I guess Victorians had better blankets

September 29, 2010 1:27 PM

Insulating my attic cathedral ceiling, but I'm trying to be (a little) cheap about it.

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'm removing the plaster Saturday, so the clock is ticking.

I have already had the spray foam quoted. 800 square feet of 3" thick polyurethane (about R18) would cost me almost $5000! That quote seemed high to me, but it was the cheapest of three quotes. I checked out the DIY urethane foam kits too. 800*4 board feet worth of the home kit polyurethane works out to $3500 (http://www.tigerfoam.ca/), which is a little cheaper but doesn't include labour.

I figure I can get R20 using 4" of rigid polystyrene foam (xps) 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?

For added detail, I was planning, if I used spray foam, not to leave any gap for ventilation (i.e. a "hot roof"), but to leave a 1" space for ventilation if I use XPS board.

Anyone have any experience with this?

I've reposted the question from a DIY forum which was getting little response. My apologies if that's not kosher

posted by Popular Ethics to Home & Garden (14 answers total)

I think I have actually done what you are describing but I did not pay close enough attention to know the answer. I seem to recall the foam boards fit snug and were probably cut to fit between the rafters/joists. I don't recall any spray foam at all in the places we used rigid board insulation but lie I said, that was the part I didn't look at much.

Many of my former neighbours who redid their Victorians got spray foam because it's insulation + vapour barrier in one package and the results are very good. In most cases the extra cost was peanuts when factored into a larger renovation.
posted by GuyZero at 1:38 PM on September 29, 2010

If you're going to DIY with the foam boards, you know you need a vapour barrier. At what point does all the foarm board cutting and fitting and gluing and taping add up to the $1500 difference between hiring out the spray and doing it yourself? I guess only you can tell for sure the value of hassle and your time. I can tell you this, though: our REEP guy told us the best dollar we could spend insulating our 80 year old house was the roof, and we didn't cheap out. Now the attic is the most comfortable part in the house (which is why we sleep there).

Our rafters are 2x8 and we put in an air gap, 8" of batts, then 1" of foam board crosswise, then a vapour barrier, then 1" barn board paneling. We evaluated spray foam but knew with our thick rafters and headroom we could get away with losing almost a foot of space to insulation.

I suggest paying for the foam. Just get it done and move on to the next project.
posted by seanmpuckett at 1:51 PM on September 29, 2010

Oh, right -- the original rafters were 2x6, I remember sistering 2x4s to them with a 1/2" spacer between that we foamed, using 3/8" plywood straps on either side to get the 10" space needed for the batts. (Power nailer FTW.) It was a few years ago....
posted by seanmpuckett at 1:54 PM on September 29, 2010

Yeah, I'd look at studding out the existing rafters and using fiberglass batts or cellulose if you're really concerned about cost.
posted by electroboy at 2:38 PM on September 29, 2010

if the last layer of foam is going over the rafters you could just tape it; tyvek tape sticks fairly well to at least the pink polystyrene. if the pieces that fit between the rafters are fairly tight i wouldn't worry about trying to seal them, air won't convect through the cracks if they are small. trying to use sprayfoam in a can to seal the cracks will generate a huge mess and not work very well...

but, honestly, i think DIY spray foam might be worth the extra $1000. do you know how evenly spaced the rafters are in your roof? has anything twisted in 100 years? if so, cutting the foam sheets to fit could be a huge pain. It will be a lot less work to do the spray foam yourself and will perfectly seal your roof.
posted by ennui.bz at 3:11 PM on September 29, 2010 [1 favorite]

Some suggestions here.
posted by Neiltupper at 9:07 PM on September 29, 2010

Best answer: Spray foam in a can is going to be an unpleasant way of sealing your foam board. If you apply it after the first and after the second layer of foam board (you are going to have three, one 2" and two 1", unless I misread your question), you'll find it will make it harder to fit the next layer of foam board (and you'll likely find yourself futzing around with a razor blade, trimming bits of foam that expanded somewhere you didn't want them to). I wouldn't bother with that. If you really want to do foam board, just cut the first two layers to fit as best you can and then tape the seams on the third layer (and seal around the edges of the room). However, cutting foam board to fit like this will be a huge pain. Unless your time isn't worth much to you, $2500 is not enough to cover your labour, especially working over your head like this. The spray foam option will also do the best job of air sealing and leave no space for convection cells.

That said, a hot roof is not a good idea. Once you get ice dams, you'll know why. You might be fine, but you are taking a significant risk of leakage in the future. I'm sure your spray foam contractor has good ideas about ways to avoid a hot roof. The contractor should be able to spray to a -0/+1/4" tolerance (with some trimming), so you have 3.75" of space to work with.

Finally, don't be afraid to fur down the rafters 1.5" or more. It doesn't take much time, costs very little, and you can do it yourself (a nailgun is a very good idea though), and, of course, it will allow you to fit a more reasonable amount of insulation. Think of the money you will save having reasonable insulation and compare it to the benefit of 1.5" or more of headroom.
posted by ssg at 10:20 PM on September 29, 2010

Response by poster: Thanks all. I think I've been convinced to go the spray foam route and suck up the dollars. The extra money will probably save me a lot of stress costs.

Re hot roof and ice dams: My roof currently has zero insulation, so anything I add will reduce the amount of heat transfer and lessen the problem. Plus the Building Code Commission has recently ruled that ventilation is not required [pdf] when an airtight layer of spray foam is used.

I spoke with the foam installer again today and he raised an interesting possibility: We might be able to fill the cathedral ceiling space with slow-rise foam. This would cost more (it would require 4" thick foam), but I would save a few grand in demolition and reconstruction costs by not having to take down the ceiling. Or it could pop off the plaster, in which case it does my demolition for me :) Whichever method we choose, I'll report back whether it's successful.
posted by Popular Ethics at 12:16 PM on September 30, 2010

My roof currently has zero insulation, so anything I add will reduce the amount of heat transfer and lessen the problem.

Be cautious here because this is not a linear problem. If you have roof at a high temperature, snow will melt off it and you won't have any trouble. If you have a roof that stays below 0C, snow won't melt and you won't have any trouble. It is in the middle, when your roof is a little above 0C, that you can have problems. You get water underneath the snow, which runs down the roof underneath that snow until it reaches the edge of the eave, where the temperature is suddenly much lower, freezes and potentially creates a situation where the water behind it is under pressure. This pressure can force water through your roofing material and then it can be wicked into parts of your house you probably prefer to keep dry. So, while your spray foam will prevent water from dripping right into the attic, there is still the risk of moisture in the rafters, sheathing, etc. There are a lot of variables to consider (your eave configuration, the type and condition of your roofing material and underlayment, the current performance of your roof when it snows, typical snow loads in your area, and so on). Even if the Building Code Commission says it is OK, I think you still are running a risk, so it is worthwhile to get all the info to assess that risk.

The CMHC has some good information about this issue. Start here.

If you haven't guessed by now, this is an issue I've dealt with first hand. A wet roof sucks!
posted by ssg at 12:43 PM on September 30, 2010 [1 favorite]

Response by poster: ssg: Be cautious here because this is not a linear problem.

Good point. In fact *mucks around with spreadsheet*, I can always construct a combination of snow depth and outside temperature which results in a > 0C temperature at the roof/snow interface, no matter how deep the insulation in the ceiling is. (temperatures closer to zero with deep snow is the worst case). Alarm!.

The online literature is pretty evenly split as to whether ventilation keeps the roof temperature low enough to prevent melting. Even the CMHC document you linked suggests "Attic ventilation is overrated" and "A well-sealed roof will not need ventilation". The convection would have to be strong enough to remove most of the heat leaking through the insulation before it got to the shingles. I'm not sure that's possible when the roof vents are covered in snow.

This seems like a pretty common problem. I'm surprised the industry hasn't solved it by now.
posted by Popular Ethics at 2:05 PM on September 30, 2010

I think I agree with you. It is just a matter of your specific conditions and how far you are willing to go to minimize your risk. It is a tricky cost/benefit analysis, because it is very difficult to quantify the benefit. Realistically though, ice dams won't form unless it is fairly cold out, so you don't need to worry to much about deep snow / temperatures near 0C conditions. To be honest, I think I'm overstating the risk of ice dams for someone in your location (I always forget that the rest of Canada doesn't get quite as much snow as we do). Sorry.

And yeah, well-sealed roofs are a really good idea (waterproof membrane is pretty nice stuff). And you are probably right that ventilation is not going to help much if you only have vents that snowfall covers (I was imagining a typical soffit & ridge or soffit & turbine venting setup, which was probably a bad assumption for the age of your house).

Hope it works out well for you.
posted by ssg at 6:00 PM on September 30, 2010

A "past the point of decision" moment here, but I just got some quotes from home depot.

4' x 8' x 3" XPS was $47.25 a sheet. (R-Value 13.5)

4' x 8' x 3" PolyIso (foil faced) was $20.35 a sheet. (R-Value 15.6)

That's a huge difference.
posted by BleachBypass at 3:56 PM on October 1, 2010

Response by poster: I couldn't find a Canadian supplier of polyisocyanurate . Where did you see that BB?
posted by Popular Ethics at 1:18 PM on October 4, 2010

Response by poster: OK, here's the post-mortem:

I took ssg's (and others') warning seriously and decided I would create a proper ventilation channel before spraying foam. I went with the cheapest channels I could find - the thin styrofoam ones with the 1.5" deep cones for separation. At the soffit and the roof, I nailed up some furring strips and painstakingly (emphasis on the pain) cut 1/2" rigid foam to block off air cavities. The whole exercise took me three weeks worth of evenings and probably $300 worth of material.

The spray foam guys came today. They were livid. Turns out the stupid little grooves and cones in that commercial ventilation channel make it really hard to eliminate voids in the spray. They finished the job, and it looks pretty good, considering. But it took them much longer than they had estimated. So here's a warning to anyone following my path: Use the (more expensive) sturdy plastic ventilation channels if you're going to spray foam.

For the foam producers / retailers: why don't you get together and come up with a ventilation system that is ideally suited to spray foam? I'm thinking little plastic spacers followed by some stiffish plastic that comes on a roll. This shouldn't have been so hard!
posted by Popular Ethics at 4:47 PM on November 10, 2010