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Discussion Starter · #1 ·
It seems in my project I will have to spray foam into a wall cavity , which is insulated (UNDER the sheating ) with a 1/2" XPS Foam. does anybody know if spray foam will succesfully stick to the XPS foam?

Another option is to simply cut out the 1/2" XPS (remove large rectangles ), and spray against the OSB (this actually will give me an extra 1/2" of room for good insulation. (this is a remodel and wall s are framed with only 2x4 so only 3.5" for insulation .

Was planning on using FrothPak 650 from DOW for this project. which is pricey.

This is in Chicago, which has hot summer and REALLY cold winters - trying to limit potential of Condensation at either side of the XPS. 1/2" of XPS has a rating of R-3 however, its probably close to R 2.5 (since XPS is really R5 per inch.) also what worries me, the 1/2 Inch XPS foam has a plastic foil backing on both side ( i think for structural integrity) ... I would keep the 1/2 foam that is under the 2x4 and other structural members.

I can provide pictures. also i should mention , i did SEAL all seams to the best of my abilities with the all weather flashing tapes. ... hoping to gain even more air tightness by using Spray foam. But also trying to not create any moisture in the wall cavity.

Thank you for your help - mark
 

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Hi Mark,
Make sure there is no vapor barrier on the inside under the drywall.

½" of foam was not a good choice. Chicago looks to be in zone 5 close to zone 6. In Zone 5 the minimum r-value for exterior rigid (2x4 walls) should be r-5. The objective is to ensure the inside surface of that rigid remains above the dew point.
Here's the link so I'll let you read.

I would need more information on what you have and what you are doing to try and make a suggestion.

Bud
 
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Discussion Starter · #3 ·
ahhhhh....

That makes perfect sense of what is happening, here is an example:

I had a fairly small piece of 3" EPS foam. (larger rectangle 1foot x 3 feet x 3" inches.

I have put it flush against the "external" ridgid foam (the 1/2" pink stuff) --- i had major condensation under the EPS - just enough warm (warmer) air got behind there and it created condensation. --- this is what pointed me to write this question in the first place.

I was going to spray foam against that PINK stuff, to build up the insulation - and that would be insulation next to an insulation . ... if any air got behind that spray foam, it would be an opportuity for condensation .... I think, not sure if I am understanding this dew point business accurately.

In our case, we were really limited to the space on the outside (there was an existing 2x4 framed wall that held up the roof,.. .and also not much space to go with 1" insulation on the outside

Our wall cavity is as follows now: (From outside to the inside)

hardie Siding >> House Wrap Dupont,>>> 1/2 OSB sheathing >>> 1/2 XPS foam >>>2 x 4 Fall Framing . (and there is nothing (no Insulation ) inside the 2x4 cavity , all exposed, no drywall just. yet.

As i said i was going to spray foam right against the XPS, but now i am thinking of pulling the majority of it to expose the back of sheathing.

Also from what I read the adhesion of spray foam will be better to wood, rather than XPS rigid foam. (it will introduce another challange of trying to keep the place warm enough (70-80 degrees when I do spray foam in early spring. )
 

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Notwithstanding everything else being discussed...

It seems in my project I will have to spray foam into a wall cavity , which is insulated (UNDER the sheating ) with a 1/2" XPS Foam. does anybody know if spray foam will succesfully stick to the XPS foam?
Yes. I can confirm that spray foam (at least Great Stuff) sticks to XPS.
 

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Here's a link on inserting pictures on this web site.
https://www.diychatroom.com/f114/how-insert-images-into-your-posts-205921/

There are too sides to the dew point issue, the amount of r-value for the rigid and similar for the cavity insulation. To move the dew point into the rigid insulation, where it will not see the inside air, you can also reduce the cavity insulation. I know, going the wrong direction for reducing heat loss, but the actual compromise will be minimal over a small area. I doubt the difference would show up on your heating bills.

How large of an area are you working on, whole house or just one room?

Bud
 

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Is that an outside wall? Looks like an attic above?

Bud
 

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Discussion Starter · #10 ·
That IS an outside wall, this is an old Coverted 3 season room, that had really LOW ceiling . I converted the ceiling into Vaulted Ceiling using LVL's, "sistered" rafters (old growth 2x6 are now attached to a 2x8 pieces of rafters.

The 4x4 posts you see on top of the perimeter boards (which are 2 by 2x8) - they sit on one top plate. .. the 4x4 posts (about 7.5" tall ) are where the ceiling joists used to be. (now removed and replaced with the 4x4. I had to retrofit the new Rafters to attach to the Top plate that runs top of the 4x4 and supports the old rafters (2x6) . There was really no other way to approach this go get 96" height for this room.

I could provide more pictures but newly constructed ceiling "Plane is Level, and True" ready for sleepers and drywall.

Plan is to Insulate the Underside of the roof sheathing with about 4" (or more ) of Spray foam , sprayed right onto the 6" T&G pine that was used in the 1950's as roof sheathing .. The remaining open cavity will be filled with ROXUL insulation. then drywall. I am actually on the fence weather to use some sort of MemBrain (Certainteed's smart vapor retardant) as vapor barrier below the drywall. - just afraid to trap any moisture in the ceiling, and not let it breathe and dry.

Yes, I know , very interesting project. If I had a chance to do it all over, I would knock the whole thing to the ground and start over. but this was not an option. =) This area is just 12 x 16 feet in size.

Thanks!
 

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Just the wall. You can insulate with foam, fiberglass or cellulose. Foam will give you more r value but expensive, esp with home owner bottles. Also it is now too cold. Foam may not cure properly esp because your exterior is cold. If you want, you may have to heat the space enough where the surface temp of the pink board is above 70 deg. I think 70 is about the number for spray foam. And as noted, you don't have to remove the pink boards.



Your eps foam test was faulty. You didn't air seal, so lots of condensation in between. Insulation test is almost by faith. You install it and air seal the insulated space. In your house, you'd seal the joints between the studs and the pink stuff, then insulation. Then you can use vapor barrier or use caulk and drywall to seal that side. Seal the electric boxes.


Cathedral ceiling should have a vent baffle between the roof decking and insulation. Search listiburek. He studied insulation and other building methods. There is some moisture and rot damages that can happen from moisture trapped because of foam insulation which does not breathe.



I'm in NJ and not really familiar with every new building practice. This is for other people who read this post. Yours is done already. Exterior sheathing is usually nailed directly to the framing studs. This way of sheathing, I think, would be best way to get the bracing capacity of the sheathing. With soft insulation in between, nails are floating in the insulation. I may be wrong about this, but I would have asked an engineer or such first.
 
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