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Foam in unvented attic

Last Post 25 Sep 2012 11:20 PM by joeboy. 26 Replies.

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tomtoth

New Member

Posts:47

11 Jan 2012 05:50 PM
This sounds like what I will do. The rigid foam, what kind? Should I do 2 layers of Dow SuperTuff which says it can be used as a membrane? Any sugestions as to what kind of vapor-permeable barrier I should be look at? Thanks again, Tom

Dana1

Senior Member

Posts:6991

11 Jan 2012 06:32 PM
There's nothing wrong with using 15# felt between the Super Tuff-R in that layup, which is semi-permeable. The foil facer on Super-TuffR is a true vapor barrier, but it's all on the outside of the moisture-susceptible wood. You will have to set up the interior to be semi-permeable so that the roof deck could dry toward the interior, which means open-cell only if you use spray foam between the rafters, and no more than ~6" thick, and no interior poly/other vapor retarder.

tomtoth

New Member

Posts:47

11 Jan 2012 07:14 PM
Thanks Dana1, So if Super-Tuff r is layered, should foil face down on both layers(one to roof deck, the other on top)? If 15#felt is sandwiched between layers, then what about on top layer under the metal roof? I assume all joints will be tapped together. so 6" max of open cell spray foam under deck, but don't I still need a fire retard or something on that? Would that be considered a vapor retarder? Much appreciated

joeb

New Member

Posts:14

22 Sep 2012 12:59 PM
New member here. Still learning he ropes of this site. Sorry if screwing up the current path of discussion here. I've been reading a ton of previous posts and thought I would reach out for advice. I'm looking forward to any insight you can provide. I live in NW Pennsylvania. We get dumped on with snow in the winter, frozen stiff in January and February and then a month or so of real humid weather in summer. I designed my own house and went for it. Its a bit Frank Loyd Wright meets Asia. It has 6-12 pitch roofs and a couple flat roof/deck sections. Basement- It has a daylighted basement with 8" block that i added extra rebar to and filled with cement. I sprayed 2lb. foam on the above grade exterior of the basement and below grade sprayed rubber sealer and covered with 2" foam board. My goal of insulating the exterior of the basement rather than the interior was to have the floor and solid cement basement walls act as extra thermal mass for the structure. I plan on covering the exposed 2lb foam outside with a flexable parge of some kind that would stick to the foam or maybe tapcon wire mesh on and then parge as normal. ? The goal was to have the exposed basement look like rock when I was all done. As you'll be able to tell I had some theories and finally just went for it. :-) Anyways, the upper portions of the home steps up in height by floor levels. Upper levels- My tallest/3rd floor tower room is 14x14 with a walk around treated cantelevered deck. It gives great veiws and a tower affect for natural ventilation. I filled all my 2x6 walls with 1/2 lb. foam then the sheeting, tyvec, 1" foam board as a thermal break and extra r-value and then fiber cement board siding with asics trim to detail it out. My roof sheeting is treated 3/4 plywood with high temp ice and water over all of it and then architectual shingles. My 4 foot overhangs are all treated materials. I used no roof venting and sprayed 1/2 foam to the underside of the roof decks. Where i have flat roofs/outside decks there is little space of course to the interior drywall ceiling below. I used 2 x 12's. I'm planniing on putting 1" foam board on top the flat sealed rubber roof and then my actual decking materials. The pitched roof portions of the house have limited closed attic space being 6-12 pitches and the 1/2 lb. foam is probably at least 8" or more thick and sprayed over the truss itself as well as thermal break. Now, i did not spray a vapor sealer on any of the 1/2 lb foam. It sounds by reading the posts on the building site that that is needed. I have no way of getting to the flat roof sections other than pulling the drywall ceiling down and then spraying it. Not going to do that. My hope was the 1" foam board on top the rubber flat roof would act as the moisture barrier plus it would act as skid sleeves so my decking material does not cut into my rubber roof. My pitched roofs are a different story as i cant put foam board on the exterior. It might be possible to cut access holes and spray up in to it to seal it. Do i really need to do this or will it be fine the way I have it? It would be a ton of headache and mess I'm sure. I have noticed on some winter days that I can see the rafter outlines. I'm wondering if that is just because of the temperature sweeps of the day when there is just a skim cote of snow sitting on the roof. I thought maybe I should also cut an access hole in the pitched areas and blow in cellulose to increase R-value as well. Is this needed and will it create extra moisture issues up there? So, my worries are roof rot or moisture trapped because I have high temp Ice/water on the outside and 1/2lb only on the inside. Even though the roof sheeting is 3/4 treated i still dont want any issues with it of course. And should , could I blow in more cellulose for more R-value. I have radiant floor heat on all levels of the home as well. I am installing an ERV whole home system in a month or so. I put 2 pvc chases in my walls, before I foamed, on opposite sides of my home and they go from basement to cieling of the highest floors. I will tap into those torun the ERV. I planned on using them for future solar/wind power cable chases as well if needed. Dana, I really appreciate any advice you can give me on this crazy house I've built. Thanks!, Joeboy....

joeb

New Member

Posts:14

25 Sep 2012 12:50 PM
Update on my research thus far. After everything I've read so far I think my plan of attack is to open up an access hole in the ceiling areas where I have pitched roofs. First off i hope it looks o.k. !!!!!!!!!!!!!!!!!! Ei., no moist foam, mold, etc. Next, i will can spray foam around any fart fans, can lights, etc that would be letting indoor humidity up into the space. After that i would like to blow cellulose into the space to further increase my R-value. My open cell foam has not been sealed to the heated space and is at least 10" thick so i think the roof deck saturation because of the high temp ice and water shield over the entire roof deck is not a problem because the deck is unable to get cold enough. Also if there was some moisture transfer it could dry inward and not be a problem because the open cell is not sealed to the inside and the small attic space is an air tight space (drywall ceiling below). I will then seal up the access holes I created. In addition, the ERV I'm installing will correct the pressure in the home to further help.

Dana1

Senior Member

Posts:6991

25 Sep 2012 01:39 PM
joeboy: First, start a new thread rather than hijack this thread. Second, 10" of open cell foam is still well over 5 perms probably over 10. This will result in wintertime moisture accumulation in the roof deck via vapor diffusion from the interior unless you keep the house & attic's humidity at extremely low (even unhealthy) dew point levels. A layer with a vapor retardency of ~0.5-1.0 perms on the inside of the open cell foam would prevent this moisture cycling and prevent your roof from rotting over time. Vapor retardent paint has be demonstrated to NOT work when applied directly to open cell foam, but you may be able to apply 1-2" of closed cell foam on it to achieve that vapor retardency. In NW PA the primary moisture risk is from interior moisture reaching the roof deck, not exterior moisture drives (particularly when there's a vapor-barrier underlayment on the roof preventing all drying toward the exterior.) As currently installed the o.c. foam is a code violation. I won't respond more on the subject under this thread title.