spray foam insulation for underside of roof
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bmancanflyUser is Offline
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19 Mar 2009 02:56 PM
I saw a product at the FL home show where spray foam is applied to the underside of the roof.  They claim to seal the attic space completely from the outside air.  There is then no need to insulate the ceiling (between the attic and living space).

Anyone heard of this?  Wouldn't this raise the amount of air space that my a/c has to cool?  They seemed very convinced that this was a huge improvement over the old way of doing things. 

Any feedback would be appreciated.
AltonUser is Offline
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19 Mar 2009 03:31 PM

I have been in the attic of homes in Florida and Alabama that had the underside of the roof insulated.  The temperature was about 10º warmer than the air conditioned space below.  Big difference in outside temperature and inside the attic.

Building scientist I have talked to encouraged me to use at least 2.5" of closed cell polyurethane under the roof to insulate\seal, waterproof and glue the roof together.  Some installers advocate using open cell foam because they are concerned with future leaks being hard to find.  I would rather the roof not leak at all instead of the open cell absorbing its weight 10 times in water like a sponge.  Maybe that is just me, but I rather replace some roof decking the next time I re-roof than to to have my sheetrock damaged with water and mold. 

Let us hear the various opinions about what to do.

Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979
buck3647User is Offline
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19 Mar 2009 03:51 PM
Foam dome in Inverness Florida
I used 3 lb closed cell foam 3 inches thick to cover the exterior of the cocrete shell. My hope is that I can maintain an interior temp of 78 degrees with little or no mechanical device.
safedomes.com
wesUser is Offline
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19 Mar 2009 09:04 PM
I agree with Alton.
We have used this system over the past few years with good success. I use the open cell foam rather than closed cell (just personal preference), but results are the same.
I'm sure that there will be much debate on this subject, but my personal experience is that the insulated attic is far superior to an uninsulated attic over ceiling insulation.
Wes Shelby<br>Design Systems Group<br>Murray KY<br>[email protected]
greentreeUser is Offline
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20 Mar 2009 06:19 AM
I think the increased conditioned space concern is valid, keep mechanicals and duct out of the attic. It's a waste of energy. My view is coming from basement country though.
Dana1User is Offline
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20 Mar 2009 09:10 AM
Unvented-insulated roof designs work in all climates- it's been tested, studied, and analyzed to a near-certainty:

http://www.buildingscienceconsulting.com/resources/4-Understanding_Attic_Ventilation.pdf

http://www.buildingscienceconsulting.com/designsthatwork/hothumid/appendix2.pdf

I foamed the rafters & sealed the attic with low-density foam as a retrofit in my own home (in MA), it cut my heating bill by more than 10%, and now I only run the AC about 20 hours PER YEAR. (I'm sure in FL you'll still be running the AC more than that... :-) )

It's easier to get a perfect air-barrier by foaming at the rafter. If you have any ducts, compressors, air-handlers etc in the attic, putting it all inside the pressure & thermal boundary of the envelope keeps any duct-leakage inside the pressure boundary (less blower-induced outdoor air infiltration), and keeping it inside the insulating boundary reduces radiant losses/loads. In some instances insulation under the roof deck might reduce your roofing life 5-10% (your 25 year shingles may only last 22-23 years), but in most cases it makes less of a difference in peak roof temperatures than the COLOR & emissivity of the roofing, or on a pitched roof, whether it's on the north vs. south side of the roof ridge.

Anyone doing this should considerthe sidebar 1 on page 13 that first document. In FL you're in zone 1 or 2A. If you go with low-density (1/2lb per cubic foot- Icynene, et al) aka "open cell" foam you need more than an inch of foam to get the air-permeance low enough (you'll probably be doing more than that anyway, since an inch is only R3.5-ish.) If you went thinner than that, best-practices would then prescribe R5 of rigid insulation ABOVE the roof decking to guarantee a mold problem can't get started. High-density foam more is pretty much both air and vapor impermeable even at an inch of thickness and you won't need anything above the deck.

Some local codes require air-vent chutes (like those commonly used with fiberglass) between the underside of the decking and low density foam in retrofits to give the decking some inward drying capacity. (Seems easier to just go with the 2lb foam and forget about it.) In my case (climate zone 5) I didn't do that, and with 6" of half-pound foam under the deck, higher than prescribed water vapor (not air) permeability. I COULD just go ahead and apply an interior vapor retarder or paint it with something with a vapor-permeability of 1.0 or less, but since I'll be re-roofing in only a few years, I'll add the prescribed R20+ to the outside of the roof decking just to be safe. (I was planning to do that anyway, to bring the total R-value of the assembly up to something more like code for new-construction in these parts.)
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20 Mar 2009 11:35 AM
I always liked the system of leaving an air gap between the roof and the insulation, and using fans on a timer to keep the air circulating fast enough to keep the bottom of the roof cooled. If it is 70 inside and 90 outside, in the sun your roof could be 130(bright white)-170 (dull black) depending on the color and the material. By circulating air, you can keep the air space at 100ish degrees, also depending. I would much rather my insulation fight against a (100-70=) 30 degree differential than a (130-70=) 60 to (170-70=) 100 degree temperature differential.

Also, putting reflective insulation will help a great deal if you are in a sunny climate. The easiest way to do both is to put spacer blocks on the roof (like 3” chunks of 2*4) put a piece of reflective foam board up against the blocks, reflective side out, and then foam over the whole thing. This gives you an air gap, reflective barrier, and air seal all in one.
ICFconstructionUser is Offline
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23 Mar 2009 10:26 PM
Will spray urethane stick to the underside of sheet metal roofing? I think they apply a thin coat of oil to the sheet metal, that may complicate things. It would also prevent condensation, limit noise and lessen the likely hood of hail damage.
Brad Kvanbek - ICFconstruction.net
wesUser is Offline
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24 Mar 2009 06:12 AM
ICF,
My Icynene installer tells me he sprays on the underside of steel building roofs all the time, with no problems.
Wes Shelby<br>Design Systems Group<br>Murray KY<br>[email protected]
Bruce FreyUser is Offline
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24 Mar 2009 07:14 AM
Posted By ICFconstruction on 03/23/2009 10:26 PM
Will spray urethane stick to the underside of sheet metal roofing? I think they apply a thin coat of oil to the sheet metal, that may complicate things.
That is a valid concern.  I have had spray-on-fireproofing adhesion problems with corrugated metal deck.  Usually the galvanizing or phosphatizing problem solves that problem, but the coil material is usually lubricated to facilitate rolling.

Bruce
greentreeUser is Offline
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24 Mar 2009 07:18 AM
Why dont i ever hear of anybody spray foaming the ceiling and then spraying cellulose over it? It's always the roof deck.
ICFconstructionUser is Offline
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24 Mar 2009 07:40 AM
To spray a couple inches of foam on the ceiling to form a good vapor barrier, an then cheap blow in cellulose or fiberglass on it is a good value and done often here (Minnesota). A house I am bidding now is drawn that way.
Brad Kvanbek - ICFconstruction.net
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24 Mar 2009 08:07 AM
Posted By ICFconstruction on 03/24/2009 7:40 AM
To spray a couple inches of foam on the ceiling to form a good vapor barrier, an then cheap blow in cellulose or fiberglass on it is a good value and done often here (Minnesota). A house I am bidding now is drawn that way.

This sounds interesting.  The bid I got to due the whole roof underside (1600 sq ft house) 5 inches thick was $6700, ouch.  A thinner layer of spray foam topped by blown in cellulose would accomplish the same thing at a much lowert cost.
ICFconstructionUser is Offline
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24 Mar 2009 08:10 AM
The house I am referring to is not a budget house either, it is a contemporary on Lake Minnetonka. This method is a standard practise.
Brad Kvanbek - ICFconstruction.net
glenfotreUser is Offline
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24 Mar 2009 09:18 AM
I know how you spray foam on the underside of the roof, but how do you blow in cellulose on the underside of the roof??????  Or do you just have a semi-conditioned attic with the HVAC running in semi-conditioned space?  By the time that you then insulate the HVAC, you may just as well do the foam job right??
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24 Mar 2009 10:23 AM
spray a couple inches of foam on the ceiling (on top of ceiling and bottom cord of trusses, over light fixtures and seal around ceiling penatrations) to form a good vapor barrier, an then cheap blow in cellulose or fiberglass on it. So it is like a conventionally insulated attic but with spray foam too.
Brad Kvanbek - ICFconstruction.net
dmaceldUser is Offline
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28 Mar 2009 10:49 PM
Posted By bmancanfly on 03/19/2009 2:56 PM
I saw a product at the FL home show where spray foam is applied to the underside of the roof.  They claim to seal the attic space completely from the outside air.  There is then no need to insulate the ceiling (between the attic and living space).

Anyone heard of this?  Wouldn't this raise the amount of air space that my a/c has to cool?  They seemed very convinced that this was a huge improvement over the old way of doing things. 

Any feedback would be appreciated.
This is the way my house is insulated. The additional volume of air adds a negligible amount of load to the heating/cooling system. A pound of air that stays in the house is cheap to heat. It's the pounds of heated air that leave the house that cost. Sealing the attic air tight is one of the greatest benefits of spray foam insulation.

In my house I designed the heating/cooling system so that the attic is the return air duct. A significant side benefit, IMO, of using spray foam on the underside of the roof is it makes the attic a much more usable storage area. You don't tromp down cellulose or fiberglass to get around, it's easy to put OSB on the ceiling joists for an attic floor, it's a conditioned space, and it's cleaner. Oh, and if you need to work on a light fixture from above, or drop a new wire down a wall, you can do it without digging around in ceiling insulation!


Even a retired engineer can build a house successfully w/ GBT help!
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29 Mar 2009 07:40 AM
Does anyone know of a house insulated this way in a cold climate?
Brad Kvanbek - ICFconstruction.net
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29 Mar 2009 09:53 AM
Posted By ICFconstruction on 03/29/2009 7:40 AM
Does anyone know of a house insulated this way in a cold climate?

Mine. I'm in SW Idaho. Approximately 6500 heating degree days.

Even a retired engineer can build a house successfully w/ GBT help!
egouinUser is Offline
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31 Mar 2009 06:39 PM
I just completed my superinsulated house and this is precisely what we did. We used 3 pound closed cell spray foam and sealed eave to peak. We have not been through summer yet, but we had a pretty cold winter and the attic area stayed the same temperature as the rest of the house (done purposefully). Heating 3,750 square feet (and much more cubic feet) cost us approximately $200/month to heat with geo.
Regards,
Ed
http://www.GouinGreen.com<br>Superinsulated SIP/Modular House (HERS = 30)<br>GSHP w/SCW, ERV, Passive Solar, Solar HW
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