Retrofitting a Low-Slope Roof
My 67 y/o mid-century modern, Seattle home (Zone 4) needs a new roof, and I’m proposing adding insulation under a new standing seam metal roof… but I have questions.
The attached PDF diagrams the proposed design.
- Insulation value: Given the air flow in the roof cavity, what insulation performance can I expect from the addition of 2 inches of Polyiso to the roof?
(My thought is that the ceiling heats the air in the ceiling cavity, which loses heat through the roof, and so reducing heat loss from the ceiling cavity by insulating the roof will keep the house warmer).
- Powered vent: 2 of the 3 roofing companies I’ve engaged to give quotes insist that a passive ridge vent is essential. I’m concerned that in the winter a vent will cool the ceiling cavity and so suck heat from the ceiling. One contractor offered the option of a powered ridge vent fan, which could be turned on and off, e.g., based on moisture content. In the winter it could be used sparingly and allow warm air to remain in the ceiling cavity. But I’m unable to find such fan. Do you know of one?
- Venting necessary: The roof has been unvented since the house was built in 1955, with no apparent ill effect. Do you see any reason why adding insulation to the roof will necessitate additional venting?
- Other: Are you aware of other solutions or ideas?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part