Devising Vented Roof in Southeastern Alaska
Hello all,
I’m finishing a build on an off-grid house in Alaska. It’s very unconventional and was framed with a chainsaw and hammer 15 years ago. All the lumber is rough cut and milled onsite.
Now I’m finishing it all as best as I can working with what is already in place. There are no building codes here but I’m trying to do things as best I can now so that I can minimize the issues that might come up down the road. It’s a relatively warm but very wet part of southeastern Alaska.
My question is around roof venting. The siding is complete and the roof is already on. There was no venting incorporated for the roof. One contractor friend suggested that we just do the best vapor barrier we can and roll with it. Another friend really urged me to put a vent in and devised the plan in the attached diagram.
The basic idea is: is to fur the rafters out and put collar ties in and create a small attic and some airflow. Then drill holes in the bird blocking and create an opening under the ridge beam on the gable ends.
My questions are:
-Does this plan make sense?
-How large does the attic space need to be? What’s the minimum that it could be?
-How large should the opening in the gable ends be?
-Is 2.5″ large enough for the air gab above the insulation?
-How big should the holes in the eaves be and how many holes should I put in each bird blocking (it’s 16″ OC)
Attached are some old photos for reference – we’ve made lot of progress since I took these.
THANK YOU!
Replies
The required venting depends on the size of the house, and quality/presence of a vapor barrier between the heated space and vented attic.
https://roofsaver.com/net-free-charts/
Planning on a good vapor barrier, the amount of net free venting should be 1/300 of the floor area of the heated structure. Split between the top and bottom.
Commercial vent products have a rated net free area quoted. If you are making one up onsite, you will have to calculate/estimate the net free area.
http://www.wovenwire.com/attic-soffit-vent-ground-ventilation-screens.htm
screen material can be obtained with different amounts of flow reduction.
When looking at the attic cross-section between joist bottoms and the top of your collar ties, you can compare the resulting area with the required venting area. I would want several times the area you need for venting. (based on a guess)
You may want to consider the utility of making this area large enough to crawl through, (access via gable end vents) in years to come to allow for inspection and other purposes.
Thanks for the reply!
I should have more than ample venting based on the math. The footprint of the insulated space is 24x31’ and the cross section of the attic will be triangular - 10” tall by about 6’ across.
Would you recommend utilizing baffles to
prevent the insulation from blocking the 2.5” of air space between the insulation and sheathing? I’m confused on whether the baffles would block moisture from being able to escape upward out of the insulation. I do plan on having a solid vapor barrier between the warm space and insulation.
https://www.buildingscience.com/
You can find engineered articles specific to your region as well as a book specific to your weather and construction at Building Science. Loads of info!