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Rigid foam between rafters - any tips?


NuclearNerd's Avatar
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09-28-10, 11:17 PM   #1  
Rigid foam between rafters - any tips?

Hi all

I'm planning to insulate the roof of my 2.5 storey Victorian house. There's currently no insulation whatsoever in the cathedral ceiling! Unfortunately the rafters are ridiculously small (real 2x4s, 16" O.C.) so there's not much room for insulation (and I don't want to lose what little headroom I've got up there).

I have already had the spray foam quoted. 800 square feet of 3" thick polyurethane (about R18) would cost me almost $5000! I figure I can get R20 using 4" of rigid xps foam for no more than $2500, without losing too much ceiling height. (The last inch would be across the bottom of the rafters.) My only question is how to ensure a good seal between the foam sheets and the rafters on the side. Would Great Stuff or PL300 work well? If so, how many inches of foam "bead" does a typical can cover so I can add it to my cost estimate?

 
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drooplug's Avatar
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09-29-10, 03:13 AM   #2  
You should look into the DIY spray foam kits. It may not cost too much more than your foam sheet idea and you would get that air tight coverage from the start.

 
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09-29-10, 08:04 AM   #3  
Are your rafters currently exposed, ie the plaster or drywall removed?

Are you trying to establish or maintain any form of ventilation. The three inches of foam isn't a fill so the other inch is either venting or a gap that needs to be filled. The air barrier and the insulation need to be in contact.

Their spray foam quote sounds high, $2.00 per board ft, should be able to get closer to $1.00.

The rigid foam board is usually about $0.50 a board ft.

DIY foam in my area works out to be about the same as having it installed. Exception is small jobs the big companies don't want to do.

Bud

 
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09-29-10, 10:43 AM   #4  
I'm planning to remove the plaster Saturday.

The plan was *if I use spray foam* not to leave any gap for ventilation (i.e. a "hot roof"). But *if I use XPS board* to leave a 1" space for ventilation.

The spray foam quote seemed high to me too, but it was the cheapest of three quotes. 800*4 board feet worth of the home kit polyurethane works out to $3500 (Tiger Foam Canada - DIY Polyurethane Spray Foam Kits), which is a little cheaper but doesn't include labour.

No advice on my proposed "hybrid" approach?

 
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09-29-10, 05:00 PM   #5  
Try Foam it Green DIY Spray Foam Insulation Kits

Contact them in an email or on the phone for a more accurate quote. They do some volume discounting.

 
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09-29-10, 07:34 PM   #6  
I have done this and used a lot of cans of spray foam. I'm sorry, but I can't even begin to estimate how many cans because I purchased as I went and didn't really track it. It also depends a lot on how tight you cut the foam. The rafters are never completely straight or parrallel and the foam doesn't give, so the gap is quite variable. You also get a fair bit of waste as it is nearly impossible to anticipate the amount of exapansion. A very rough guess is that the small can would do about 30 linear feet. I used the window and door stuff because it has a little more flex and I figure the rafters will move a bit. It also expands a little less. I wonder about using sheet foam for most of the thickness and then a thin layer of the doityourself spray to seal ? I used polyiso BTW. As suggested above, I also left a vent space against the roof sheathing (i.e. above the insulation so the insulation and drywall are in contact. Another thing to consider is using foil faced iso with the foil facing the ventilation space, so you also get radiant insulation in the summer.

 
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10-03-10, 07:30 AM   #7  
Polystyrene insulation

In the normal way five inches of polystyrene sheet should give you a good result. I am running* a test on a room since last autumn.

I put three inches of polystyrene sheet between the joists of the ceiling and two inches of polystyrene sheet below the joists.

The polystyrene was carefully measured and cut with a knife to a tight push in fit. It was pushed up from below.
The polystyrene fitted below the joists is a tightly butted fit .

That room went through the winter and all of this year with no heating, there are three outside walls and one internal wall into our rear hall.

The only heat getting into the room is through the open door into the hall, the hall is kept at 22C the year round.

That room has shown a lowest temperature of 20C when the outside air temperature was -8.5C and the lawn a horrible -28C (the lowest I have ever recorded)

I would suggest that you use a knife, go for a tight fit and enjoy the result.

Note: Over time spray foam loses 15% of its volume, this can lead to cold patches where the foam has shrunk back.

 
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10-03-10, 10:17 AM   #8  
On a cathedral ceiling the ventilation between the underside of the roof and the top of your insulation is very important.

By using spray foam insulation you have no control over how much ventilation space you get. Since the rafters are only 2x4s (which gives you 3.5 clear for insulation) a 3 Fiberglas insulation will not give you a good R value and so I would use Blue Styrofoam

 
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10-04-10, 12:30 PM   #9  
Roof ventilation is over rated. Don't take my word for it, take the CMHC's word. It's mostly there to prevent condensation from soaking porous fiberglass. The Building Code Commission has recently ruled that ventilation is not required (pdf) when an airtight layer of spray foam is used.

My rafters are "real" 2x4s (ie, they were made before lumber sizes were reduced to 3.5"). That said, I've decided to fur them out with (modern) 2x2s to get 5.5" of rafter bay space. The foam technique I was asking about is going to be too much of a hassle I think, so I've decided to fork over the extra dollars and pay for 5" of spray polyurethane (with no ventilation).

 
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10-04-10, 12:33 PM   #10  
Thanks for the encouraging reports Perry525 and Hironomous. I measured my rafters this weekend (after removing the plaster) and found that the bay sizes are really different. I think now that cutting the polystyrene as close as you did will be too much hassle. But I'm glad to hear some people have had success!

 
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