Hi We are taking advantage of a tax rebate to install double glazed windows in our 1913 (uninsulated) house. The interior is all nice varnished fir panelling - the original walls were shingled - then someone put cedar siding over the shingles... This seems like the right time to add some 2" R12 polyisocyanate external foamboard to the walls ontop of our existing cedar siding and then another layer of cedar siding to finish. We might also get the 2x4 wall cavities foamed as the new siding will cover the necessary holes to do this (random horizontal boards in the cavities make this difficult otherwise). Any suggestions or things to watch out for? (climate is damp, cool - Vancouver BC) Thanks, Graham

It is indeed a good time to add inssulation while you're trimming out windows.

There's some details that need to be just right. Do tons of research if you're doing it yourself and/or make sure that your contractor is up to date on current tech. Make sure that any water that gets in can drain. No horizontal runs of caulking!

I imagine the the key details are in the fastening of the foam board (long screws into furring strips over the foam).

Here the code requires a rain screen layer under the siding so it is common to use preserved plywood strips for furring strips to make a space under the siding for rain to drain out.

I was wondering if I need to apply some kind of housewrap over the foam to cut down on infiltration or just tape all the foam joints?

Tighten up the sheathing as much as possible with foam & caulking, then install housewrap onto the sheathing, tape all joints. Pay special attention to the bottom of the wall where air can get in. This will be your air barrier; then install your exterior insulation, tape those seams (this is the drainage plane & rainscreen) & install your strapping/furing strips. See:(http://www.buildingscience.com/documents/digests/bsd-104-understanding-air-barriers?full_view=1)

Thanks Bob

Now I just have to convince my wife that it is worth spending the money on an insulation upgrade!

Graham