Hi,


I live in a 1964 4 level split here in Canada.

This house is about 1600 sf and has cathedral ceilings in the living, dining and kitchen.

The roof joists are 2 x 6, with R14 fiberglass insulation between the heated space and the roof deck.

I am replacing the roof shingles and believe that this is a good time to consider insulation of these ceilings from above.

I have heard that one can do this by tearing the roof sheathing off, removing the fiberglass and the exposed poly and spraying polyurethane foam in (2 lb, medium density).

It has been suggested by contractors anywhere from 3-4.5 inches for an R value of 20 - 30 depending on thickness.


Since my imagination tells me that there will be some vapor movement from the living space into the joists during the winter my question is:

should one be concerned about wood rot when sealing against the joists with closed cell foam?

Input from anyone with experience regarding this type of application of spray foam in a cold climate would be greatly appreciated!!

tia

You are on the right track with the idea of using spray foam. Spray foam has an R value of approximately R6 per inch depending on the brand. In Southern Ontario you need at least R28 in a cathedral ceiling. Which is just under five inches. We have sprayed many homes from the top down when the roof is off. A small air space can be created above the foam and below the roof deck, although it is not crucial with this type of Insulation. I am located Near Barrie, Ontario, If you are in Southern Ontario, I may be able to help you.

As far as your concern about moisture: the spray foam is only going to be sprayed on two sides of the lumber, the top and bottom faces of the 2x6 will still allow for drying if any moisture were to accumulate. Wood completely encapsulated with foam will rot as it has no drying ability. If one face is left un-insulated the wood will be fine. You will actually have far less moisture in the ceiling space with spray foam than with the fiberglass.

Good luck

Georgian Insulation Systems, 705-329-3972.

This approach could work, spraying the SPF on the gypsum board, but leaving a 1.5-2cm ventilation gap between the SPF and (new) roof deck and ensuring/preserving the soffit-to-ridge ventilation flow. Vapor migration through the foam occurs at about the same rate as through the rafter timber (which is not very much), and with a ventilation gap the (new) roof deck should remain dry as long as you give it a couple of cm of space and haven't blocked the vents. A 12cm layer of closed cell would then yield an R value of ~R30.

Alternatively using nailbase panelized iso or EPS insulation (eg: http://www.hpanels.com/2009/pages/pdfs/Lit_Prod_Color/H-Shield-NB.pdf) or XPS sheathing + OSB nailer deck through-screwed into to the rafters above the roof deck keeps the roof & rafters warmer & drier. Filling in the remaining interior rafter cavity could then be done with, dry blown cellulose, or high-density sprayed fiberglass by drilling hole in the roof deck (not ripping it up) compressing the existing batts. That would yield a center-cavity R of ~ R20-R22. 5cm of XPS above the roof would then yield ~R30 center cavity, but it would outperform the SPF since it more than doubles the R at the thermally bridging rafters. If you went with a 9-10cm nail-base iso panel, you'd hit R40 center-cavity, and R25 at the thermal bridges. The condensation potential at the structural roof deck of any leakage air that made it around the interior vapor barrier would be minimal, and seasonally buffered by the cellulose. Indeed, if you went R20 or higher on the exterior, with a ventilated nailbase iso (eg: http://www.hpanels.com/2009/pages/pdfs/Lit_Prod_Color/Cool-Vent.pdf ) the assembly would probably perform better from a humidity point of view if you removed or ripped the interior vapor barrier, but that would be between you and the code inspector. Nailbase iso (any thickness) or XPS at ~5cm is more vapor-permeable than 12cm of closed cell SPF, but having a ventilated nailer deck above it would be a good idea if the existing cavity is retroactively filled, and has an interior-side vapor barrier like poly or foil-faced batts.

There are several vendors of nailbase rigid foam insulation (JM, Atlas, Hunter, etc.), with similar performance characteristics. It's more often used in commercial construction, but is beginning to penetrate the retrofit residential markets for situations exactly like yours (and mine): Insufficient rafter depth to achieve current code for total R value.

Other options:

http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

For most of central & eastern Canada the stackups in the tables on P.11 that work for Int'l Falls MN are reasonable.

Combining a few inches of SPF with high-density sprayed fiberglass gives it substantially more inward drying capacity than a ever deeper interior SPF (at a lower cost per total R as well.)

A layup of shingles/sheathing/5cm ccSPF/9cm high-density spray fiberglass yields a total center-cavity R of R26.

A layup of shingles/sheathing/12cm ccSPF yields a total center cavity R of R30- a modest improvement.

But the whole-roof thermal performance of the SPF/spray fiberglass stackup can be improved to exceed the all-SPF approach by a significant margin with as little as 2.5cm of iso or 3cm of XPS on the exterior, since it doubles the R value at the thermal bridging at the rafters. The more insulation you put above the roof deck, the less significant the thermal bridging of the rafters becomes.

Spray foam during roof replacement is an excellent application and one that we have done many times. I actually have a photo of an installation in Winnipeg that was taken last year.

Moisture is not a significant issue because of the difference between foam and fiberglass. Because foam (and I am assuming you will use a 2lb medium density foam) is a recognized air barrier and vapour barrier, spraying it from top down actually creates a drainage plain whereby any moisture that were to ever get through your roof sheathing will evacuate to your eves on what amounts to a plastic channel (the foam). This is the greatest concern on the west coast if that is where you are.

Vapor drive from below is a lesser concern though as pointed out above, two exposed faces of your framing members will allow for drying.

Depending where you are at, perhaps we can assist you in addressing your specific concerns.

Give us a call at 1-888-880-8420. We serve Western Canada from Winnipeg to Vancouver.

ecologicfoam.com

Hi tia,

When we renovated our 2000sqft house in 2008, we popped the roof deck and sprayed open-cell foam directly over the existing batts to form an airtight foam shell over it.  You can see pictures of this phase of the project here:

open-cell spray foam install

Click on "original size" under the picture to see more detail.  To read more about our renovation project, you can read the following thread:

3-ton Hallowell installed in northeastern PA

Feel free to ask questions!

 - Chris

Posted By chrispitude on 23 Aug 2010 09:42 AM
Hi tia,

When we renovated our 2000sqft house in 2008, we popped the roof deck and sprayed open-cell foam directly over the existing batts to form an airtight foam shell over it.  You can see pictures of this phase of the project here:

open-cell spray foam install

Click on "original size" under the picture to see more detail.  To read more about our renovation project, you can read the following thread:

3-ton Hallowell installed in northeastern PA

Feel free to ask questions!

 - Chris

It looks like you did the spray-on-batt on the attic floor sections, not the cathedralized roof, and that you did an open-cell spray under the roof deck, which is quite different from what was being proposed above.

BTW: It looks like you sprayed ~1-2" closed cell in the wall cavities against the wallboard, leaving the rest of the cavity empty between the cc foam and the SIS sheathing, then used insulating siding over the SIS?  What do you figure you ended up with for clear-wall (or center-cavity) R-value? (Filling the rest with cheap blown fiber might have been cost-effective since it looks like you had ~2-3" of dead-air, which would add a low-cost R7-R12 to the stackup, blocking the convection loop & impeding infiltration in the cavity.)

+1

We have almost exactly the same issue. Central NJ 1964 house with 2x6 rafters and rotted deck and skylights. We'll be ripping off all the deck, esp. since the interior is 4" T&G.

How important is the thermal bridging issue? Is the heat flux due to the conduction path significant? What is the R-value of the rafters alone? Budget is tight just with foam, deck and shingles, and that's doing all the work ourselves.

Is there any reason to use closed vs open cell in a top-down approach as long as 1-2 inches of ventilation gap is maintained? Is an internal impermeable vapor barrier necessary? We'll have to use something, since there are gaps in the T&G that any foam would extrude through. The manufacturer of the foam our intended contractor uses, LaPolla, suggested 30# felt as an internal barrier for 4.5" of open cell. From what I've read, 30# felt has a high permeance, 5 perms. This seems an insufficient barrier to prevent moisture drive into the roof.

Any thoughts?

Thanks to all...

The R-value of the rafters is ~ R1 per inch. A 2x6 is 5.5", so the R-value for that 1.5" of rafter edge is between R5 and R6. If the rafters are 16" on center it's about 9% of the total area, but more than 1/4 of the heat loss (assuming R19 between the rafters,) and the better insulated the rafter bay is, the greater the fraction that thermal bridge represents.

With a ventilated roof deck the permeance of the interior is less consequential- making it AIR TIGHT on the interior is more important by far. It's unlikely you'd achieve air-tighteness with felts of any weight, and they all have variable permeance with humidity in the WRONG direction to be useful as an interior vapor retarder in a heating-dominated climate. The permeance of felt rises with humidity, and you're trying to keep the moisture from conditioned space air OUT of the rafter bay. With felt the more humid the interior air, the more readily the moisture gets passed on up into the rafter bay- not good! Standard 30lb felts start out at ~3 perms in bone-dry conditiones, but rises pretty rapidly to 25+ perms with higher humidity. There are special-rated 30lb asphalt felts that are lower-perm, but IIRC they too pass more water vapor the more humid it gets.

Closed cell foam sprayed directly against the exterior-side of the interior t & g ceiling is probably the best solution- it's air-tight, and low perm, high-R. In a 4.5" lift you'd get ~R27 out of it, and it would be well under 1-perm. By contrast 4.5" of open cell stuff gives you only about R15, and run more than 10 perms. If it's done well and is fully air-tight without creating obstructions to the ventilation gap it could work, but you're counting on the ventilation.

Alternatively, if you put down R10 of XPS sheathing (2 staggered layers of 1" XPS sheathing with sealed/taped seams, and a non-structural nailer deck long-screwed into the structural roof) or nailbase-iso panels atop the roof deck and dense-packed the 2x6 rafter bays fully with cellulose (with NO interior vapor retarder) you'd have a nominally ~R30 roof that would have very low risk of condensing temps the roof deck in a central NJ climate. It would dry toward the interior, and the intermittent periods during cold snaps where the roof deck was below the temp of the interior air any moisture that formed on the interior of the roof deck would be wicked away & buffered by the cellulose. What's more, those R5.5 thermal shorts would then be ~ R15, and the roof would perform more like a 2x12 rafter roof with ~R38-R40 cavity fill. Don't go lower than R10 on the exterior without doing a closer analysis on the climate conditions, but more is fine. To make the interface with 4" t & g air tight (further minimizing winter moisture from reaching the roof deck) stapling & taping housewrap to the interior fully covering the sides & exterior edges of the rafters as well (no cuts) would work, but it might be just as easy to do a 1" closed-cell flash-seal before insulating with cellulose. (If you're filling a cathedral ceiling with cellulose it's best to keep the interior side vapor permeance up so the assembly can still dry- a 2" lift of closed cell on the interior side would be too much. Air-tightness is the goal here, not water-vapor tightness.)

This is far more reliable than counting on a ventilation gap to protect you (especially when there's snow on the roof deck potentially blocking the ridge vents, and leaching it's own moisture inward), and it's likely to price out cheaper than a 4.5" closed cell solution, while beating it's thermal performance hands-down. A 4.5" open-cell solution wouldn't even meet code for R-value, and without a true interior vapor retarder of 1-perm or less would put the new roof deck at higher risk. (I s'pose you could lay down some poly.)