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diybrad 08-02-2011 01:12 PM

using XPS and spray foam together in stud bay
I posted this in the insulation section but i need to get some fast answers and i know not a lot of people wander over there so i am going to post it here as well.

I have four of my rooms taken down to bare studs and i am about ready to insulate the exterior walls. I am really like idea of the spray foam but i hate the price. Can i first use 2" XPS board and then 1-1.5 inch of closed cell spray foam to get 3.5" of insulation and still retain the value of spray foam? This may be a stupid question but i have searched and searched and i cant find anything about it anywhere.

I am looking at the DIY spray foam kits and they are around $1400 for 1200 board feet. That comes to about 1.16 per foot not counting waste. Foam board is about $.50 per foot and is closed cell. If this would [COLOR=blue !important][COLOR=blue !important]work[/COLOR][/COLOR] would the spray foam stick to the smooth surface of the XPS?

by the way I am in nashville tn and the home is 1200 sqf main floor and a 1200 sqf basement (70% below grade) all brick/block.


jrepp44 08-02-2011 02:51 PM

Hi Brad, I think you are on the right track using a combination of foam and some other type of cheaper insulation. Research the following terms on the internet: "Hybrid Insulation System", "Flash and Fill Insulation" and "Flash and Batt Insulation". Also for anyone doing new construction check out this method of insulating:

diybrad 08-02-2011 03:14 PM

i have seem the flash and batt type of hybrid but i still havent seen a XPS version of it or if it has been tested. The problems i have read about with the flash and batt scare me with moisture issues. what i read talked about the vapor barrier being behind the fiberglass insulation and that could cause condensation in the wrong place.

My rooms are already gutted due to a moisture and what was starting to be a mold problem. I have corrected the moisture problem but now i am looking for the most efficent i can get will a medium budget. That is where the XPS and spray foam idea came from. Moisture can get past that (or should i say probably wont).

has anyone ever used or seen it before? In theory it sounds good.

Gary in WA 08-05-2011 10:08 PM

What kind of sheathing is on the walls- OSB, plywood, Celotex.....?

Is there a gap- air space between the sheathing/brick?

Use the AC much?


diybrad 08-06-2011 08:26 AM


Originally Posted by GBR in WA (Post 701280)
What kind of sheathing is on the walls- OSB, plywood, Celotex.....?

Is there a gap- air space between the sheathing/brick?

Use the AC much?


Its not none of the three. It is some sort of MDF or fiberous sheeting that is 1/4 thick. The there is about a 1/4 to a 1/2 air gap between the brick.

So from the outside to the inside it goes: brick, air gap, 1/4" board then the stud bay. there is no type of house wrap or air barrier.

The air gap goes up to the soffits down to the breather holes before the block foundation.

we do use the AC. we keep it set on 72-73. outside temp at worst gets to 95-100 with high humidity. in the winter we keep it around 68 if that matters.

If you have any reason that i shouldnt do this let me know. I already put the Dow XPS foam board up and i plan on spray foaming with closed cell next weekend.


Gary in WA 08-06-2011 08:08 PM

Your Zone, if #4, requires only R-10 cavity insulation (minimum). I would be worried when adding the SPF after the rigid of being too low in the permanence rating of drying to the inside effectively. Rigid foam at 2" = 0.75 perms (about same as asphalt paper faced f.g.), SPF at 1-1/2" = 0.9 perms, combined let very little moisture through. Why so much foam? You want a vapor open inside (no poly), to dry both ways. When sunshine drives the water in the brick inward, you want a permeable cavity insulation not to stop the moisture in the cavity;

You are in Zone 4, not really a cold climate:

Read these recommendations and follow the links given throughout, in a location similar to yours;


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