Hey Guys, I have done much research and searching but cannot find the best answer to this question. I am having a poured basement completed this week. It is going to be 8ft on the one end and about 13ft on the other, its a walkout basement. I have read a couple documents off of buildingscience.com or whatever the site dana1 posts from but it is still unclear to me. The basement is also going to be a radiant slab. This is in Zone 5. What is the most efficient method of insulating this space. From the research I did you can either insulate the outside and use the whole slab and basement foundation as thermal mass or insulate on the inside and lose space but achieve relatively the same thing. I would like to know: 1. What is the best method, interior or exterior? 2. For Zone 5 how much insulation do I want beneath the slab? 3. EPS seems to be pumped as being more environmentally friendly while still giving a good cost to efficiency ratio, is EPS still the best? 4. What is the newer method, interior or exterior insulation? 5. What is tried and true i.e. really works and works well? From what I understand the best method is interior with 2 - 3" spray foam and then roxul / batts to make up around R20+, is this correct? Thank you much guys, you guys are awesome.

the main issue with exterior insulation is that it can become a highway for ants or termites unless properly coated and sealed and maintained for the life of the house. Otherwise, interior insulation is "bulletproof" and the best way to go. 2" of spray foam and 5-1/2" of Roxul will give you R32; 3-1/2" Roxul will give you R25. the spray foam also minimizes moisture getting into the basement. Two other important issues: painting the footings with UGL Drylock keeps moisture from migrating into the basement walls, and isolating the basement slab by placing foamboard completely under and around the slab - including between the footings and slab and between the slab and the wall - will keep the heat in the slab instead of dissipating into the earth.

Whatever you do, check carefully that there are no thermal bridges.

For US zone 5 you'd want at least R7-R8 on a high-R house even without the slab being the radiator. With a radiant slab, add at least R5 to that. R10 (2" of XPS) would be an absolute minimum.

EPS is usually cheaper than XPS per unit R, and is more than an order of magnitude more enviro-friendly from the greenhouse gas potential point of view due to the blowing agents used. But XPS does a better job of retaining staples for the PEX tubing, and is less subject to damage from people walking on it during construction, which is why some radiant slab folks prefer it. There are purpose-built high density EPS products out there with molded-in knobs for retaining PEX tubing, but I've never priced it out. Going with 3-4" of Type-II EPS (R12-R16) would be my personal recommendation. At 3" or more it's less likely to break off at the corners and a few big dents from work boots aren't going to change the performance much.

A 2" spray-foam treatment at the band joist & foundation sill is a good thermal break while air-sealing those seams, and is sufficient to allow use of fiber insulation to fatten out the R. Using ICFs automatically gives you a thermal break between the slab and foundation wall, as would using rigid foam at the edges. Either would potentially be a termite-highway into the interior in extreme-risk areas though.

I wouldn't hesitate to put 1" of XPS over 2" of EPS if you need some XPS for the above reasons.

Note that the way most ICFs are done creates a thermal bridge from the footing to the interior of the ICF. Not a big deal at full basement depths, but wasteful.

Going 2- layers adds labor cost to the foam installation, maybe as much or more as the higher labor of making the PEX sit still on EPS.

The thermal bridging of the footing is real but not of much consequence in this application/location- the loss through a heated slab even at R12 would be a much bigger number than the loss through the ~R10 between the room and the footing 1' above the slab. At 3' above the slab the R-value of the foundation wall to the footing is more than from the room-air through the slab. The slab area is just so much bigger, and the fact that it's running above room temp during the heating season that it swamps the heat loss to the footing. To meet PassiveHouse spec on energy use you'd have to address it though.

Dana1 can you simplify your last paragraph for me, I have read it several times but it doesn't completely make sense to me. Can you provide a pictoral view of spray foaming the 'band joist' and 'foundation sill' that you spoke of further up? Thank you all much. From what I understand Spray foam is triple or more the cost of ridgid insulation here in canadia...prolly won't be using much of that given my very low / small budget.

The concrete adds about R1 per foot, so when trying to figure out the consequences of not having foam under the footing, you have to add the R value of the path through the concrete + wall foam. It's a much lower number 1' above the slab than 3' or 5' above the slab, since there is only ~R1 of concrete above the footing low on the wall, but ~R5 of concrete between the section of wall 5' above the footing.

At 3' above the slab with an R20 ICF you have R10 of foam + R3 of concrete, which is more than the R10 below the slab, and far less exposed area. But if the footing were completely wrapped in R10 the way the wall is, the heat loss at the lower part of the wall would be a bit lower than with an un-insulated footing.

You can go to google-images and search on [spray foam band joist] to come up with pics like this one, showing somebody foaming the band-joist & sill to the top of some XPS on the interior:

http://www.energyvanguard.com/Portals/88935/images//crawl-space-encapsulation-band-joist-insulation.jpg

or this one, with no wall-foam insulation:

http://www.isenbergsprayfoam.com/images/band-boards.png

You can also cut'n'cobble rigid foam if you seal it with 1-part gun foam around the edges, but it's far more labor-intensive:

http://www.house-energy.com/images/SealingRimJoist.gif

When installation labor and scrap rates are factored in, the delta between rigid & sprayed isn't as big as the per-board-foot price might indicate, and nowhere near 3x if you're using virgin-stock rigid foam. As a DIY it's cheaper to go with rigid if you discount your time, but going all spray foam isn't always a cost adder in a "time is money" construction situation.