Hi all

I am planning to insulate my Victorian 2.5 story house's attic.  I've already torn down the plaster, and am gearing up to spray on the polyurethane.  The problem I'm having is that

a)  The rafters are only 4" deep (real 2x4s).
b)  I'd rather not furr them out any more than 2".  Any more will reduce the headroom unacceptably.
c)  I'd like to get at least R30.    The only real hope I have of reaching this (within the limits above) is 5" of SPF, or 6" of XPS with a vapour barrier.

I've researched the concerns and benefits of "hot roof" (aka un-ventilated) cathedral ceiling assemblies (where the foam is sprayed directly to the underside of the roof sheeting) on this site and others and I've found the following:

i)  A 2" or thicker layer of SPF is much better at eliminating condensation than simply ventilation
ii) SPF Hot Roofs might be more prone to ice dams than ventilated attics, but usually this is more than compensated by the improved insulation performance compared to fiberglass with an unreliable vapour barrier
iii) The Ontario Building Code (and I think the Residential Building Code in the US) now allows unventilated Hot Roofs.
iv) SPF Hot roofs might not dry as quickly or show symptoms after a failure of the shingles / roof membrane.  As a result, a leak may go undetected until the roof is severely damaged.

The first three arguments are reassuring, but the last one raises enough doubt that I'm wondering if I shouldn't try something new.  Here's my thought:  "if the only problem with SPF Hot Roofs is that you can't detect a leak, why can't we design a channel just for that purpose?"

To that end, here's my proposed roof assembly, from top down:

- Existing Shingles
- Existing Decking (with existing vents)
- 1/2" thick Corrugated Plastic Roof Sheets between rafters.  Possibly sprayed with water-soluble paint
- 3-3.5" of spray polyurethane
- 2" of wallboard over the top of the rafters
- drywall

The idea is that I don't need the ventilation channel to remove much heat or vapor, so it doesn't have to be very thick.  It's just there as a "backup roof", to carry any leaks down to the soffit where they will be detectable.  Furthermore, I could paint the exterior side of the currogated sheeting with water-soluble paint, so that any leak will stain the soffit noticeably.

The only disadvantage I can think of is this:  If, (in 15 years or so when the current roof wears out) I want to replace the shingles - I can't improve the insulation by adding above-deck foam (the ventilation channel would negate that benefit, unless I could seal it up).

Is this a great idea or a terrible one?  I have to decide within a week or so!

FYI, my research sources are here:
http://www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf
http://www.cmhc-schl.gc.ca/en/co/maho/gemare/gemare_001.cfm
http://www.obc.mah.gov.on.ca/Asset1199.aspx?method=1
http://forums.jlconline.com/forums/showthread.php?t=50365

If the corrugated sheeting is vapor impermeable you could still end up building up rot levels of moisture on the rafters, since it eliminates all drying of the rafter sides through the SPF into the ventilation channel. A better bet might be to use 1/4-1/2" XPS instead of corrugated roofing as the substrate for your SPF. (I assume cutting in the corrugated or XPS be a DIY project?) A few well placed spacers to maintain a 3/4" standoff from the roof deck would be sufficient, even if it ends up a bit wavy on that side after spraying. If roofing nails protrude or penetrate, it's still not a problem. For sure venting the roof deck keeps it a lot drier. Any "backup roof" that's fully ventilated doesn't need to be perfectly waterproof the way a main roof needs to be, since it's never fully saturated with water, and it's always in a drying-mode. SPF or XPS are both pretty water-resistant- waterproof by most definitions, and are not damaged by repeated wetting. Most ventilated nailbase iso panel systems have nothing but bare OSB on the inner-deck- if you want it to be a bit more rigid you might just use 1/4" OSB to form your vent channel, but thin XPS would give you a higher total R.

Sealing the vent channel and putting in above-the deck insulation later isn't a problem- easy to do. Sealing & insulating the soffits & vents with low-rise foam 4-6" into the vent gap would be enough to ensure you don't end up with measurable convective losses.

I was under the impression that closed cell SPF was impermeable after 2", so I didn't expect there to be any drying through the foam. I like your suggestion for using XPS for the ventilation channel - I was just looking for something easy to install (I could just staple the plastic roofing). The price I've got for SPF is about the same per board-foot as XPS, and I would much rather have someone else spray than have to cut a lot of rigid panels.

Thanks for the tip about sealing the vent channels with low-rise foam. That would work great. I think I can put that anxiety to rest.

At 2" ccSPF drops below 1 perm to semi-impermeable, but it's far from being a vapor barrier like vinyl or poly. ccSPF has about the same perm-rating per unit thickness as fir or hemlock timber- that stuff will dry, but it takes a lot more seasoning to dry an 8" log for cordwood that a 4" log. With 3-4" of SPF in there all of the rafter is within 2" of air that it can dry into within the seasonal time alloted. If you seal one one side with a vapor barrier, there's still a slight risk of moisture accumulation (although I think it's a pretty small risk in your case.)

Good point Dana. Thanks for the clarification. I'll reconsider making up some homemade xps baffles.

Creating a thermal break; ie: disconnecting the (cold) 2x4 rafter from the interior heated surface will make a big difference in heat loss and can be done easily here. Do your vent channel if you don't want a hot roof, then spray 2" of SPF. Add a piece of 1x3 strapping under and paralell to each rafter, separated from the rafter with a piece of 2x2 every 16". Use pieces of the 1/4" XPS to block off the bays from each other & blow in DPCellulose. This gives you a vented cold roof and a mostly termally broken R-26 roof. Another 1" will get you to R-30.
Bob