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Dennis Doucette 07-20-2006 03:08 PM

closed cell insulation in attic
We are currently building a new home for our daughter. We would like to use the spray foam application, but we are getting conflicting opinions from the closed cell spray foam contractors. We are located in central Minnesota, with extreme cold (can be as low as -40 F) and heat (last week 102 F). We are acting as our own general and our primary concern is the insulation in the ceiling areas. The living, dining and kitchen areas have a 6/12 vault outside with a 4/12 vault inside with energy heel trusses. The bedroom, bath areas have an 8' flat ceiling. One contractor says to place cardboard chutes up the rafter bays in the vault, up to the roof vent and to spray against the cardboard chutes at the roof of the house. The second contractor says we don't need an airspace because the foam is a vapor barrier and that 2" of foam will not allow any heat or cold to pass through. The roofing manufacturer feels this may cause a "hot roof condition" and may void the warrenty. Our concern over the vault is that we will have approximately 48 inches of air space above the taped sheetrock ceiling before reaching the spray foam insulation, and we will be heating that area in the winter and that we will have trapped moist air above the sheetrock that will not be allowed to escape. Similarly, the flat ceilings in the house create differing opinions by the contractors. Ths first feels we should spray above the sheetrock and the second says we should blow in fiberglass over this area. We are terribly confused, but we feel this product will offer the greatest energy savings, but we do not want to make a huge mistake with the foam. Is Foam the proper application or should we revert to a blown in product. Has anyone used foam in an attic situation and how was it completed and are you satisfied? Any help anyone can offer will be greatly appreciated.

manhattan42 07-20-2006 08:37 PM

I am not a big fan of foam because of its exorbitant costs.

Foams can easily cost 3-5X more than conventional cellulose or figerglass insulations while providing no more energy saving benefits.

In fact, high density fiberglass can give you an R value equal to most open and closed cell foams and many times less the cost.

Foam insulations used in accessible attics must be also specially isolated from combustion by way of covering them with drywall, mineral wool insulation or other approved thermal barriers. Foams will ignite in a fire causing the flames to rapidly spread and produce noxious fumes including cyandie gas if not thermally protected for ignition. This can also add to the already high costs of their use.

The biggest ultimate drawback with foam insulations is that they can take decades to pay for these exorbitant materials before seeing that money applied to any real dollars saved on energy costs...

And they still do not address all heat loss issues in the structure.

That doesn't make for a very cost effective or sensible way to insulate especially when money is a factor.

That said.

There are two ways to insulate an attic or rafter spaces:

1) By insulating the floor of the attic, or lower portion of your scissors trusses and then ventilating the attic..

2) By insulating the rafters right up against the roof without having the attic ventilated at all.

It is actually #1 that offers the least energy efficient design and most potential damage to shingles. The reason is that the deck immediately under the shingles in such designs will get excessively hotter in summer than if you insulate with a closed attic design. Placing the insulation directly against the underside of the roof deck makes for a cooler roof deck and cooler shingles. This is a perfectly acceptable means to insulate for shingle manufacturers.

Also, despite technically having to heat the attic space when insulating the underside of the roof deck and rafters, studies have shown the closed design in #2 above to be highly more energy efficient than conventional ventilated and insulated attics.

Moisture is not an issue with closed cell foams. Closed cell foams have perm ratings less than 1 making them true vapor barriers. For this reason they can be applied directly to the underside surfaces of rafters or to the joist cavities without worry about condensation.

When placed against the rafters, you are technically 'trapping' gaseous water vapor from the house below in the attic space, but so what? The foam keeps the surfaces in the attic well above the dew point at which condensation can occur, and the attic space is conditioned from below, so there is no worry about water vapor condensing at all.

Using closed cell foam in joist spaces also keeps water vapor out of the colder attic above because as a vapor barrier, no moisture can pass through it... so again no worries about condensed liquid water..

So regarding closed cell foams and water vapor issues, it is a win-win situation whichever place (joist or rafters) you choose to install.

But you should seriously rethink your use of foam altogether and in my professional opinion.

The best insulated homes use combinations of insulation types to deal with heat losses and gains from convection, radiation, conduction and air infiltration...and no single type of insulation, especially foam, can address all these insulating issues.

Here is a helpful "cold climate" home insulation design from the Building Sciences Corporation that could easily be adapted for use in Minnesota:

Dennis Doucette 07-21-2006 07:50 AM

Great information. That is what I have ben looking for. We will rethink our choice of foam. Thanks again.:yes:

jpgick 07-23-2006 07:10 AM

I do not mean to start a big argument here, BUT!!! I would like to see some documentation from Manhatten22 on where he is getting his information. Now depending on whose foam you are looking at, there are a number of them out there that have a class 1 fire rating. Meaning that they will not ignite. Your studs will have gone up in smoke before the Class 1 foam does. As for the heat loss, you can put a R-50 fiberglass up there and the air will move through that just as easy as if it were a R-13. There is no air sealing done by Fiberglass. Test can be proven that the air sealing will make a hugh difference in your utility bills. The blower door does not lie!! If you have a good foam contractor, he will tighten your home up, much better then any of the other insulations. The blower door test will tell you if it is to tight, and then you can address that to keep the Air changes per hour to .35 pascals, which is the minimum needed. If there are a lot of windows then you wont need to do anything probably.
As for the Shingle temperature. Putting foam up on the roof will not make the shingle's cooler. Test have shown that they will increase a average of 13 degrees. It makes more of a difference if you change the color of your roof then if you foam it. I think that ELK shingles says that they dont mind if you do this method.
Your payback will not be over several years, it should be in about a 2-3 year's. You should not concern yourself with the payback anyway. Example:

Normal insulation will cost you $3500.
Foam Insulation will cost you $11,000.
Added cost to your mortgage $70. ( I did not do the math on this but I am close)
Normal utility bill $250
Foam utility bill $125
Difference $125
Positive Cash Flow of $55 every month. Who cares if it takes 30 years to pay back, you lay out less money every month.

I am a Foam contractor, I have seen the difference stated above. We have put into writing the difference above.
If you look on the building america website, you will see that they recommend using foam. This is a Department of Energy website.
The only problem I have with what you described is putting closed cell foam on the roof line. If you ever had a leak, then you would not know it until it is to late to fix. Look at using open cell up there. If you really want closed cell in your walls make sure that it is atleast 2" Don't let them do a Flash and Bat, Opens up several other issues.

All in my opinion.


joasis 07-23-2006 09:01 AM

Good post jpgick. I am getting into ICF home construction and one of the issues is the insulation of the ceiling. If 4 inches of foam is placed above the ceiling, the whole house has an effective (acts like) an r-60, which to say the least, very impressive.

In Oklahoma, they do contend ICF's and typical r-40 values are overkill, but my view is you pay only once for insulation. The spec homes we currently build utilize a metal panel roof and we place a "double bubble" foil back insulation directly inder the metal, and blown cellulose in the attic. Since the metal is attached to purlins and there is not a deck as in traditional home construction, it works great.

I have been quoted (in my area) $1.00 to $1.30 a sq/ft for foam installed, How does this compare to your rates? I have looked at the equipment and it is expensive.

Bpawb 03-16-2007 11:36 AM

[quote=joasis;The spec homes we currently build utilize a metal panel roof and we place a "double bubble" foil back insulation directly inder the metal, .[/quote]

Hi joasis, i found this old thread & am considering using a raidant barrier/double bubble underneith galvalume panels....unlike the spec homes you speak of ... our house has full decking on the roof...i'm unsure if i should use strips/battens or not? or just apply the metal panels directly to the double bubble?


AtlanticWBConst. 03-16-2007 02:01 PM


Originally Posted by Bpawb (Post 37178)
Hi joasis, i found this old thread & am considering using a raidant barrier/double bubble underneith galvalume panels....unlike the spec homes you speak of ... our house has full decking on the roof...i'm unsure if i should use strips/battens or not? or just apply the metal panels directly to the double bubble?



You can also PM (private Email) .... Jay (Joasis) thru this website directly - to get answers to your questions....

joasis 03-16-2007 09:08 PM

If you strap the roof with 1X4's, you will have enough "air gap" for the solar gaurd to work more effectlively then just compressing the 1X's 3 to 4 feet will be satisfied with the results. Good luck.

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