Expanded foam attic insulation

[Richard Moore]

A first for me. REALLY nice 4800 sf, 1991 home today. Major renovation/upgrade in 2006. The two huge attic spaces were totally insulated with expanded foam. I saw the invoice for this and it was done by a licensed insulation contractor, also in 2006. The order specified a minimum of 5½ inches. There was at least that as far as I could tell. I've checked a couple of tables and the R value of 5½" would be right at 38...so that's good.

Any input on attics insulated this way? Pros, cons?

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[Marc]

I wonder if any wiring that's immersed within the foam has to be derated? There's no convection 'escape' for heat generated by the wiring. I know wiring can penetrate it without derating, but what about long parallel runs totally immersed within the foam?

Marc

[Mark P]

Can you walk on that?

If there ever is a roof leak, you'll have a lake in the attic before the ceiling below shows a stain.

[hausdok]

I wonder if any wiring that's immersed within the foam has to be derated? There's no convection 'escape' for heat generated by the wiring. I know wiring can penetrate it without derating, but what about long parallel runs totally immersed within the foam?

Marc

Would you derate it if it were buried under 18 inches of blown-in fiberglass or 12-inches of fiberglass batting or rockwool or 14 inches of cells or silvawool? If not, why derate it for foam?

ONE TEAM - ONE FIGHT!!!

Mike

[kurt]

I've never seen it installed on the attic "floor". It's usually up on the underside of the sheathing.

[Richard Moore]

Can you walk on that?

That was one disadvantage. I was very hesitant to step on it even directly over the bottom chord of the trusses as it seemed my weight would transfer to the sheetrock below. So I transversed the spaces by walking in the crotches of the trusses.

If there ever is a roof leak, you'll have a lake in the attic before the ceiling below shows a stain.

An interesting point, but probably not enough to say it's a problem. The roof was wood shake, but very recent and in excellent condition. I earned brownie points on this one by doing the roof (and decks) a couple of days earlier due to expected snow fall. We never did get anything heavy but there was still enough snow on some slopes that it made the prior visit worthwhile and much appreciated by the client.

[Jim Katen]

I wonder if any wiring that's immersed within the foam has to be derated? There's no convection 'escape' for heat generated by the wiring. I know wiring can penetrate it without derating, but what about long parallel runs totally immersed within the foam?

Marc

The wires will probably stay cooler, overall, than they would if they were exposed to the hot attic air.

- Jim Katen, Oregon

[hausdok]

Richard,

There's a whole slew of Bennet homes up in Issaquah Highlands that have several inches of foam like that with another 15 inches of blown-in fiberglass on top of that.

Really tricky attic to cross!

ONE TEAM - ONE FIGHT!!!

Mike

[Richard Moore]

There's a whole slew of Bennet homes up in Issaquah Highlands that have several inches of foam like that with another 15 inches of blown-in fiberglass on top of that.

Really tricky attic to cross!

ONE TEAM - ONE FIGHT!!!

Mike

Wow, that sounds like they have a R-idiculous value. I've done a bunch of homes up there (haven't we all) but have yet to see that.

This home had "wood cathedrals" for attics, so walking the truss crotches was fairly easy. I don't think I want to come across it in a smaller, lower sloped attic.

[Bain]

I wonder if any wiring that's immersed within the foam has to be derated? There's no convection 'escape' for heat generated by the wiring. I know wiring can penetrate it without derating, but what about long parallel runs totally immersed within the foam?

Marc

The wires will probably stay cooler, overall, than they would if they were exposed to the hot attic air.

- Jim Katen, Oregon

I was told the exact same thing by my local electrical inspector a few years ago when I called to inquire about a situation similar to Richard's. I didn't know if encapsulating the wires was kosher or not, and didn't want to screw up.

[hausdok]

There's a whole slew of Bennet homes up in Issaquah Highlands that have several inches of foam like that with another 15 inches of blown-in fiberglass on top of that.

Really tricky attic to cross!

ONE TEAM - ONE FIGHT!!!

Mike

Wow, that sounds like they have a R-idiculous value. I've done a bunch of homes up there (haven't we all) but have yet to see that.

This home had "wood cathedrals" for attics, so walking the truss crotches was fairly easy. I don't think I want to come across it in a smaller, lower sloped attic.

Yeah, they're in West Highland Park. If you do a home in there, be very cautious where and how you place your feet when you step through that blown-in. Finding the foam under the blown-in was a little bit of a shocker and then I had to go very slowly in order to be sure I was on truss chords and not the ceiling before I placed my weight on a foot.

The home had a broken whole house air exchange timer so some dildo removed the timer and left the fan on 24/7. The house is foamed everywhere - top, bottom, sides - so the client was relatively comfortable and didn't even realize that most of his air was being replaced with exterior air round the clock in the dead of winter.

I hope to hell when they came back and repaired that whole house system that they showed him how to set up the programming properly, and that he uses it enough hours of the day, or he's going to be living in a petrie dish.

ONE TEAM - ONE FIGHT!!!

Mike

[inspectorreuben]

There is no better way to insulate an attic. This is the gold standard.

While the calculated insulation value might come out to about R-38, it will perform far better than an attic with any amount of loose fill insulation. I'd take five inches of spray foam over any depth of fiberglass or cellulose. While loose fill insulation will allow for convection, foam will pretty much eliminate it. This will also seal up every possible bypass perfectly, as long as it's installed properly.

I inspected an attic today with about 2 - 3 inches of foam insulation on the attic floor, and the owner told me not to worry about walking on the trusses; he said it would hold his weight just fine, and he had spent plenty of time in the attic. He was about twice my size, so I gave it a shot. He was right.

[Marc]

There is no better way to insulate an attic. This is the gold standard.

While the calculated insulation value might come out to about R-38, it will perform far better than an attic with any amount of loose fill insulation. I'd take five inches of spray foam over any depth of fiberglass or cellulose. While loose fill insulation will allow for convection, foam will pretty much eliminate it. This will also seal up every possible bypass perfectly, as long as it's installed properly.

I inspected an attic today with about 2 - 3 inches of foam insulation on the attic floor, and the owner told me not to worry about walking on the trusses; he said it would hold his weight just fine, and he had spent plenty of time in the attic. He was about twice my size, so I gave it a shot. He was right.

Not all SPF has the same density. Density determines it's strength.

Marc

[inspectorreuben]

There is no better way to insulate an attic. This is the gold standard.

While the calculated insulation value might come out to about R-38, it will perform far better than an attic with any amount of loose fill insulation. I'd take five inches of spray foam over any depth of fiberglass or cellulose. While loose fill insulation will allow for convection, foam will pretty much eliminate it. This will also seal up every possible bypass perfectly, as long as it's installed properly.

I inspected an attic today with about 2 - 3 inches of foam insulation on the attic floor, and the owner told me not to worry about walking on the trusses; he said it would hold his weight just fine, and he had spent plenty of time in the attic. He was about twice my size, so I gave it a shot. He was right.

Not all SPF has the same density. Density determines it's strength.

Marc

True, but if it has an insulating value of R-38 for 5 1/2", that comes out to about R-7, which means it's the dense stuff. Around here we call it 'closed cell foam'. I've also heard it referred to as 2 lb foam.

[David Meiland]

There is no better way to insulate an attic. This is the gold standard.

It is quite possible to air-seal an attic with caulk and small amounts of canned foam, and then insulate it with cellulose. This application performs very well, the cost will probably be far lower, and the whole thing is serviceable and can easily be remodeled later without destroying the insulation. There are possible failure issues with foam as well--sometimes catastrophic--that may or may not become apparent immediately (i.e. before the whole thing is covered with drywall). Foam is a petroleum product, and it will go up as the oil price increases, and of course the embodied energy of the material is extremely high compared to most other forms of insulation.

While I suppose it's possible that the foam job would be slightly more airtight than anything else, remember that air exchanges are necessary, mechanical or otherwise, and turning the house into a beer cooler is not the goal. The only place I see a need for foam in new construction is under concrete slabs.

[Jim Katen]

. . . While I suppose it's possible that the foam job would be slightly more airtight than anything else, remember that air exchanges are necessary, mechanical or otherwise, and turning the house into a beer cooler is not the goal. . . .

I disagree. All of the current trends in building science are pointing toward the beer cooler model as being superior. Ventilation is, of course, mandatory. But leaks are not the same thing as ventilation. We've moved past the paradigm of using sloppy construction to provide air exchanges.

- Jim Katen, Oregon

[inspectorreuben]

It is quite possible to air-seal an attic with caulk and small amounts of canned foam, and then insulate it with cellulose. This application performs very well, the cost will probably be far lower, and the whole thing is serviceable and can easily be remodeled later without destroying the insulation.

True, but I've found that it rarely gets done right, and this does nothing to prevent convection. You're right when you say the cost is lower though.

There are possible failure issues with foam as well--sometimes catastrophic--that may or may not become apparent immediately (i.e. before the whole thing is covered with drywall).

Sure, as with many other products, it can fail if it's not installed properly.

It is quite possible to air-seal an attic with caulk and small amounts of canned foam, and then insulate it with cellulose. This application performs very well, the cost will probably be far lower, and the whole thing is serviceable and can easily be remodeled later without destroying the insulation. There are possible failure issues with foam as well--sometimes catastrophic--that may or may not become apparent immediately (i.e. before the whole thing is covered with drywall). Foam is a petroleum product, and it will go up as the oil price increases, and of course the embodied energy of the material is extremely high compared to most other forms of insulation.

While I suppose it's possible that the foam job would be slightly more airtight than anything else, remember that air exchanges are necessary, mechanical or otherwise, and turning the house into a beer cooler is not the goal.

I disagree. Turning the house in to a beer cooler is another way of saying that you're doing a perfect job of preventing unwanted air leakage, especially in to the attic. Air exchanges should be provided by mechanical equipment, not holes between the living space and the attic.

It definitely depends on your climate, but here in Minnesota, tiny air leaks in to the attic can equate to huge moisture problems with the attic.

[kurt]

Yep, beer cooler is where it's at.

If closed cell foam is out of the price range, then tight construction, caulking and sealing all floor penetrations, no recessed lights, vapor barrier paints, etc., etc., and dense pack cellulose is my next choice.

[Chad Fabry]

Around here, to reap the benefits of foam and to minimize costs, many builders are using a hybrid approach.

They're applying an inch or so of high density urethane based closed cell foam everywhere. In the walls they're adding R13 or R19 kraft faced FG and in the ceilings, they're blowing in either cellulose or fiberglass using the foam as the VB.

[David Meiland]

We've moved past the paradigm of using sloppy construction to provide air exchanges.

What I am seeing--so far, and in the few foam-insulation houses that I've looked at--is that foam is being used to try to make up for sloppy air-barrier work. Guys think that if they spray foam into the stud bays there will be no air leakage at the walls. Unfortunately, it still comes in under the mudsill, at the corners, between stacked framing members, etc. And of course there is leakage around window and door weatherstripping.

[David Meiland]

True, but I've found that it rarely gets done right, and this does nothing to prevent convection. You're right when you say the cost is lower though.

......Turning the house in to a beer cooler is another way of saying that you're doing a perfect job of preventing unwanted air leakage, especially in to the attic.

So then a foam attic is the right choice for a builder who doesn't want to learn how to do the air-sealing right. Hopefully he's not also using pull-down stairs, or if he is, he's building a really nice insulated/weatherstripped cover over them. I'd be interested to hear from Richard how he got into the attic he photographed, and whether air leakage actually was near zero.

Convection in fiber insulation is certainly an issue, but I think that it's very minor compared to the air sealing mistakes that are common. I have not heard or read anything that makes me think that convection is a big enough issue to make foam the only workable option.

[David Meiland]

Yep, beer cooler is where it's at.

If closed cell foam is out of the price range, then tight construction, caulking and sealing all floor penetrations, no recessed lights, vapor barrier paints, etc., etc., and dense pack cellulose is my next choice.

What I am reading is that the few guys out there who have really learned the details are getting around 1 ACH50 using air-sealing and dense packed cellulose everywhere. That seems pretty good to me. It would interesting to compare a house like that to a foamed house where the same attention was not paid to the small details.

[kurt]

I'm seeing what Fabry's talking about. The walls are flashed with 1" of foam, then batts or dense cel to fill the cavity.

I'm not sure about Meiland's aversion to foam as an air barrier. The jobs I see, it's superb as an air barrier. All those places Dave notes are sealed with foam in any decently applied application.

What's not to like?

[David Meiland]

They're applying an inch or so of high density urethane based closed cell foam everywhere. In the walls they're adding R13 or R19 kraft faced FG

I think that blown fiber in those walls is going to be a better choice. Batts laid against lumpy SPF are going to have air gaps all over the place.

[Marc]

True, but I've found that it rarely gets done right, and this does nothing to prevent convection. You're right when you say the cost is lower though.

......Turning the house in to a beer cooler is another way of saying that you're doing a perfect job of preventing unwanted air leakage, especially in to the attic.

So then a foam attic is the right choice for a builder who doesn't want to learn how to do the air-sealing right.

I'd suggest that the only right choice for a builder who doesn't want to learn is to do something else for a living. Houses cost too much to mess up.

Marc