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foam & fiberglass combo?


kurt
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New construction house; builder proposes insulation method.....

1) 2x6 wall cavity

2) plans on spraying in 1" of closed cell foam on interior side of sheathing.

3) followed by R19 batts, which will of course, end up compressed.

Anyone heard of this? The supposed benefit is getting the increase R value (1"=approx. r6 + r19 = approx. r25).

I'm researching, but am interested in opinions (other than "whadda goofy f***ing idea").

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Hi Kurt,

Unless it's a high-density batt, an R19 batt is sized to fully fill a 2 by 6 cavity. If he blows in an inch of closed cell, he'll compress the batting an inch and reduce it's R-value by roughly 3.55, which will result in a wall of only 15.45 + 6 or R21.45. He'll come closer if he uses an R15 high-density 3-1/2inch batt and two inches of closed cell.

ONE TEAM - ONE FIGHT!!!

Mike

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That's what my quick head calc says too. When he told me what they planned, I just sort of blanked. I'd never heard or seen of such a thing. That's why I'm asking.

There's no benefit other than air barrier, no?

I don't know why they don't go all foam. That's tomorrows conversation......

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Another good point.

The guy is a good guy, and he's honestly trying to do "right" by his (and my) client, but he's in over his head. He's trying to satisfy the clients request for "lots of insulation".

This is one of those where i wish I'd been included at an earlier stage.

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Hi,

The advantage to the closed-cell foam is that it will stop more air movement than insulation; so, you might get an R25 but you end up with a wall that costs energy wise like a leaky wall with R30 or better batting.

I'm not so certain that you'd get condensation on the back of the FG with an inch of foam there. first, he really wouldn't need a vapor barrier because of the foam and, unless the temp at the face of the foam is low enough to cause condensation, I don't think it's going to be an issue. If the foamer does a good job and the wall is properly draft sealed, the warm air migrating in there should be enough to keep the foam warm enough to avoid condensation. Two inches would be better, but one might work fine. It's becoming pretty common to use a 1-inch layer of foam on the undersides of roofs and then use unfaced batting below it in unvented catedral ceiling roofs, so it might work fine.

ONE TEAM - ONE FIGHT!!!

Mike

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Unless it's a high-density batt, an R19 batt is sized to fully fill a 2 by 6 cavity. If he blows in an inch of closed cell, he'll compress the batting an inch and reduce it's R-value by roughly 3.55, which will result in a wall of only 15.45 + 6 or R21.45. He'll come closer if he uses an R15 high-density 3-1/2inch batt and two inches of closed cell.

Hi Mike, from what I understand compressing the R19 batt is essentially the same as turning it into a high density batt. There will be a loss from the compression, but not the full R3.5 loss that one would expect.

They'd save money and get the same gains ( in my mind) by just detailing with foam and installing the VP directly behind the interior wall plane.

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What Randy said.

R-21 batts are not much more costly than R-19. Properly detailed construction will not leak much air, and R-21 would perform as well as the proposed idea, without the concern of a double vapor barrier. Foam is a lot more costly and is probably the reason for the idea posted.(Trying to get the best of both worlds)

In my opinion there would be no benefit, but possible trouble. Use one system or the other.

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I have R-21 in 6" walls with R-48 blown overhead, built in 08 and it's been very reasonable to heat. If I want to make changes or modifications later, I don't have to deal with the foam mess. I think if I were going to foam, I would consider going all the way and do the whole house with ICF's instead of conventional framing. Maybe next time.

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Originally posted by Chad Fabry

Unless it's a high-density Batt, an R19 batt is sized to fully fill a 2 by 6 cavity. If he blows in an inch of closed cell, he'll compress the batting an inch and reduce it's R-value by roughly 3.55, which will result in a wall of only 15.45 + 6 or R21.45. He'll come closer if he uses an R15 high-density 3-1/2inch batt and two inches of closed cell.

Hi Mike, from what I understand compressing the R19 batt is essentially the same as turning it into a high density batt. There will be a loss from the compression, but not the full R3.5 loss that one would expect.

They'd save money and get the same gains ( in my mind) by just detailing with foam and installing the VP directly behind the interior wall plane.

Hi Chad,

I don't actually know what the loss in total R-value would be by compressing insulation an inch or so. What I do know is that R-value is supposed to be dependent on the amount of trapped air that you have in the insulation; and, that since air is never really trapped in fiberglass insulation - just sort of slowed down as it tries to pass through - it seems to me that there's no way that a layman can accurately guess. Therefore, in my mind it makes sense to deduct the full 3.55 you'd get by losing an inch of insulation. I figure that if they want accuracy, they can install a high-density batt that's not as thick and they'll have the manufacturer's lab testing to back them up.

I still don't think that you're going to see moisture condense in that wall. After all, if you wouldn't have seen condensation take place in a conventionally insulated wall with nothing but plywood or OSB for sheathing covered by Tyvek or some other type of polyolefin barrier, why would you have condensation on the inner face of closed cell that's eliminated all telltale drafts and has moved the inside face of that outside wall further into the heated zone?

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by gtblum

Originally posted by kurt

While possible, I'd put condensation way down the list.

Why?

My biggest question about closed cell has always been, about condensation. How does moisture dissipate? What about the difference between the indoor and outdoor climate?

The moisture can't dissipate, but if the cavity is full of closed cell there is no place for the condesation to take place. There is no air to condensate moisture from, or very little anyway.

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Sounds like a dangerous experiment to me, tell the builder to try it on his own house and let us know how it turns out. If they still insist on mixing systems, they would be better served with advanced framing and foam sheathing, and a little spray foam for added air sealing.

I say let the manufactures and engineers do the experiments, they actually gain something from the money spent regardless of the outcome.

Tom

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