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Old 08-12-2010, 01:11 AM   #1
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Insulating & Ventilating Vaulted Ceiling

The family room in our home has vaulted ceilings and gets very hot on a warm day – to the point the A/C can’t keep the room cool. The current construction of the roof & insulation ceiling is:

The roof is dark composition shingle, over ½” CDX ply wood. The framing is 2x10 rafters 24” OC. 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 R30 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 removing the roofing and CDX sheating to re-insulate with POLY ISO 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.

Option 1 - Use 2 sheets of 3” PolyISO, providing R36. Create a 3” venting space between the insulation and the top of the cavity (9" Cavity - 6" Insulation).

Option 2 - In this option we increase the insulation by 1.5” getting to 7.5" total insulation – increasing the RValue to about R45 - Create a 1.5” venting space between the insulation and the top of the cavity

Option 3 - 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.

Would appreciate input on best approach to reduce thermal gain.

Thanks in advance for your help – Frank
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Old 08-12-2010, 09:14 AM   #2
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You didn't say but I assume you have a good ridge vent. There should have been the styrofoam chases against the underside of the decking to get a good air flow also.

This may not interest you but when I built my house with a vault, I had the same problem, I looked into installing a "skin roof". A skin roof is a roof built above your existing roof with an air passage between. The skin roof is built with the chimney effect in mind.

Hot air rises drawing in cool air at the bottom and discharging at the top. This would keep the existing roof much cooler in another way, the sun would not hit the existing roof. I don't know if this would work or not as I have never seen one personally, maybe someone on the forum has. Just a thought.
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Old 08-12-2010, 02:51 PM   #3
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I believe R-30 is 9-1/2", R-25 is 8", are you positive on your R-values for the inch thickness?

22-1/2" rafter blocks with only 3- 1-3/4" holes (older) have a of 7.22, less than 1/2 of the required 18 NFVA.
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