My century old Victorian house suffers from ice dams and I suffer from high heat bills. Talked to a spray foam contractor who said I only need about 2″ of foam on the inside of the roof, to form a complete seal, and I will have a greater reduction in heat loss than with much more fiberglass or cellulose. He implied that the “R” is less important than forming a complete seal.
I have tried to find a site on-line that would give a good, and honest, explanation of the physics/mechanics of this with no success yet.
Would love to hear from those experienced with this type product of who can guide me to a reputable site or provide their own insights on this type of insulation and installation technique.
Thanks, Marshall the NSpecter
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I se you have been lurking since January.
Welcome to Breaktime.
Try http://www.buildingscience.org and then use the search function on this site for previous discussions. You may get lucky and not get the copper pipe thread to start with, but if no keep trying:)
If you are going to be up in the attic of a century old home, you may want to seal all the pipe and wiring penetrations in addition to the insulation project. Old homes are notoriously leaky, and by sealing the highest leaks first you reduce the chimney effect, and thus reduce some heat loss.
Dave
We started with a 1872 2nd Empire Mansard... and sealed the entire roof deck with 6-8 inches of Corbond after replacing the asphalt roof with Grace ice&water shield below + Duraslate above. Where we had ice dam issues before in the whole mansard region, we only have ice formation now on copper sections of the roof where insolation causes freeze & thaw action.
I made the decision to go with Corbond and a unvented attic based on the research of building science corp. We are happy with the roof and none of the shingles show any signs of distress (like warping, etc.) I would avoid mixtures of insulation types like FG and corbond because the dewpoint could occur somewhere outside the Corbond.
Google "Walls tht Work" which ontains all the research Corbond has done.
The installer has it 60% right.
I'd go with 4"
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Nice site but I didn't see any real comparison between the various insulating products to help in a cost/benefit analysis.
Maybe they don't have averything on site that they put in the book.
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got more time now -
R-Value is a standard used to measure objectively the Resistance to heat loss in a given situiation.
The problem is that very few house heating situations are exactly like the conditions used in the testing set-ups. They are probably a better measure of resistance to heatlost via radiant and conductive transfer, and far less a measure of heat lost via convectyion, which is one of the factors that varies most from house to house.
Corbond has studies demonstrating that it is important to control infiltration in the whole house. That means air flowing out the top takes heat energy with it.
but wait - there's more!
To replace the air that flows out the top, more air has to come in at the bottom. That means sealing the perimeter joist or sill beam, as well as the rubble foundation - or the under floorboars if it has an open crawl space. When the installer knows his product, he will push to sell you the base as well as the top. That is when his statement has much more truth to it.
The other part ofd the storey if you are comparing one type of insulation to another is the convection currents withing parts of the house.
Think for a moment how poor an old window set-uyp is when you have your double hungs with single pane glass and a combination storm unit installed to the exterior.
There is an air space between the two. The air against the exterior storm cools and grows denser, so it sinks to the bottom of the space. The air near the interior is heated by radiation loss from the interior. It grows lighter and rises. A convectio0n loop is established where heat is carried up and out towards the storm panel, then chills and sinks to the bottom, radiating its heat energy out through the glass as it sinks. The cycle goes on and on, losing energy faster when the temperature differential is greater between in and out.
It is easy to understand this with open air space between glass. The fact is that with fibreglas batts, in sealed stud spaces, it still happens. Not as fast, but it still happens and it still losses a measurably greater amt of heat as the temperature differential between in and out increases. FG batts is the worst insulation you can usebecause of this convection current it allows. Foam will totally deny this. Cellulose dens packed will nearly do so. Chopped FG BIBBs will come almost as close.
so when insulating a house, you need to think in terms of denying heat loss through all three methods, radiant, conduction ( thermal bridging) and convection. Foam well installed is best at all three.
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Taunton University of Knowledge FHB Campus at Breaktime.
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