[Greenbuilding] exulation

[Chris Koehn]

I am planning some work on a 1970's single storey home here on Vancouver Island. Replacing windows is the impetus. The home has vertical cedar channel siding over tar paper over sheathing (both ply and solid sawn recycled form wood in different areas) over 2X4 framing with fiberglass batt insulation, over 6 mil poly vapor barrier over GWB. We are in zone 4C, "mixed marine" with roughly 5400 f HDD. Our heating season is long but rarely severe. <http://www.greenbuildingadvisor.com/sites/default/files/H-T%20Zones%20-%20legend.jpg>

I'm considering removing exterior siding and adding a layer of rigid insulation while I'm at it. Some siding needs to be removed anyway for window replacement and some in other areas needs to be replaced due to flicker damage, so taking it all off (and re-using it) doesn't seem to me to be a substantial added burden. 

Quoting the U.S. DOE site: "Foam board insulation is commonly placed between the exterior finish (i.e., siding, brick) and the studs of exterior walls. To prevent air infiltration, you should place rigid insulation boards tightly together and seal the seams with tape or caulk. However, this practice may worry some builders in cold climates since the foam board may act as a second vapor diffusion retarder. Studies have shown, however, that condensation rarely occurs in these areas unless something else is seriously wrong with the wall assembly (i.e., massive uncontrolled air leakage into the walls from the house). If the assembly is constructed correctly, the inside surface of the foam board stays warm enough to keep water vapor in its gaseous state long enough for it to escape."

In the few areas I have opened the walls up I have seen no evidence of condensation, though in some areas near the top plate some of the insulation has turned grey, indicating air moving through it. It is possible, I suppose, the air that's moving through the wall is not coming from the inside.

My questions to the group are:
* Given our climate and wall profile, is adding insulation to the outside a good idea?  
* Is there a type of insulation recommended for this work, specifically in light of any concerns over moisture drive, as well as a desire by the client to keep the thickness build-up minimal, and of course cost.
* What are suggestions around detailing: joints between insulation panels, treatments at the bottom (siding currently laps directly over brick, which is applied to a concrete foundation), fastening schedules (insulation and siding), drainage plane. The house has min. 2' overhangs. 

Thanks as always,

Chris Koehn
TimberGuides
Cobble Hill, BC


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