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cww1 11-21-2012 09:48 AM

Insulating walls from exterior
We are purchasing a home that will need to have all the siding (stained wood) and sheathing removed due to insect and woodpecker damage. We live in zone 4 and the walls are 2x6 on 16" centers. All the windows are going to be replaced as well. I am torn (monetarily) on closed cell spray foam but have concerns about settling on FG or Cellulose. The plan is insulation, 1/2 ply, housewrap, 1/4" XPS (R1.6) , then foam backed vinyl siding (R1.6). My first instinct is 3" of closed cell (R18) sprayed direct to back of drywall but thats a bit $$$ so am looking for any all ideas or flaws in plan. Thanks,

joecaption 11-21-2012 10:02 AM

I'd strongly suggest againt the insulated vinyl siding. Use a .045 high grade siding instead. The money saved could go toward having the spray foam done.

I've done a few home with the insulated siding, (customer insisted) and they all came back and report it made 0 differance in there heating and cooling cost.
There's still going to be air gaps where the seams are and around all the inside and outside corners,windows and doors like with any siding.

Sure hope your getting this house for a steal, a whole lot of work and money to do what your suggesting.

Gary in WA 11-21-2012 09:48 PM

I'd use R-5 insulating sheathing to stop thermal bridging/cavity condensation:

Read up on it:



cocobolo 11-21-2012 11:28 PM

I think you will get far more bang for the buck if you use 5 1/2" Roxul batts in the cavities. Then cover that with a 1" or 1 1/2" foam board (usually EPS) and then your 1/2" plywood. Spray foam is quite costly.

I agree with Gary concerning the thermal bridging, which is VERY important to keeping your overall wall R rating high.

cww1 11-22-2012 07:26 AM

Thanks for input all and the links GBR. Should it be foam then ply or ply then foam ? Putting the ply on the outside will make it easier for siding but not sure local code will allow without additional bracing on walls, at least the corners.

Gary in WA 11-22-2012 04:35 PM

Where are you located?


cww1 11-22-2012 08:54 PM

House is in St. Louis, MO. And EPS or XPS, I think XPS has a higher r value per inch.

Gary in WA 11-24-2012 08:33 PM

Plywood sheathing before foamboard as per structural code. 1" of XPS (R-5) will warm your sheathing and prevent condensation there, up to 56%RH at 70* in the room. With just 1/4" (R-1.6), you are safe only to 48%RH. Local EPS may be rated at only R-3.5 per inch thickness, but I would get rid of the insects around the house first, as they love foamboard. If using ESP, install the housewrap exterior of it to control water intake/storage. If XPS, install housewrap first, tape the vertical joints in foam, and get the window/door flashing correct.

At annual low temp at 47*---
your sheathing temp is at 49* with only R-1.6 f.b. or 52* with R-5.


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