Spray foam under roof decking.

[swampratt]

No pics needed I have seen many buildings spray foamed.
I just have an attic in the 2 story house I live in that is just inches away from my sheetrock ceiling.
I have insulation packed in there but not against the roof decking.

My sheetrock in the heat of a summer day can be 90+F along the edges of the room for about 1" to 3 feet from the wall in the upstairs bedrooms and bathroom.
That sure makes it hotter upstairs.

It's a tight space to get to, I have to belly crawl to get to the edges.

I was thinking spray foam and then this Rockwool brand stiff insulation looks like it may do the trick.
I will research it more.
Thanks for the mention of that @TinkerTanker I did not know it existed . The video in the link was very informative.

When I hear Rockwool i immediately think of that itchy black blow in stuff but this is not that product at all this is a stiff form fitting batt.

 

[Bocephus123]

A cousin of mine experimented with spray on insulation and what is called blue board down in SE states. He did a wood frame roof, 1/2 spray in and 1/2 blue board. He determined over time the spray in due to humidity caused moisture to collect in the decking leading to rot. The blue board didn't experience the same issue. It was cut to fit tightly between rafters. R value was a little less than spray in but no issues over time. Both were pest attractions burrowing into it to make their homes but with pesticides they were controlled.

PS: This was done many years ago so material composition today may differ and have differing results.
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The bomb put 2” down before or as you deck. They have 4x8 sheets at maxwell supply used it at a freezer warehouse we built. And we’re backing 8 yard concrete trucks across it didn’t even hurt it

 

[yukonjack]

One draw back is the asphalt shingle mfgs. were wiggling out of warranty issues claiming the foam retained
or trapped excessive heat and that it promoted deterioration of shingles. That was years ago though.

That’s very true. It traps the heat and degrades the shingles. I’d never insulate the roof decking.

 

[Parks 788]

Im on a FB group called Oklahoma Barndominiums and most on the site will recommend about an I"-2" of closed cell and if you want more R value then spray on open cell. Seems the R-value per inch of thickness is far superior to fiberglass and others. They state the closed cell foam doesn't capture/retain water so I'm not sure how things would rot or rust. Whether you have a leak with fiberglass or foam you still have a lead. I'd probably rather not want wet fiberglass that soaks up moisure and then potentially start having a mold issue. Foam is quite a bit more $$ but buy once cry once.

 

[Big50]

I put down 21 inches of fiberglass for a R-60 rating. With 3600 SQ feet of living my electric bill runs around $100 per month on average. Yes that's with electric heat in the winter.

 

[swampratt]

I put down 21 inches of fiberglass for a R-60 rating. With 3600 SQ feet of living my electric bill runs around $100 per month on average. Yes that's with electric heat in the winter.

If I had that much attic space above my upstairs sheetrock in the hot areas I would be golden.

 

[-Pjackso]

I put down 21 inches of fiberglass for a R-60 rating. With 3600 SQ feet of living my electric bill runs around $100 per month on average. Yes that's with electric heat in the winter.

^^^ This is how I would do it.
I would over-insulate the house using fiberglass insulation.

Batt insulation can be removed/replaced if maintenance is required. Spray foam is almost a permanent install.

If people want spray foam benefits, then I would strongly consider thick foam boards.
Foam boards are removable, if required.

Plenty of YouTube videos on foam board, but here's 1 example:


@swampratt , if you go this route, maybe consider foil-backed foam board for lower emissivity.

 

[BrandonM]

I thought the real benefit to spray foam was the tight air seal, or envelope, as they call it? Only doing a roof line would have limited effectiveness.

 

[swampratt]

This is my issue here. There is minimal attic space and the radiant heat will soak into the insulation from the roof decking.
I need to eliminate the radiant heat from the decking.
Not enough attic space.

When I need another replacement roof I will add another roof over this one just for the upper portion above the rooms and a ridge cap vent and sort of soffit vents at the bottom to allow air flow.
That way the sun will never hit the original decking below the new roof.
That is what I thought would be the fix.

My access door to the attic 19x21" and just about 18" before your head hits the roof decking so no fat non bendy guys go in this attic and I outlined in black where the room ceiling and walls are .

Horrible design as far as efficiency goes.

Just imagine the heat you get on the edges of those rooms closest to the roof line.

 

[HoLeChit]

This is my issue here. There is minimal attic space and the radiant heat will soak into the insulation from the roof decking.
I need to eliminate the radiant heat from the decking.
Not enough attic space.

When I need another replacement roof I will add another roof over this one just for the upper portion above the rooms and a ridge cap vent and sort of soffit vents at the bottom to allow air flow.
That way the sun will never hit the original decking below the new roof.
That is what I thought would be the fix.

My access door to the attic 19x21" and just about 18" before your head hits the roof decking so no fat non bendy guys go in this attic and I outlined in black where the room ceiling and walls are .

Horrible design as far as efficiency goes.

Just imagine the heat you get on the edges of those rooms closest to the roof line.
View attachment 386287View attachment 386288

Looking at your picture I see a vent in your siding, and it appears that there’s soffit vents, at least one next to your chimney?

I would make sure that neither of them are blocked, and would definitely consider adding a fan to the wall vent in your siding. Maybe throw a few of those rigid foil backed foam boards up in between your rafters to block some of that heat from the deck. I wouldn’t make a complete seal or anything, but if you can cover 80% of the affected gaps with foam board wedges in, then you’ll have that much less radiant heat and still have room for your house to breathe as it was intended. Yeah, radiant heat sucks, but if you have an air gap between your rafters and your insulation/house framing, then most of your heat problems will likely be coming from the trapped air in your attic. Some of that heat in the top corners of your room will be unavoidable, as that’s where your roof comes in contact with your walls, and creates a thermal bridge.