Insulating and vapor barrier installation before drywall in a basement
JWS_90
| Posted in Building Code Questions on
I would like to know the install method if one wanted to both insulatate and apply moisture barrier to basement drywall, before hanging the drywall sheets? Also, is there specific code requirements for Prince Georges County, MD in 2025? This install should consider a very humid climate in the summer, and colder temps in winter time. I am mostly curious on how to install both a vapor barrier and foam board insulation before putting up drywall. Thanks.
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If you use a foil-faced foam product before the drywall, simply tape the seams with foil tape and use flexible sealant at top and bottom. The same is true with rigid XPS insulation. For MD, 2" of any rigid foam will meet code for foundation insulation. You can use construction adhesive to hold the foam in place. Then build your stud wall nearly in contact with the rigid. You can bump up the R-value of the wall by insulating between the studs with unfaced fiberglass if you want to.
You should be able to find used foam for a serious discount in MD to reduce the climate impact of new foam insulation.
Your primary moisture challenge in that climate is to keep water vapor away from the (relatively) cool walls where it can condense and cause problems. The vapor barrier on the face of the rigid insulation does this. You should still run a dehumidifier in the summer to keep the RH of the basement air below about 50%. You probably won't need to dehumidify in winter.
Before insulating and finishing the basement, make sure all penetrations through the foundation walls and through the lumber band joist area are sealed. You can use caulk for small holes and canned foam for the larger ones. This is super important for lots of reasons. It is also worthwile to insulate the band joist area, though this is a bit more work. This is one of the few places where I like to use spray foam because it conforms easily to the complex geometry, wires, pipes, etc. I apply the spray foam after the rigid foam on the walls so that it can seal from the top of the rigid foam all the way to the underside of the subfloor above. This seals any penetrations you missed as well as the gap between the sill plate and the foundation wall and it helps to prevent condensation against the band joist in winter.
You can also search this site for how to insulate basement and foundation walls. There are several articles with graphics and careful explanation.
Hi Peter. I am in a very similar situation to JWS as I live in MD as well. As far as specific rigid foam product at the big box stores, does it boil down to just getting the most R value for your money? Conflicted on xps vs foil-faced poly iso vs cheaper foil-faced EPS products. Also if you leave a little gap between the foam and stud wall, doesn't that create a need for additional fireblocking?
Taylor, Yes, to a large degree it doesn't matter much what flavor of foam you use - most foam insulation materials are impervious to moisture and as long as the foam is covered with drywall somewhere in the wall system, there are no fire code issues. The only slight exception is polyiso insulation that can soak up moisture and swell a bit if it's really wet. For that reason, many people use XPS for the first few inches, with polyiso above because of the risk of basement flooding swelling the polyiso if in contact with the floor. And as far as the "gap" is concerned, I didn't mean to suggest much of an actual "gap." Just enough space to keep the stud wall flat and plumb. Any gaps between the foam and studs will effectively be firestopped with the fluffy insulation in the walls if you install any. Either way, the stud wall can be firestopped at the top of the "gap" with rockwool if that makes the local AHJ more comfortable.
Thank you very much and that helps a lot. My strategy now is using membrane plastic vapor barrier directly on basement wall sealed with caulk or double sided construction tape, then purple R-3 foam board insulation (1/2 inch), tape and seal cracks on all sides, then 2x4 stud wall with mold resistant drywall. Thanks again, and hopefully this will work and I will consider your suggestions also.
Justin
This is what I usually do. I put some 6 mil poly up on the wall, then I insulate over that with rigid foam. I secure the poly to the wall using polyurethane sealant, which is usually sufficient on it's own to hold the poly in place, but sometimes I'll use some of the push-in plastic anchors made for the purpose. I secure the rigid foam panels over the poly using tap-in plastic anchors that go into holes drilled through the foam, through the poly, and into the wall. This has worked pretty well for me.
I don't really trust tape to seal to concrete or block well, which is why I prefer the polyurethane sealant.
Bill
That would not be my first choice. I never like putting poly film directly against the foundation walls. Once you insulate on top of the film, you keep the film at outside ground temperature which is below the dewpoint of interior air in spring and summer. Because of their connection to ground temperature, foundation walls are almost always colder than the interior air in a finished basement. Remember that moisture flows from warm to cold. There are almost no typical indoor conditions where foundation walls "dry to the inside." Just doesn't happen. Poly film has no value when installed against a foundation wall. If the foundation wall is wet, you should fix the outside conditions that are making it wet. Apply a waterproofing paint if it makes you happy.
Also, your foam board is probably not thick enough, especially where the foundation walls are exposed above grade. Your foundation walls require R10 continuous insulation or R13 Cavity. With R3 foam and studs filled with fluffy insulation, you have a mixed condition but it does meet the total R-value requirements. For above grade walls, R3 continuous on the outside of a 2x4 stud wall is enough to prevent condensation. Below grade you have less vapor drive, so R3 foam meets that test, but you do have to fill the stud walls with unfaced fluffy insulation. I'd still be more comfortable with 1" foam, which also acts as a better vapor retarder if you are using XPS. Foil facing on any foam also provides vapor barrier at the warm face of the foam, so I suppose foil-faced R3 foam would technically be OK. Bumping it up to 1" doesn't cost much and the extra R-value will make the basement more comfortable.
So what specifically is the danger of a poly film against the concrete?
A basement wall is a wrong-side vapor barrier and has to be insulated as such. But what's the harm of making it a perfect vapor barrier rather than an imperfect one?
It's common practice to run the poly up the walls to the mudsill when doing a crawlspace encapsulation project. The poly helps to seal out humidity and radon this way, which is what I'm after. I still tape the insulation on the interior for a clean finished look (I use polyiso painted with a fire retardant white primer).
I do agree that the poly is not the answer to bulk water problems though, which ideally need to be address on the exterior. If there was no other option, I'd use a blockfill-type interior side water proofing paint (Drylok, etc.) to deal with the bulk water. I have sometimes used thin dimple mats behind the polyiso, with the lower part of the dimple mat tied into an interior perimeter drain (I seal the top with polyurethan sealant). This can also work pretty well if the water problem can't be address on the exterior.
Bill
Thanks guys! One last question, is there a "double vapor barrier" danger with both plastic and foam board install, like plastic to the wall, then foam board (r-3) over it? Some say it can trap moisture in between. I had basement waterproofing done along the entire edge trim so I'd like to tuck plastic vapor barrier into the 4" trim riding up from ground as a catch. If we put a dehumidifier in the basement rooms it shouldn't be much an issue I am guessing.
If the materials are impermeable there's no danger of trapping moisture. The double barrier is only a problem with permeable insulation like fiberglass batts or blown cellulose.
It would be possible to trap some moisture, but it wouldn't matter because the materials involved don't care, and the moisture would be encapsulated so any potential mold issues would be contained as well. With a tight installation, there would be very little air space in between for moisture to accumulate anyway, which would also limit the issues you could potentiall have. I wouldn't worry about a "double vapor barrier" in this case. As DC mentioned, the usual issue with double vapor barriers is when you have fluffy insulation (usually batts) between two layers of impermeable materials, which can result in the batts getting saturated over time which is a problem due to both reduced R value and mold growth.
Bill
I would highly recommend having insulation on the outside of the basement wall. Ideally, Rockwool Comfortboard. Foam products may end up being eaten by bugs at some point. A vapor barrier on the inside of a basement wall sounds like it's asking for trouble. Basement walls will tend to pass moisture into structures, so sealing a wall cavity against one might result in mold and eventually rotting.
Basement walls only tend to pass moisture into structures when they are not properly drained and waterproofed on the exterior. That's the important part. After that, insulation can be installed on the exterior or interior if you use the right products and processes. I agree that exterior insulation has some benefits, but it is much harder to install as a retrofit. Once the grading and landscaping are finished, few people are going to dig it all up to install exterior insulation. The OP asked about interior insulation options, so that's what we've offered.
Itchy,
What mechanism allows "basement walls pass moisture into structures" when they have an interior vapour-barrier of either foam or poly? And what would rot?
I agree - exterior foam is much more vulnerable to damage from pests than mineral wool, but if you are insulating on the interior then you need to use something impermeable, so mineral wool is out.
Another option for inexpensive walls is to glue 2" foam directly to the foundation walls, and then glue the drywall directly to it. No studs at all. If you want electric outlets, you can cut grooves in the foam where you need to. This works best if your walls are very flat and dry.
It's also possible to run electrical outlets "commercial style", using EMT conduit down from the ceiling to a surface mounted 4" square box and a raised cover. This doesn't give you a nice look though (it's an industrial kind of look), but it works, and it's easy. You can even used drywall anchors on the glued up drywall, since you can usually push them into the foam behind the drywall and still get the anchors to hold properly.
Bill