Hi,

We are building a house on an existing slab on grade in Connecticut, Zone 5.  It is a frost slab, with the foundation walls going down to ledge.  It was a garage, and we ripped out the garage slab, left the original walls, and added more frost walls as we expanded the footprint.  Since the wall heights are all different, we need to pour the new slab on top of the walls in one pour, to level everything to the new additional height.  This prevents us from adding a thermal break from the slab to the walls.  Due to this, we abandoned our plans to install radiant heat in the slab, and made a change.  We proposed building an insulated subfloor to meet the code insulation requirements for the floor (which is R-10). We are ready to pour, and now the inspector wants us to furnish documentation that this is an approved method.  I found an article online from jlconline.com for June 2018, that has the same method described.  I wrote and called the author and have not heard back. "Insulating Over a Structural Slab, Skip the sleepers and float the subfloor over the insulation" is the title of the article.

We want to lay rigid insulation directly on the slab, no sleepers, and install plywood over the insulation, and then a pine finish floor on that.  A floating subfloor, probably using construction adhesive for the insulation and the subfloor, and nailing the finish floor to the subfloor.   This is an open floor plan, but we plan to build all of the walls directly on the slab, to avoid any compression issues.

The inspector is concerned that we will still get cold/moisture penetration through the sill plates, and condensation will accumulate where the rigid insulation meets the interior of the sill plates.  He may have us insulate the exterior of the foundation to prevent this issue.  In my humble opinion, I think the insulated subfloor is more effective and a better method. 

QUESTION:  Can anyone please help us find approvals to demonstrate code compliance? 

We were planning to pour this week, so any help would be extremely appreciated!

What constitutes an "...approved method...", for your code inspector? Approved by whom? The IRC?

The IRC doesn't call out ANY sub-floor/sub-slab insulation for unheated slabs, but DOES call out R10 slab-edge insulation in your climate (zone 5) down to 2' below grade or the footing, whichever is shallower, independently of whether the slab is insulated from below. Even if you go with the slab-top floor foam, it's "worth it" to go with slab edge insulation.

https://up.codes/viewer/wyoming/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2

If one uses an EPDM sill gasket to prevent exfiltration paths under the sill or EPDM flashing tape covering the bottom and interior face of the bottom plate, the amount of moisture accumulation in the sill from interior moisture drives is effectively zero.

Going with 1"/R4-EPS floor foam would keep at least 1/2" of the sill above the floor insulation, which is probably enough to prevent moisture accumulation without the slab edge foam, and would provide about the same level of wintertime under-foot comfort, and a negligible difference in heat load for an otherwise code-min type of house. (Hitting PassiveHouse performance would require more floor foam than R10, but that's not your project.)

With non load bearing walls there are no compression issues with the foam. The subfloor distributes the weight, staying WELL within the elastic limits of the foam, even for a heavy plastered wall. Installing partition walls above the subfloor is better, since with stud plates directly on the insulated slab the slab would be in the 50s F, well below the average outdoor dew points, and you would get summertime moisture accumulation in the bottom plates. With the bottom plates on top of the insulation, it's fully inside of conditioned space, and stays dry, even if the indoor air's dew points rise into the 70s in summer. (The sill plates of exterior walls don't have the same issue, since they are at a much higher temperature in summer.)

Using EPS rather than XPS for your floor foam and/or slab-edge foam is far greener, due to the differences in blowing agents. EPS is blown with low-impact hydrocarbons, usually a variant of pentane, with a 100 year global warming potential (GWP) of about 7x CO2, most of which escapes the foam and is recaptured at the factory. XPS is blown with a combination of HFCs, all of which have very high GWP. The predominant component of the mixture is HFC134a, with a GWP of about 1400x CO2. As the HFCs diffuse out over a few decades the thermal performance eventually falls to that of EPS of similar density & thickness. The performance of EPS on the other hand is not dependent upon blowing agent, and is stable over time.

There are several vendor of reclaimed and factory seconds foam board in southern New England, the largest two are Green Insulation Group in Worcester, MA and Nationwide Foam in Framingham, MA, but there are others. (https://hartford.craigslist.org/search/sss?query=rigid+insulation ) Used roofing EPS is typically "Type-VIII" 1.25lbs per cubic foot density, R4.17-ish per inch, and walkable even under a membrane roof, and would work well under your subfloor or slab edge, but you'd need 2.5" to hit R10. Foam reclaimers also tend to have a lot of roofing polyisocyanurate (about R11 @ 2"). Don't use polyiso for slab-edge insulation since it can wick ground moisture over time, but it's fine under your sub-floor as long as you have a polyethylene vapor barrier between the concrete & foam.

Hi Dana,

Thank you so much for your input!  Today I called Huber and spoke with their key guys today, and they were absolutely fantastic.  I am waiting to hear from their building scientist. 

I like all of your suggestions, and have forwarded them to my designer as we await the call from Huber.  Thank you again, and I will post an update when I have one.

Craig

A code-min wall in zone 5 is 2x4/R13 + R5 continuous insulation on the exterior, or a 2x6/R20, no insulating sheathing. If adding insulating sheathing to a 2x6 wall it needs to be at least R7.5 to provide sufficient dew point control a the sheathing. That could be 2" of EPS (R8-ish) or 1.5" of polyiso (R8-R10-ish, depending on product).

A 2x4/R13 wall with 3" of fiber faced roofing iso (recycled & cheap) would roughly DOUBLE the performance of a code-min wall.

With 3" of slab edge EPS (R12-ish) that would align perfectly with 3" of insulating sheathing, assuming the sill is at the exterior edge of the foundation. Using EPDM flashing tape as Z-flashing to direct any bulk water that makes it to the structural sheathing to the exterior side of your foundation EPS there is very minimal thermal bridging at the sill, and a nicely vertical drain-plane. Using a crinkle type housewrap (eg Tyvek Drainwrap) allows bulk water to drain, and provides at least a modest capillary break between the foam and the sheathing. The 3" wall foam can be initially cap-nailed in place, then secured to the studs with 1x4 furring through screwed to the studs 16" o.c. with 5" pancake head cap screws 16" o.c., onto which the siding is mounted.

The exposed slab edge EPS can be cap screwed to the foundation, and finished with a cementicious EIFS such as Quikrete Foam Coating to protect it from sunlight, lawnmower damage, etc.

I like the idea of adding the external insulation to both the sheathing and the foundation. 

Lots of tips & details on installing exterior insulating sheathing can be found here:

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing

You may need to take a free 10 day trial subscription to download all of the construction detail drawings and view other links, but the main article is a freebie.

Thank you!