Inset sill plate when using exterior foam?

9 years ago

WeÃ¯Â¿Â½ll be building a house in Illinois next spring. Hubby is acting as the GC and wanted me to get some feedback from everyone :) Here's his thoughts:

We'll be adding 2" of exterior XPS foam. Please take a look at my proposed wall assembly.

Typical for our area would be a 2x8 sill flush with the outside wall of the foundation. Because of the 2" foam, doing this would push the siding out beyond the stone foundation causing an aesthetic issue.

My plan is to inset the sill(2x6) so that the foam is flush with the outside wall of the foundation. Does anyone see a problem with this approach?

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Comments (4)

amberm145_gw
9 years ago
Why would you stop the foam at the concrete wall? We put our foam all the way to the footings, which was a little over the top for our area, but most go at least to grade, or a couple of feet below.

Also, exterior foam above the concrete is not common here. Instead, most builders do 2x6 framing, and add an extra 2" of insulation inside the studs. We're doing both. But I wonder how the rigid foam exterior compares in cost to the 2x6 framing? I know that the foam on the outside allows you to insulate over the studs, so maybe that's the reason rather than costs. But if you're worried about where to stop the foam, maybe this is something to consider?
laurensmom21
Original Author
9 years ago
amberm145,

Thanks for the reply. My initial plan was to use foam from footings to the soffit, as you mentioned, but this creates an exposed foam foundation and we really want to use a stone product to cover.

I initially considered a 2x6 wall, but I like the idea of exterior foam to reduce thermal bridging.

I'm just curious if anyone else has ever inset a sill plate - and if so, does it structurally make sense?
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worthy
9 years ago
Looks fine to me. Only, under brick and stone, I forego the housewrap altogether and use 30 lb. felt. Also, I frame with 2x6 @ 16" o.c. and use fibrous insulation as well. Without further insulation, 2" of XPS won't meet Codes.

You can forego structural sheathing, but will have to provide alternative means of bracing.

With that much external insulation, you may have to go to deeper window jambs. Keep that in mind when ordering windows.

BTW, all the cultured stone I'm acquainted with has to be kept a certain distance above grade.

Here are some alternative sections from Building Science Corp.

This post was edited by worthy on Wed, Sep 10, 14 at 16:11
laurensmom21
Original Author
9 years ago
Good to hear. We will have blown in cellulose in the stud bays. And the deeper window jambs are a bonus in my opinion. More expense, but its only money. (My wife will get a kick out of that one).

Thanks for the info about cultured stone. I had assumed it would be ok slightly below grade, but I better ask about that.

Anyone else see a problem with this? This is one of the last big hurdles before we go ahead with final plans.

thanks!