Insulating and ventilating a vaulted ceiling with polyiso foam
The family room in our home has vaulted ceilings and gets very hot on a warm day – to the point the air conditioner can’t keep the room cool. The current construction of the roof & insulation ceiling is:
The roof is dark composition shingle over ½” CDX plywood. The framing is 2×10 rafters 24” on center. And the room is finished in sheetrock. Each rafter bay is stuffed with fiberglass insulation. 3 small ventilation holes are drilled at the end of each rafter bay. The rafter bay cavity is 9″ high. Ventilation is minimal.
While we have R-30 insulation – the room still gets unbearably hot. Our guess is the lack of ventilation allows heat to build up and ultimately overwhelms the insulation.
We are looking at re-insulating with polyiso boards and increasing ventilation. We have 3 different options and trying to find the best approach for minimizing thermal gain in our home. We live in central East SF Bay Area. Summers are dry and can get very warm – 100+ degree days not uncommon.
In each option we increase ventilation in each rafter bay.
Use 2 sheets of 3” polyiso, providing R-36. Create a 3” venting space between the insulation and the top of the cavity (9″ Cavity – 6″ Insulation).
In this option we increase the insulation by 1.5” getting to 7.5″ total insulation – increasing the R-Value to about R-45 – Create a 1.5” venting space between the insulation and the top of the cavity
In this option use two 3″ polyiso sheets – but we Increase ventilation flow around insulation boards by putting 1” furring strips between each 3” sheet of polyiso. Our goal is to increase airflow between polyiso sheets to reduce the overall temperature in rafter cavity. Net result is 1″ of airflow between boards, 2″ between insulation and top of the cavity.
We would appreciate your opinion on the best approach.
Thanks in advance for your help – Frank Galdes
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