[Greenbuilding] attic insulation in Seattle house

[Chris Koehn]

Sacie-

I live on Vancouver Island- a very similar climate to Seattle. Couple of thoughts:
• Moisture drive in our climate is quite low when compared to more sever climates.  For example, we still have a few single glazed windows in our home- in both baths and in the kitchen- and they only produce condensation in extreme conditions, like when our 10 year old forgets that the shower isn't his personal spa.. So even if a bit of indoor air finds it's way to the attic our climate is pretty forgiving, whereas in a more extreme heating climate this could spell disaster. 

• Outside air at Seattle winter temps, even in the middle of winter at 100% or near humidity, has an RH which is quite tolerable when warmed up to indoor temps. For example: Outdoor air at 100% RH and at 4 C (40 f) when warmed to 21 C (70 F) has an RH of 33. See <http://www.hpc.ncep.noaa.gov/html/dewrh.shtml> for a simple calculator. So introducing a bit of fresh "moist" air to the attic shouldn't be a big concern either.

What shape is the roof on your son's home- gable or hip? Could you simply incorporate gable attic vents? These can work well in my experience in our climate, and are especially efficient in a retrofit situation. Soffit vents could be added at a later date if he feels that the ridge vents aren't doing the job for some reason. Most attic venting is purposed to keep the attic cool in summer, so the test will come next july / august.

Best,

Chris
TimberGuides

Sacie wrote: "I would appreciate advice.  I am going to Seattle to help our son
install insulation in an unconditioned attic in a 1906 house that has been
completely upgraded with insulation everywhere except the attic..."