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brent lay
Posted: Dec 21, 2013 09:59 PM
roof decking question
I have a couple of questions for the experts:
1- I have a 25 yr old home that is insulated well in the ceiling area, I am considering using closed cell on the roof decking in attic!
QUESTION 1- will the foam possibly cause odor issue since I have both fiberglass bats and cellouse spray in my attic now? ( and the foam will be sprayed on roof decking above the other which lays between ceiling joists)
questions 2- if I spray the roof decking, should I leave the gable end vents open so some air does come in to the attic area for some minor ventilation.?
I am concerned about having odor problems after all I have read.
I have read that the ?? carbons in the foam can react with the current installed insulation and create a smell.
I like the idea of having foam under the roof decking to help prevent the heat from getting into my attic , yes it gets hot there in summer. and I understand it will increase the life of my shingles.
John Shockney
Posted: Dec 22, 2013 09:41 PM
Well the only time that I have sprayed foam to the underside of the roof and left the venting for the attic, was a condensation problem with a metal roof. Normally to get any advantage from spraying foam to the underside of the roof you must create a closed or sealed attic.

Properly sprayed foam should have no odor; most of the odor problems (in my opinion) come from not spraying enough foam to the roof deck combined with high temps caused by dark colored roofing.

I would recommend removing most of the present insulation and spraying 8-10 inches of open cell or 5-6 inches of closed cell foam, if you want to install foam. The only real advantage to spray foam to the roof deck is if you have HVAC equipment or ducting in the attic and you probably won’t gain any savings by spraying the roof over your present insulation.

brent lay
Posted: Dec 24, 2013 08:57 PM
Thanks for the opinion air pro,
so, you don't think I would have any gain by spraying the underside of my roof decking, since I have good insulation in my attic?

and, if I did decide to spray the underside of decking with closed cell, you would not be concerned that the existing insulation would react with the chemicals in the foam and cause some odor issue?

I have dark shingles , and my attic does get hot in summer, I installed an temp controlled attic fan few years back to help with that. it did a lot to help.
Tom Eletto
Posted: Jan 07, 2014 01:11 PM
When you say the attic floor is well insulated, what exactly is there and has great attention been paid to all penetrations into that space from below? That's a major source of heat loss due to the stack effect. Is there any duct work in the attic area as well?

I agree with AirPro's suggestions except that you could leave the old insulation there if you wanted, it will be vestigial for the most part but shouldn't cause any issues going forward. You won't need that attic fan any more either and all exterior venting to the attic will need to be sealed.
brent lay
Posted: Jan 08, 2014 10:02 AM
the house is 1.5 story with some duct work to service the 2 upstairs bedrooms ( split unit)
the duct work is rigid, and has insulation inside

the ceiling above this area has batting insulation and also blown in on top for an RE factor of 50's if I remember correctly. I have plenty of insulation, but the attic area does get hot as hades in the summer ( reason for fan) and I know its not good for house and/or shingle life. My thoughts were to put a couple of inches of closed cell on the decking under the shingles to prevent the thermal transfer of heat into the attic, which should extend shingle life and have some small effect on the R factor of my house. ( i am not concerned about shingle warranty, if you read it, its not worth much)
The reason for my concern about foam, was after reading all the posts, I was concerned about a possible reaction of chemicals with my existing insulation, creating the "fish" smell. Which I do not what to take the chance on.
Mark Mouton
Posted: Jan 10, 2014 09:09 PM
Your house would probably do very well making it an unvented attic. A story and a half house responds the best. When we do one, we pull the fiberglas batts out of the sloped ceilings, then install open cell@5-6" from the ridge down to the top plate, sealing it off from the top plate to the decking. Insulate the gables and bring the attics into conditioned space.
Take your fan out, patch the hole, and seal up any gable vents first.
It will take the heat load off of your ceilings, upstairs attic walls and ductwork.
We've never had a problem with an odor. I do know the fish smell you're talking about. I've smelled it in fiberglas batts that were new.
The only thing you have to watch for, is if you have gas heat, and the furnace is getting it's combustion air from the attic. I wouldn't recomend doing this job if that were the case.
Another thing, the spray man needs to be good when you backfill sloped ceilngs so as to not bust sheetrock. We install a cannon tip, then lower the reactor pressure where the foam almost comes out like a water hose.
(Note: I hope you can decipher this. My thoughts run kind of erratic.)
Posted: Jan 11, 2014 07:03 PM
The issue with leaving the existing insulation is not that the sprayfoam has a chemical reaction with it. It is more a case of the existing insulation acting like an absorbing material that absorbs the odors that come from the spray foam reacting and release them at a later date. In addition to that, those materials have odors all of their own that could now be trapped in the building envelope. The odors from those materials are not so noticeable when the attic is vented and fresh air is coming in and out, but when you seal that air, that odor has nowhere to go. Aside from that, rodents love fibrous insulation materials and if rodents have been in there, their odors are now trapped in the building envelope as well. Industry best practice are to remove the existing insulation if you are going to spray the underside of the roof and gable ends.

brent lay
Posted: Jan 11, 2014 08:40 PM
Good answers but still lots of variables. taking the existing insulation out would not be cost effective, plus I have a great R factor to begin with. I was looking just to put some on the roof decking to keep the heat out of the attic in the summer. I still believe that's possible, but I would probably want to cover up my existing insulation with plastic until foam is cured and aired our. And just in case I think I would still want to leave my eve vents open for some light ventilation.
in reality, I don't think its even practical for me to even to consider this, but it was a good thought. !!.

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