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Using Solid Foam insulation in rafter areas?


Homerluvsme's Avatar
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11-30-02, 09:30 AM   #1  
Using Solid Foam insulation in rafter areas?

I will soon be finishing off the attic in my older (built circa 1948) home. I only have 2x6 rough cut rafters up there now. I want to make this space into a master bedroom and master bath so usable head space is at a premimum.

I do have a ridge vent and undereve vents already installed, but there is no insulation up there. Originally I was thinking that I needed some of the foam W shaped rafter vents and then R-13. This would pretty much fill up the stud cavity.

So my question is this, can I use solid foam insulation instead?
http://www.dow.com/styrofoam/na/iso/value.htm Can I use this instead of regular loose fill fiberglass insulation? If so what do I need to know, or be aware of? I really like the idea of having an r-30 ceiling instead of an r-13 ceiling.

Your help is greatly appricated.

 
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12-01-02, 06:58 PM   #2  
toddmn68
I'm in a similar boat. However, I furred out my 2x4s with 2x2s, added baffles, and went with R19 FG (~6.5"). Yes there is some compression but the local inspectors were ok with that.

I never saw the dow stuff-- those R values look pretty sweet, but I don't know where I would buy it. Local HD & Menards don't carry. Plus it is probably REALLY expensive, when you consider the 2" pink foam (Owens corning) is about $16 for a 2" R10 4x8 sheet of theirs -- and with no foil or other barriers.

And to be honest, one of the reasons I went with FG instead of the Owens Corning pink is because I have 16" OC rafters and the damn pink sheets are scored at 16". No thanks on cutting all that up.

Sure there are better ways to do this, but my house is old like yours and will never be a thermos, given the old windows, 2x4 const, and 50 yr old loose fill settled in the walls. The FG is cheap, and with the proper venting, I know my config will be much better than most of my neighbors.

Hope this helps...

 
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12-01-02, 07:49 PM   #3  
I feel your pain. I haven't priced it yet, but has been suggested that is use 2" sheets of fiberglass board with foil on both sides and then tape all the seams. i like the idea. but haven't priced anything yet.

A problem I have is that my joists have a good bit of variance in the spacing. The cavities vary from 13-18 inches on center. I know this sounds odd but trust me so I know that I will have to be doing some cutting anyways. How much compression did you have in your width?

Also did you use any rafter vents running from the soffit to the ridge vent? How did that effect the compression if you used them?

 
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12-01-02, 08:08 PM   #4  
toddmn68
Sorry about your spacing variance.

I had no width compression -- my rafters are 16" OC and good. I used the foam baffles/vents which stick out about 1.5". The compression is really not too noticable once you staple in the craft edges-- it looks like anything else. The compression I have (depth) is not very significant or "violent" which I also realized when observing how the batts are compressed to about 1/8th their thickness when packaged.

Again I know I'm not really getting R19, but it seems to work ok in the 4.5" of depth I have available.

 
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12-01-02, 09:21 PM   #5  
Did you bother to make an air tight seal so the air from the soffit could only go up the vent? If not have you had and back pressure or moisture problems? I have heard of others having such issues.

 
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12-03-02, 10:23 AM   #6  
toddmn68
Yeah I did seal down at the edge. I have loose fill so I had to find a way to keep that stuff from falling back into the soffits over tiem. Click on the search button by this post and you should be able to find the thread.

I'm not too worried about moisture-- I think my methods are pretty sound (famous last words). Plus, on the ceiling below is 50yrs of paint, the paper facing of the original FG is caked to the sheetrock of the ceiling, and the blown cellulose on the top 8" or so always was dry in my dealing with it.

Haven't got far enough to feel any back pressure -- it may happen, but like I said, the house is old-- it'll never be a thermos.

 
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12-04-02, 11:15 AM   #7  
wingnut2
I'd go with the 2" foil lined polystyrene sheets.
$$$$ ....but worth it.
I wouldn't bother with the ventilation baffles. Buy an extra 1" thick polystyrene sheet, and cut it into 1" wide strips. Attach two strips in each bay where your roof sheathing meets each rafter. Once your first piece of polystyrene goes in place, these strips will hold it out away from the sheathing, thus creating space for ventilation. If you are really worried about the moisture problems, adhere everything together with an adhesive/sealant as you go.

 
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01-03-03, 05:54 PM   #8  
I have the same issue. Irregular spaces between 2x6 beams in a finished attic.

I propose to use R-21 insulation that is designed for 2x6 walls. Then place an inch or two of foam under neath the rafters to get an overall R-31.

My questions are:
1) Can the sheetrock be screwed to the 2x6s with long screws throught the foam?
2) Should I install faced or unfaced R21 insulation. I think it should be unfaced and let the foam act a a vapor barrier.
3) My house is a low sloop roof town house that does not have soffits and ridge vents. The bottom half of the roof covers an unfinished attic that has insulation on the floor. The area is vented by one simple vent on the roof. The top half of the roof is above the finished attic. Vents are to prevent ice damming, which is not an issue on a low slope roof, correct? The roof is covered with a modified bitamnan rubber material, not shingles.

 
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01-29-03, 11:11 AM   #9  
toddmn68
Update

I ended up using R21 faced (5.5" thick) instead of the R19 (6.5"). I kept the baffles because its so tight that they are necessary to hold up to the pressure of the slightly compressed FG. This I had to order from HD but it was only lieke $26/bag.

I guess you could use the sheathing as a vapor barrier, but I'm not sure what the rules are. I'm pretty OCD so I like (maybe just the illusion of) the security of the plastic sheeting, overlapped on all corners and taped in cuts and holes.

I used the faced FC pretty much to just hold the rafter insulation in place not because the paper is a real vapor barrier.

Hope your project is going well also. I hope to turn on the heat up there soon! Brrr!!

 
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