Exterior Basement Wall Insulation
JoeT1
| Posted in General Questions on
Been thinking about insulating my walkout basement. I have a bunch of plumbing and electrical wires that would make it very difficult to get a good seal with Rigid. Also, my understanding is that it generally considered better from the exterior if possible.
Anyone know if it be stupid expensive to dig up 160 linear feat x 4 ft deep around the exterior to have a dimple map and rigid foam installed? House is only 2 years old and I don’t have landscaping yet, and it’s not a development/neighborhood so excavator wouldn’t have access or space issues.
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Replies
I've installed both exterior and interior foundation foam; interior is definitely my preference - faster, easier and a better insulation job. Exterior foam is subject to chewing by insects and rodents. Go around the plumbing drains, and move the wires which should not be much trouble (although you can notch the foam for conduit) . Well worth it, as well as much faster and less expensive. I typically use polyiso foam on interior walls; it stops moisture from getting into your basement. If you do go with exterior foam, the foam (EPS or XPS which are both moisture resistant) must be tight against the concrete; the dimple material would go outside of that.
thanks.
would you expect a substantial improvement in humidity levels with poly that is well sealed and caulked?
Anyone know how much, if any, humidity reduction could potentially be expected by insulating a walk out basement with Theramax?
Although it would really suck...in the long run is it better to just insulate from the exterior if trying to mitigate humidity with insulation?
Either way will work IF you seal the wall up with a vapor barrier of some sort. Basement walls are usually different from "regular" above grade walls in one major way: above-grade walls primarily deal with condensation issues involving moisture from the interior of the home. Basement walls are different, because basement walls can actually introduce NEW moisture to the home from the moist Earth on the exterior side. Because of this, you want to have a vapor barrier tight against the wall, and that barrier can be on the outside OR the inside. Exterior insulation is usually better for overall performance, but more complex to install as a retrofit. Interior insulation should be fine. I would use polyiso here with seams taped, and it wouldn't hurt to put a layer of polyethylene against the masonry first with foam over that tight to the wall.
Bill
Joe,
The insulation won’t alter humidity levels, it will just mean less chance of condensation on cold surfaces. Adding an vapour barrier to the floor and walls is what will reduce humidity, and that can be a variety of materials, including foam.
I don't believe that Polyiso is generally recommended to be used underground.
krackadile,
Bill recommended EPS or XPS for exteriors. Polyiso is fine for the interior. "On the interior side of a basement wall, all three common types of rigid foam insulation—polyisocyanurate, expanded polystyrene (EPS), or extruded polystyrene (XPS)—perform well." The quote is from: https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
Polyiso isn't recommended to be used "underground", but that means "buried in the ground", not just "below grade". Using polyiso on the INTERIOR side of a foundation wall is fine. You wouldn't want to use polyiso on the EXTERIOR sode of that same wall though. The reason is that polyiso can wick up water, EPS doesn't do that nearly as much, and XPS does it even less than EPS. That is why my preference is XPS if you are insulating the EXTERIOR of the foundation wall, or polyiso if you're insulating the INTERIOR side.
Bill
Thanks All, I would just add that there is quite a bit of plumbing/electrical fastened to the walls, which could impede the tight fit. Would that change the recommendation in any way?
Joe,
It may. There are complications each way. Interior foam needs to work around existing services and requires protection from fire. Exterior insulation has to be continuous up to the framed walls above, meaning it needs protection for the parts that extend above grade.
In terms of price, the only way of getting a good estimate is from contractors in your area.
Sometimes you can sneak some thinner insulation behind things like drain pipes, if they're not tight to the wall. Ideally, you want to relocate everything out enough that you can fit full thickness foam along the entire surface of all of the exterior walls, then put the mechanicals up tight to the outer surface of the foam. That's not the ONLY way to go, but it's usually the best way.
Sometimes you can cut foam between things on the wall, then fill in the gaps where the mechanicals are with spray foam to "connect" the sheets of rigid foam on either side, encapsulating the mechanicals in the spray foam. This works well too, but requires the use of spray foam, and impedes access to the mechanicals if you ever need to do any work on them in the future.
The most important thing is that your VAPOR BARRIER is completely sealed behind everything. The insulation isn't as demanding.
Bill
Thanks Very helpful!
Seems to be quite a debate about polyethlene on interior basement walls.
Mostly that depends on where the poly is placed. Stackups that are masonry wall / studwall with batts / poly / drywall are BAD. The poly is now between the interior and the wall, with batts and framing in between. That means the poly traps moisture within the stud wall, so mold can grow and wood can rot. That's a Bad Thing.
Poly TIGHT against the MASONRY WALL keeps moisture in the masonry, where it won't cause any problems. The poly blocks the moisture from getting into whatever else you put on that wall, making a safe assembly. My preference is poly tight to the wall, then rigid foam (usually polyiso, but sometimes EPS) tight against the poly. I usually drywall right over that, but the finished surface can be things other than drywall if desired, or even a regular studwall.
You just want to make sure the poly layer doesn't trap moisture in an area of the wall that could have problems if kept moist. Since concrete block and poured concrete don't have any problem staying damp forever, the poly causes no problems when applied tight to the wall. Any mold that could potentially grow right on the wall is sealed in by the poly too, so it's not a problem. You could always paint the wall with mold killing primer prior to putting the poly up if you want to minimize the chances of mold growing behind the poly anyway.
Bill
Joe,
Much of that debate on GBA stemmed from a misreading of the advice Martin gave about poly in those walls. He meant no interior poly, not that poly was a problem against the concrete.
Thank You all for the clarification.