Roof ventilation & insulation stops.July 12, 2005
Q: I have a two story, 20 year old home (1150 sq ft on each floor) in the South Delta area. I will soon be having the cedar shakes replaced with asphalt/fibreglass shingles. The contractor stated that the roof will be much tighter than the original one so he is going to install more roof vents. However he said that I must have more soffit venting, so I am in the process of cutting in 18 more 8" X 16" vents under the eaves. I noticed when cutting the holes that the original contractor did not install insulation stops so the fibreglass insulation is up against the underside of the roof, impeding air flow. In Alberta (where I come from) this is a problem. Is this an issue here? It is very hard to install the stops through the holes in the soffits so the contractor would have to take a board or two off the roof to install them from the top. Should this be done and is it critical in this climate? I have trouble getting the radio station you are on and I am heading back to Alberta soon.
A: Yes, the roof cavity will be tighter with the new laminated asphalt shingles. Roof ventilation should be calculated using the following formula: For every 200 sq. ft. of insulation area one square foot of ventilation is required. 50% should be on the roof (preferably on the ridge) and the other 50% in the soffits. Your roofing contractor can easily install the insulation stops from the top by removing and replacing the appropriate roofing strapping.
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