This is the standard in N. America for sub-slab insulation.

http://www.menards.com/main/building-materials/insulation/insulation-panels/f250-2-x-4-x-8-r-10-extruded-polystyrene-insulation/p-1444450504310-c-5779.htm?tid=-9197785561416543961

5. Caulk all inside cavities before insulation

Once you have a completely sealed exterior shell (and preferably another sealed interior shell), I don't know what one gains from this - just more redundancy?.

I'm suspicious of those flat, 1/4" foam sill gaskets that have a large bearing area. Low PSI load means little compression. Little compression means that it takes very little wood warp to create an air gap. The shaped EPDM ones make sense.

A residential concrete slab has less than 1 PSI load. Well within the specs of any EPS foam. Wood over foam is quite different - it's not so rigid.

jour: Agreed - the common foam gaskets keep the wood from direct contact with concrete, but do no air sealing, so are pretty useless. (Except for making builders think they are functional gaskets) The EDPM rubber gaskets from ConservationTechnology.com have a "bubble" at each side which compresses & do a decent job of sealing.

Talking to the GC this morning, he's concerned about doing 4" of foam under the basement slab, he thinks that they will have to excavate too deep which will be low enough that the foundation drains won't work properly. Not sure I'm understanding that one. He said usually they do 2" of foam for radiant, and that still leaves 2" of footing drain to do it's job before the foam could get damp. Why can't the drain just be put 2" lower?? Maybe I'm simplifying.

has he started excavating? Do you know how high (thick) the footing is/will be?

No, and no. Hoping to start digging this week that's why I mentioned the 4" to him. Maybe it's already permitted and it can't be changed. I have the prints but unfortunately they are at home.

4" of XPS under a basement slab in IA is waste of money.. 2" of 15psi XPS is more than enough. I used a vapor barrier under my garage side and none for the office. Neither showed vapor under a plastic sheet for the last new years. It is not wise to changer everything in a stick built home to satisfy the latest fad in GREEN building.

The quality of construction is the key to comfort and fuel efficiency.

BBNM: Depends on your goals. If they include a very high level of energy efficiency, then the 4" (and R values in general) makes a lot of sense. If not, then you're right, it does not matter. Also is important to separate "fads" from science. I build fad free science based "net zero ready" houses which work. We could get to a utility bill (heat, AC, HW, lights & plug loads) of $13/month (actual cost) by increasing the #of PV panels, but the math doesn't make sense compared to the cost of foam and insulation. Leaving out important stuff under the slab makes no sense and is impossible to do later. I've built houses of high quality for many years, but not to the level of efficiency we're doing now. But to get here you have to change the way you look at construction and have an open mind.

In my mind, with what is happening in the world today, it makes no sense not to build this way.

Posted By greentree on 03 Oct 2016 11:53 PM
The menards eps shouldnt be used under your slab, it wont have adequate compressive strength. Its really low grade stuff with recycled content compared to virgin stock eps.

And if you caulk your stud bays your going to put in alot of man hours and go through an enormous amount of sealant, 1 tube will go maybe 1 1/2 stud bays, around 30 lf, a can of one component foam will put out like 700-900 linear feet.

Can you recommend a particular product for the one component foam?

When you insulate 8' below grade in Iowa with a deep ground temperature roughly 60°F and an indoor design temperature of 68°F adding more sub-slab insulation would only make sense if your goal is to waste money.

Anyone can over-engineer a residence.

Rgonyer,
First the concern of insulation depth: most footings are 8" thick. You put a 4" drain tile to the bottom and the math says you could put 4" foam without taking away from your headroom. In reality without undermining footings generally the drain tile ends up 2-3" below top of footing. To do 4" just takes more time and money from standard practice, which is a common theme in house building unless you sacrifice headroom, and as a bonus insulate on top of the footer which you could do, especially with 9' walls.

For 1 component foam I like touch n seal all weather formula. I do buy dow great stuff orange sometimes because its readily available and the dap draftstop is ok. I dont like the brands that have really tall cans, it really gets in the way, titan used to be like that, not sure if they've changed. Windows and doors i use dow blue Or dap 912.

The trick to applying effectively is injection. Too much output on the gun and the foam shoots out and rolls, too little output and it doesnt inject either, but with right output and tip angle you can get the foam to inject into the crack. You can verify by ripping off the expanded bead and youll notice the crack is still filled.

Sorry for the long reply, ive been monkeying with this for so long and its working per blower door tests, i get passionate about it, thats why i dislike the lazy tape approach, when it fails good as the tapes are, and it will, you're back to square one. Fill the openings. Gaskets could work great too, but dollar for dollar you will not beat 1 part done right.

Posted By Bob I on 04 Oct 2016 11:58 AM
jour: Agreed - the common foam gaskets keep the wood from direct contact with concrete, but do no air sealing, so are pretty useless. (Except for making builders think they are functional gaskets) The EDPM rubber gaskets from ConservationTechnology.com have a "bubble" at each side which compresses & do a decent job of sealing.

Ive done a lot of blower tests on newer standard housing and have found the cheap sill seal to be decently effective unless the foundation contractor left a wavy top, which isnt the sill seals fault. If we are getting to passive house levels, or even anyone building within this community its still sealed cause its gonna be encapsulated with spray foam anyway.

At least part of the underslab foam should be over the footings to prevent thermal bridging there, so 2" under and 2" over should be good and also doable from a drainage perspective.
Badger makes a valid comment as the delta T should always be considered when designing insulation. 4 inches is good but could you get a better return with using 2" and spending the rest on something more important?

I'd like to see more data on ACH50 values before and after a year or two of wind induced racking, moisture/temperature changes and age related shrinkage of foam (spray or rigid).

Various people report that tape and gaskets beat sealants and spot spray foams short term - my guess is that this is even more true long term.

Posted By jonr on 05 Oct 2016 11:55 AM
I'd like to see more data on ACH50 values before and after a year or two of wind induced racking, moisture/temperature changes and age related shrinkage of foam (spray or rigid).

Me too. Most will never need that level of air exchange.

Posted By FBBP on 05 Oct 2016 11:23 AM
At least part of the underslab foam should be over the footings to prevent thermal bridging there, so 2" under and 2" over should be good and also doable from a drainage perspective.

I'm not sure what you mean by 2" over the footings? Foam on top of the footings before pouring the foundation walls?

An additional question, although I feel like I should ask this in another thread but it seems related. I'd like to have a blower door test done before insulation, but how is that achieved? Drywall obviously won't be up yet, so there will be no effective ceiling at that point.

Honestly what I'm leaning towards doing right now is ZIP sheathing/tape PLUS caulking the inside cavities of the framing. I know it's a lot of extra work, but caulk is cheap, my labor is free, and my new place is conveniently located very close to where we live now. I feel like it'll be an inexpensive investment. There is a local retailer for ZIP, so I should be able to talk my GC into using it instead of OSB.

Zip code 52726 is in US climate zone 5, which need R7.5 on the exterior of 2x6 framing for dew point control on fiber cavity insulation, or R5 for 2x4 framing. An IRC 2015 code min is 2x6/R20 or 2x4/R13 + R5 continuous insulation. To go better than code-min without much learning curve you could go with Huber ZIP-R (http://www.huberwood.com/assets/pdfs/ZIP-R-Sheathing-Installation-Guide.pdf ) using the 2" ( R9.6) or 2.5" (R12.6) versions of the product. It's more expensive than foam + sheathing + housewrap, but it doesn't take a crew of rocket scientists to assemble- it's about the same as installing 1/2" CDX, but it requires some attention to fastener spacing & specifications.

The best greenest & cheapest sub-slab foam would be RECLAIMED EPS or XPS, which is typically less than 1/3 the cost of virgin stock goods, sometimes less than 1/4 the cost. If you keep checking the local craigslist searching for [rigid insulation] you will usually find somebody in the foam reclamation biz or a barn full of used roofing EPS/XPS. I bird-dogged a large cache of used 2.5" XPS for someone not too far from your neighborhood that way about a week ago, but it's gone now (he bought it all). https://quadcities.craigslist.org/search/sss?query=rigid+insulation It's out there, it's cheap, and if you have a place to store it, buy it when you can. Otherwise you can get it from places like NationwideFoam.com at a steep discount from virgin stock, but depending on how far they have to truck it. Be flexible on the thickness- use EPS is so cheap that stacking 2 layers of 3" (R25-ish) is comparable to or cheaper cheaper than 2" of virgin stock goods (R8-ish).


If you're going to staple the PEX to the foam, making at least the top inch out of XPS may be "worth it", even if it's virgin-stock. With used XPS the environmental hit has already been taken, and installing it under the slab gets another 50+ years of service life out of it.

All sub slab foam needs a vapor barrier on top of it. Staple holes have very little cross section, and barely affect it's vapor permeance once the concrete is pressing the vapor barrier to the vapor-retardent foam.

When looking at used XPS, for design purposes you need to derate it to it's fully depleted R4.2/inch performance. It only takes a few decades for XPS to drop to that range as it's damaging blowing agents leak out.

Huber ZIP-R

Which is fine for thermal bridging, but with the foam being on the wrong side of the OSB, doesn't do anything for the usual concern - minimizing condensation at the sheathing. It may be easy, but it is higher risk than best practices walls. And if you are considering accepting such risk, you could stick an inch of EPS or XPS on the exterior of regular ZIP (to reduce thermal bridging and warm the sheathing somewhat). Plywood would be even better (more breathable to the outside).