To Foam or Not to Foam
We are in zone 5, central Illinois and have just purchased a large Victorian home with an unfinished attic. The attic is huge: 1800 sq. feet of ceiling and 600 sq. feet of walls in three of the four dormers (windows on all four sides). The floor is original boards and I've detected some regular old fiberglass insulation poking through in various places. There are currently vents in the sophists (although not as many as would be expected in such a large space), vents in both of the larger walls as well and three roof vents on only one side of the pair of largest roof angles. So lots of vents, none doing much.
We've had three insulators come out and now have three separate opinions on what to do. 1st man wants to spray closed cell foam over the whole space and seal it all up (old 2x4 rafters will only allow us a max of 3" of foam). My question to him was what do you do if you have a leak in your roof - it is undetectable because nothing comes through the closed cell foarm. His answer was that you need to have regular roof inspections. Not a great answer since my roof is now nearly four stories up (house was moved to current location and put on 8ft. foundation). Roof was done in 2004 by the way and looks to be the easy to install and cheaper versions of shingles).
2nd man wanted to just stuff as much rolled insulation as he could get in there and not do any venting and cover up our existing venting. his opinion was that we didn't need any vapor barrier, that because of the angles of the overwhelming amount of gables that most don't even reach to the overhang and aren't vented anyway.
3rd man wants to use Icynene open cell foam, which he says would allow water to come through but the product would dry and retain its integrity. I liked his approach the best, but am now reading about mold and wondering about the material holding water.
We'd really like to convert this great space into something useable, but are now more confused than ever.
Any insights into dos and don'ts would be much appreciated.
Heidi (Urbana, Illinois)
Do you want to finish the space and make it conditioned?
If not, leave the insulation level at the floor and vent it more properly.
3" is closed cell is less than half of what you need.
Rolling out insulation and not addressing the venting is equally as foolish.
Icynene guy might be the best but I didn't hear anything about an intumescent layer on the foam? That will be required.
I would also air seal the floor regardless to prevent the bulk of the moisture from getting up there. If you foam the roof deck, you need to figure on conditioning the space at a minimum.
No ocSPF in Zone or above; http://www.buildingscience.com/docum...d-roof-systems
Use the proper thickness; http://resourcecenter.pnl.gov/cocoon...r/article/1520
Air seal the ceiling plane first; http://www.buildingscience.com/docum...sealing-guide/
I have read many of the guides at this point, some over 40 pages long. It's looking like in zone 5 we can go with 2" of closed cell foam in a sealed attic situation. we may then choose to increase the R value by adding foam boards to fill the rest of our rafters (they are narrow - only 3 1/2" max and i'd rather not fir out the rafters, although we may have to to get a better thickness for the boards).
The part I'm not clear on yet is how to ventilate the conditioned space. We will have a classic Unvented Cathedral Ceiling Attic. I've read we need a free vent area to insulated ceiling area of 1:300 ration. If we have 2000 sq. feet of ceiling than we need a pretty decent sized exhaust system (nearly 3ft?). I'm wondering what this exhaust system really needs to be? It seems counter intuitive to seal a space and then open it up, so I'm assuming there must be vent systems that vent only as needed and are mostly closed up when not in use, and there must be more times of the year when venting is crucial.
Any insights in venting would be great.
You do on vent conditioned spaces.
Venting is for the unconditioned and uninsulated roof plane.
If you are converting your attic to a conditioned space, it needs to be done properly and it will no longer require venting at that point.
I would not put ccSPF tight to the roof deck because it makes roof sheathing replacement in the future a more expensive undertaking. You need to find an insulator that understands the dynamics and comes up with a well thought out approach that will get you the results you need and that are code and energy compliant.
insulating cathedral attic in old house
That's so true, about replacing the roof in the future because if you have the foam sprayed right to the underside of the roof deck how can you ever pull off roofing materials? Perhaps the best method is to install the plastic vents up the gables and then use good old batting - of course that leads us to the fact that we only have 4" of rater depth, so in order to get anywhere near a decent R value (30 being out of the question) we'd need to extend the rafters with 2x2's at the minimum. This sounds like the safest and the least expensive option, but not very energy effective.
My daughter (who had been living in the attic of our previous bungalow) asked a great question: "How was the bungalow attic finished". Because she had been able to live up there with a window unit in summer and electric radiators in winter and the roof was old plaster and who knows what above that - all I know was the roof got very hot to the touch in the summer and that open crawl spaces revealed only bats in the floor and nothing up above...)
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