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Horizontal roof strapping for ventilation?

Hello,
I am building a 16'x24' house in Alaska, 12/12 gable roof using 2X12 rafters, with a cathedral ceiling. A friend has recommended venting the roof by strapping the rafters horizontally with 2x4's at 16" o.c., thereby allowing cross ventilation from gable to gable. The roof would be sheathed on top of the 2x4's. I haven't been able to find any information on this technique on the web.

This approach is meant to improve on soffit-to-ridge venting using Cor-A-Vent-type ridge venting and AccuVent baffles. My concerns with that technique are that the insulation baffles create a vapor retarder on top of the roof insulation and the ridge vent won't be effective when covered with snow.
Moisture issues are a huge concern here in the coastal rainforest, where winters are a constant mix of rain or snow and the cool summers don't provide good drying potential. Also, using wood heat means that the interior temperature fluctuates considerably.

Fiberglass batts are the only insulation option in this remote area. If I strap the roof for cross ventilationI will have to accept the losses created through wind-washing. However, I would be able to fill the rafters completely, allowing R-40 insulation because I won't need a 1" ventilation channel.

Any thoughts on the wisdom of building the roof this way?
thank you,
JS

Asked by Justin Smith
Posted Jul 15, 2013 10:07 AM ET

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6 Answers

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1.

Justin,
If you are insulating your cathedral ceiling with fiberglass batts, you need an air barrier on top of your fiberglass batts. Plywood with taped seams would be best; but if you are planning to install vent channels on top of the plywood, you probably don't want to pay for two layers of plywood.

You could use housewrap with taped seams for the air barrier on top of the fiberglass, but that can be tricky to install on an unsheathed roof.

Another approach is to install plywood above your fiberglass batts, followed by strapping installed parallel to the ridge as you propose. If you install metal roofing on the strapping, you won't need a second layer of plywood.

Answered by Martin Holladay
Posted Jul 15, 2013 10:41 AM ET

2.

If insulating with batts is the only option, it's worth the extra cost of high density "cathedral ceiling" batts, due to the much higher air-redardency. Better yet would be rock wool batts, if that's an option, since they have comparable air-retardency to high-density fiberglass, but without the airborne fiber issues, and have much better fire-resistance than fiberglass (which can melt at timber-fire temps.)

Answered by Dana Dorsett
Posted Jul 15, 2013 2:11 PM ET

3.

As Martin states, you want an airbarrier on top of the insulation. I would argue you would want this to be waterproof too. So not housewrap (leaks standing water) but a good roof underlayment (like the vapor open SOLITEX MENTO 1000 membrane I import and supply to AK. This can stand a water column of 32', and is very airtight.

Installing this will applying vertical battens (for venting) followed by plywood works well as you are building you own ladder while you work your way up with the 60" wide membrane (remember to only to staple in overlaps or under battens for extra waterproofing of the subroof.

Sealing the insulation on the inside (or actually all six sides) will make the most out of your batt insulation. Hence the interior airtight layer in the mock up photos (INTELLO taped with TESCON Vana). All available at www.foursevenfive.com

New American roof 1 475 sm.jpg New American roof 2 -475 sm.jpg
Answered by floris keverling buisman
Posted Jul 15, 2013 7:16 PM ET

4.

Thanks for all the advice.
After some research, I've become interested in the idea of using some sort of house wrap over the rafters, such as the Tyvek "furring method" of installing attic wrap instead of horizontal strapping:
http://www2.dupont.com/Tyvek_Weatherization/en_US/assets/downloads/AWIns...

This is a last-minute change, so I probably have to stick with materials in stock in Juneau. This would probably even mean using regular Tyvek instead of AtticWrap.

It would seem the installing the housewrap over the rafters as you suggest, then furring up before installing the roof deck would create a vent channel, allow for a full R40 in my 2X12 rafters, and create an air barrier over the batts, while allow faster vapor diffusion than a layer of plywood in contact with the batts would provide.
Are you aware of any moisture (or other) risks with this approach?
thank you,
Justin

ps I will look into the higher density fiberglass or rockwool batts when I reach that stage.

Answered by Justin Smith
Posted Jul 17, 2013 9:07 AM ET

5.

I am genuinely curious about whether Floris' detail above would be more robust over time as shown or with the plywood/strapping layers reversed.

Answered by Ethan T ; Climate Zone 5A ; ~6000HDD
Posted Jun 15, 2018 11:01 PM ET

6.

Ethan,
The vapor permeance of the plywood is lower than the vapor permeance of the Solitex Mento, so the stack-up shown by Floris allows better upward drying than a stack-up with the ventilation channel above the plywood.

Answered by Martin Holladay
Posted Jun 17, 2018 6:13 AM ET

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