I have a roofing / deep energy retrofit question
I have a roofing / deep energy retrofit question. I’m just beginning a deep energy retrofit as a retirement home on a permaculture garden-farm project in northern Alberta, Canada (ASHRAE Zone 8). Tomorrow we are moving a 42’x28’ bungalow 200 km.
As it needs a new roof (old shingles are stripped to sheathing and house tarped at old site), the current plan is to chainsaw the eaves, then apply air/vapour barrier, insulation (either Roxul ComfortBoard or EPS) to exterior walls, preserving as much interior space as possible.
Exterior cladding is a mix of lightweight metal sheeting and locally milled [lumber], treated using “shou sugi ban” technique both on vertical furring (metal is recycled, locally milled [lumber] is from trees harvested on site). Windows will be installed with a strapping/built-out buck system and flared walls on the inner wall with attention to air circulation, passive solar in winter, daylighting and connection to the outdoors.
Just not sure what insulation/air barrier method would be best for the roof? A metal roof is planned (not standing-seam due to budget) but good quality. It’s a simple roof, 15% slope (5.6/12). The ceilings are low (8’ from top of subfloor). It’s built with rafters – kind of a weird design – collar ties with vertical struts angling toward center fastened to the joist (this is no heritage home, but an old sturdily built prairie shack). There is very limited space between top plate of eave and underside of roof deck. I’ll know exactly how much in a couple of days. So far, everything is sound.
While I’ve reviewed many resources, the most recent and helpful has been http://www.buildingscience.com/documents/guides-and-manuals/gm-mass-save-der-builder-guide. I recognize the most effective upgrade might be an unvented attic with exterior roof insulation. However, I’ll be installing a wood cook stove with a chimney that will exit near the ridge. And that makes me wonder if an unvented attic is the best? I’d like to hear any and all feedback on roofing insulation/air/vapour barrier assemblies.
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Replies
Janet,
There is no reason to cut off the eaves (using the chainsaw retrofit approach) unless you are planning a true chainsaw retrofit.
You do know what that means, don't you? It means that you will be installing an air barrier and a vapor barrier on the exterior side of your sheathing (typically, a peel-and-stick product like Grace Ice & Water Shield), followed by a thick layer of rigid insulation on both the walls and the roof, followed by rainscreen furring strips and siding on the walls, and new 2x4s furring strips on the roof. The 2x4 furring strips on the roof are installed so that they create new roof overhangs at the rakes and eaves. Above the 2x4 furring strips on the roof, you will install either purlins or another layer of plywood roof sheathing to support your new metal roofing.
This method is called PERSIST. For more information on these details, see:
Getting Insulation Out of Your Walls and Ceilings
The History of the Chainsaw Retrofit
It sounds like you are concerned about the details for installing a new metal chimney through your roof and your roof insulation. If you are installing a new metal chimney, (a) make sure that the chimney is rated for wood stoves, (b) make sure that you purchase and install the roof penetration kit provided by the chimney manufacturer. Follow all of the installation recommendations (and clearances) provided by the chimney manufacturer, and you should be fine.