would seemingly serve to increase noise transmission fairly significantly. This is a big concern for my customers.
In conclusion I am trying to determine if the addition of FG would interior to the rigid board would be problematic. In my particular situation the 2x4 wall has already been built with 1.5'' of ISO attached directly the basement wall, putting R value around 10. I need to increase the R Value and also address the noise transmission issues. Adding FG would address both.
Ron,
I was under the assumption.......... and it may be an incorrect assumption......... that a unfilled stud cavity, regardless if the wall is butting up to a concrete wall, that unless filled with cellulose or FG, would be conducive to transmitting sound. I am planning on following up with folks over at Soundproofingcompany.com on Monday regarding this issue. They have several diagrams that are extremely helpful and confident I will get more clarity on the the issue. However, thought I would see if anyone here had any any insights.
Regardless, I still have the issue of needing to increase the R value from 10 to R15+. FG would get me there. Though curious on Martin's or others comments saying FG is not preferred even when interior to rigid board. I always thought this was the one exception and seems to be endorsed by Buildingscience.com article I referenced above.
MS,
People always talk about limiting sound transmission by adding fiberglass to interior partitions, but I don’t see a point in using it for that purpose on a basement wall. I can’t imagine much, if any, sound coming in through the soil and concrete.
Aside from having water actually flow into a basement, there are two moisture issues. One is that the concrete is always damp from the ground moisture. Both the ground and the concrete are like sponges. They absorb rain and snowmelt, and hold onto some of the water. Usually this moisture will always be drying to some extent into the basement interior. It is my assumption that the interior air contains mold varieties that can seed the porous, damp concrete, and grow; thus releasing mold back into the living space. Maybe some mold experts can shed some light on this.
The other moisture issue is that if the basement wall is un-insulated, it will be relatively colder that the living space in the winter and much of the summer. Both the normal interior vapor in winter and the high interior vapor from summertime humidity can condense on the cooler interior side of the concrete basement walls. This condensation also feeds water into the concrete “sponge.” I suspect that sometimes this condensation can actually make the interior concrete wetter than it would be just from the migrating ground moisture.
If you have fiberglass against the cold concrete and no vapor barrier, interior air can condense on the concrete and wet the fiberglass, which will also act like a sponge. However, if you have foam on the inside of the basement, it will present a surface to the interior that will be warm enough to prevent condensation on it. However, if the foam board joints are not sealed, vapor will go through them and condense on the cold wall behind them. Moisture from that origin and from the general ground moisture can migrate back through the same joints in the foam and find its way into the living space. In that case, you will have the fiberglass acting as a wet sponge on the living space side of your foam after you have isolated the sponge effect of the concrete.
So the key is to seal the joints in the foam. If you did that, I don’t see any problem with fiberglass. But I am not sure I understand why you want to use fiberglass. I am not sure if I understand your existing details of the basement wall, insulation foam, and framing.
I do have one other question, though believe I know the answer. As you can see in the picture above, the basement wall BELOW GRADE has the Poly ISO attached to the basement wall.
However, the ABOVE GRADE portion has polyethylene (vapor barrier) over the unfaced FG. Its my understanding that unless I am in Canada. That polyethylene should not be used in above grade wall assemblies especially homes that are air-conditioned and it should be taken down.
However my question is, whether the FG insulation pictured above ( which is above grade) should have been Kraft faced.
OR
Can the unfaced remain in place (poly removed) with a layer of drywall and a good latex paint suffice....
Ron
The existing details are simple......1.5 Poly ISO already attached to the interior of basement wall. A 2x4 wall interior to that. This puts me at R-10. Code is R 15 (Zone 6). Taking down the 2x4 wall to add more rigid insulation is not practical at this point.
Therefore adding FG seems like a reasonable compromise to achieve the proper R value. If the FG is installed in the stud cavaties (interior to the rigid board) with all seams taped etc, still unclear why Martin seems to be shying away from this detail in the article I referenced above.
Trying to get a better understanding of why.
As for the sound absorption theory that adding FG would help. I am no expert. I will however report back once I connect with the folks over at soundproofingcompany.com
MS,
I would definitely remove the poly from the above-grade framed walls.
Q. "I'm still unclear why Martin seems to be shying away from this detail in the article I referenced above."
A. It would have been better to make the polyiso thicker; but you went ahead and framed a 2x4 wall against the 1.5 polyiso already.
You can go ahead and install rock wool or fiberglass between the studs if you want. The main reason I argue against doing so is that so many basements have flooding events at some point -- and the fiberglass is disgusting after that happens. Chances are, however, that your basement will avoid that fate.
Thanks Martin for checking in. Wondering if you can elaborate on something. On above grade wall details in zone 6, no exterior insulation, when using fiberglass What are the disadvtges/ advantages of using kraft vs unfaced assuming both would be covered with drywall and latex paint
More specfically, can you satisfy code by using a good latex paint over unfaced FG or does it require kraft faced (class 2 vapor retarder) and the latex paint to achieve a code compliant vapor barrier.
T
MS,
On your above-grade walls, fiberglass batts are a poor choice of insulation, because they usually don't perform as well as other types of insulation.
Nevertheless, if that's what you've got, there's no reason for you to worry about outward vapor drive. Here's what's important: pay attention to airtightness when building your wall assembly. Don't worry about outward vapor diffusion.
Your local building inspector will probably want to see a vapor retarder on the interior side. You can use either the kraft facing on the fiberglass batts or vapor-retarder paint. Here is a link to a spec sheet for a vapor-retarder paint: Benjamin Moore Moorcraft Super Spec Latex Vapor Barrier Primer 260.
Martin,
If you don't have to worry about outward vapor diffusion, why use vapor retarder paint? Why not just use an air tight drywall assembly as an air barrier and let it go at that?
Ron,
That's fine with me. But as I explained to MS, "Your local building inspector will probably want to see a vapor retarder on the interior side." It's a code issue.
Okay, I understand.