Basement finish – existing exterior R-5 – questions on interior insulation
I am finishing my basement in Minnesota. It is a walkout design. The walkout portion of the wall is a 2×6 wall with typical fiberglass bats and poly on the interior. All rim joists have 2″ of foam spray.
My exterior concrete walls are covered with R-5 foam board protected with a thin layer of sand/rock pebbles.
I have been told and read so many ways to finish my interior insulation that I am dizzy. I have been told:
– No insulation, R-5 on exterior good enough.
– Put poly on concrete and get R-15 Kraft faced insulation between studs
– No poly at all, R-15 between studs
– Rigid foam 2″ thick on walls, then studs, then Kraft faced R-15 then poly
– Just rigid foam 2.5″ thick and then studs
You get the picture.. Minnesota city web sites, codes, etc are all pretty vague.
Hoping to get some help here as I am trying to submit final plans and they require my insulation be noted on my plans.
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In Minnesota, you are in Climate Zone 6 or 7. Building codes require at least R-15 of basement wall insulation.
If you want to install all of the insulation as rigid foam, you need to install at least R-10 of rigid foam on the interior side of your basement walls.
If you plan to install some fiberglass batts in a 2x4 interior wall, you need at least R-2.5 more interior rigid foam (in Zone 6) before you frame the 2x4 walls, or (in Zone 7) you need at least R-5 more interior rigid foam before you frame the 2x4 walls (following the guidelines for above-grade walls, which can also be used for below grade walls, as explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing).
For more information, see How to Insulate a Basement Wall.
-- Martin Holladay
Minnesota's building code specifications for foundation insulation have traditionally varied quite a bit from the IRC, and seem to be in a state of flux, seemingly trending toward standard IRC stuff but not completely. Run anything you plan on doing by the inspectors first.
https://www.revisor.mn.gov/rules/?id=1322.0402
But the minimum thermal performance requirement is the equivalent of R15 continuous insulation, and if I"m reading the above 2015 amendments correctly, you need at least an inch of space insulated with air-impermeable insulation between any stud edges and the concrete.
So it appears I can get by with something like Owens Corning FOAMULAR 2" R-10 rigid foam up against the wall and then frame against that. I will clear this with the city inspector before I spend money and time doing so. Total value should be R-15 which is code for my zone. Ty guys.
XPS such as OC Foamular is labeled R10, but that's only temporary. As XPS loses it's climate damaging blowing agents, over time it's performance drops to that of EPS of similar density (R4.2/inch, so R10 XPS really becomes R8.4, over the lifecycle of a house.) They warranty that it retains 90% of the labeled R over time (R4.2/inch would be 84%), but even though it probably won't, they are unlikely to ever have to pay a claim, since the costs of removing it from service and testing it are daunting relative to the material cost, and only the latter is covered. From a design point of view the conservative thing to do is assume 2" XPS is only R8.4, since the available evidence indicates it will eventually get there, warranty notwitstanding. (Do you have 40-50 years to run the test? :-) )
EPS is blown with low-impact pentane, most of which is gone before it leaves the factory, and it's labeled R is stable over decades. It's also usually cheaper per R than XPS, so from a value and lifecycle performance point of view, 2.5" EPS (R10.5, if 1.5lbs or higher density "Type-II" EPS, or R9.75 if 1lb density "Type-I" ) is a better deal than XPS, at the cost of forfeiting 1 square foot of interior space for every 24' of foundation perimeter.
If the loss of floor space is an issue, 2" of foil faced polyisocyanurate (also blown primarily with pentane) is labeled ~R12, and while it won't quite hit that number for a seasonal average in MN, it'll still beat R10 when it's the interior 2/3 of the total R. It's easy to reliably seal the seams of foil-facers with foil tapes too. The only thing you have to be careful about is to not rest the cut edge at the bottom on a concrete slab of unknown moisture levels, since polyiso can wick moisture should the slab ever become damp, losing some of it's performance.
Hi would the folllowing product work well: Rmax-Thermasheath-3-2-in-x-4-ft-x-8-ft-R-13-1-Polyisocyanurate-Rigid-Foam-Insulation-Board
Yes. As I noted in my article (How to Insulate a Basement Wall), it's fine to use polyisocyanurate to insulate the interior of a basement wall.
You might want to leave a 1/2-inch gap between the bottom of the rigid foam and the slab to make sure that the bottom of the foam doesn't get damp.
-- Martin Holladay
Martin - thank you very much. I finally met with the city inspector and he confirmed this would work as well. Very excited to be moving forward. Cheers!