One of my rentals (2 storey gable house, 2 upstairs bedrooms) has had serious ice damming problems.
Improving the existing attic insulation seems like a logical solution, but I’m concerned about causing dew point problems, with resulting ceiling damage on the ground floor.
This is a partial attic, on either side of two upstairs bedrooms, each room with knee walls and a small ceiling. Each side has a small access panel for inspection purposes. There is no ventilation in these areas. The tiny triangular peak above the bedroom ceilings is very well ventilated at each gable end, and reasonably well insulated. This attic ceiling insulation at the pony wall/roof intersection prevents full airflow of the attic areas.
The ice damming is caused by ground floor heat/moisture escaping through numerous ceiling penetrations (pipes, wiring, flue, chimney). The ceiling has no vapour barrier. There is no ventilation to let the waste heat out for the reason mentioned above.
Getting a commercial foam contractor to spray several inches urethane over the current R12 batts and 4″ of zonolite would be least amount of time/work/disruption. The foam would block all penetrations, cutting out the main supply of heat/moisture rising up from ground floor living spaces. I like not having to remove the zonolite (toxic) plus the foam keeps it in place.
Would this approach cause a weird dew point in either the fiberglass batts, or in the zonolite? I am concerned about this moisture eventually soaking through the ceilings and stains being seen in the rooms below.
Would I be better off forgetting about the urethane foam contractor, opening up airflow where the attic ceiling insulation blocks the pony wall/roof intersection, in order to promote full airflow of the attic areas, and hand-detailing the attic penetrations?
Ideas? Suggestions? Stories?
Replies
Generally, the first step in dealing with ice damming is to assure adequate ventillation.
Of course, when ice damming is severe it usually indicates that insulation/air sealing is inadequate. But sometimes the problem is a relatively small number of significant ceiling penetrations. Dropped or coffered ceilings are an especially likely place to find problems.
Ice damming is caused ONLY by heat loss through the ceiling regardless of whether it is moist or not. If it was moist, you may likely have condensation on the underside of the roof sheathing as well. Lack of this may mean the moisture is readily escaping (i.e. you are venting ... but you also have heat loss).
I've never heard of foaming over e.g. batts, zonolite, etc.
I guess I would focus on two things maybe: 1) getting ventilation - you don't have eave vents? You can vent the gable ends of the attic spaces just like up at the peak. 2) Look for air leakage into the attic ... since you are already insulated, this likely means that you have some MAJOR leaks somewhere ... also may make finding them or working to seal them difficult w/ the zonolite.
If you have major leaks, if your foam thing isn't perfect, the weak spot will be found and you may not improve your situation much. I'm guessing the foam spraying would stir up the zonolite (or is it under the fiberglass?).
Theoretically and technically, you shouldn't create a problem w/ the foam in terms of the dew point. Usually moisture doesn't just condense anywhere in insulation. It looks for a surface ... e.g. sheathing.
Do you have lots of penetrations of this ceiling? e.g. recessed lighting? I've seen MAJOR ice daming occur in brand new houses because of the lack of details around light fixtures. Look for other sources of major leaks. Plumbing, mechanical/duct chases, fireplace flues, etc.
Food for thought.
I think you are asking the wrong question here.
Anytime you have enough of a quality insualtion, you eliminate a dew point.
Your problem seems to be more one of air flow and infiltration. Start by locating that multitude of leaks you mention and sealing them off.
I would be concerned about spraying poly foam OVER a FG and Zonolite insulation. The Poly will act as a VB on the wrong side of the batts, trapping moisture under it. That will render the batts ineffective. If I were to uinsulate with the spary foam, I would rake back first, and then spray
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There's a couple of details I need to correct ....
The moisture will condense where it cools ... if you seal atop the zonolite, you guarantee moisture will be trapped there. Hello, mildew. Your vapor barrier, generally, needs to be on the warm face of the insulation.
Zonolite is NOT toxic. It is simply expanded mica. It is NOT asbestos. Despite a fair amount of media / lawyer driven hysteria (because ONE mine had and asbestos deposit adjacent to the mica deposit), follow up testing of existing zonolite installations have come "clean." Just vacuum / shovel the stuff out if you want to spray foam.
As for your ice dam problem ... as others have mentioned, your solution lies in having a cold roof. Perhaps increasing attic ventilation, and applying the foam to the underside of the roof is your best bet.
PS: Your location might be excessively arid this time of year. Foam needs moisture to cure properly. You might even consider adding humidity during the application and curing times.
Edited 9/6/2008 2:14 pm ET by renosteinke
Thanks guys.
I'll ventilate the attic spaces behind the kneewalls, and stop airflow at the ceiling perforations.
Pierre
If you have a few inches of foam there is no way condensation is going to happen above or below it because it prevents the temperature differential where the air meets the foam.
Nothing will be trapped below it because anything below it, insulation or otherwise, is now within the building envelope and roughly the same temp as the interior of the house, or at least enough so that condensation isn't going to happen. The existing insulation wont mold because it won't have any more moisture than any interior wall.
Personally, I'd have the foam sprayed to the underside of the roof and eliminate all air flow into or out of the attic.
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