How To Insulate A Hot Porch Ceiling?

candlerMarch 14, 2011

Howdy. I'm closing in a porch. I have a gable porch ceiling. It has 3.5" rafters. In the summer it gets very hot. Last year I put a rotating vent on the roof. It didn't seem to help.

To try to reduce roof radiated heat I'm thinking of installing a ridge vent and use r-13 fiberglass insulation between the rafters. However, it seems the insulation would stop the air flow from the eave vents to the ridge vent and the room would still get hot.

I feel others have come across this problem. I'll appreciate any suggestions, ideas, etc. Thanks, Gene

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With a continous run ridgevent as well as continous run eave venting, you can run radiant barrier on the underside of the rafters. The airspace between the roof sheating and radiant barrier is stopped by the radiant barrier. The heated/trapped air is directed up and out the ridgeventing. No other insulation type would be necessary in the rafter spaces , as you mentioned, would block airflow. If you want some form of insulation in there, you could install 1-1/2' foam board panels inbetween the rafter spaces keeping it down at the lower part of the rafters and keeping a 2' airspace between the roof sheathing and the foam board, but dependent on your location, imo, you wouldn't need it. You will also want to remove or disable the rotating vent that is on the roof.

Below is a link to radiant barrier rolls and you can cover the underside of the rafters after the radiant install with drywall, tongue & groove materials, or any lid covering that you have planned on without losing any effect of the radiant barrier. This method works great in attics as well to keep attics cooler as long as you have the continous run ridge and continous run soffit/eave venting. Below is only as an example, you can check your local suppliers. The rolls I use are 50' wide by 1000', have a kraft paper backing on one side, the foil facing on the opposite, with a fiberglass mesh inbetween for strength, and is non- perforated. You will want to foil tape all seams as well as any rips or tears before covering over with your ceiling material of choice.

Here is a link that might be useful: radiant barrier

    Bookmark   March 17, 2011 at 11:08AM
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Hello, SE. I appreciate your response and the clarity of the information. We've had a few days of warmer temps in these WNC mountains meaning I need to really be attentive to the ceiling. I studied the link you sent. Thanks. My closed-in porch (use to be a deck) is only a little over 200 sq.ft. I'd heard the radiant barrier stuff was expensive but the link site seems reasonable. With buying and installing eight double hung windows for that size room about anything else almost has to seem reasonable. Question: A while back I had a furnace go belly-up at a rental house. The contractor gave me a few small pieces of left over duct board. They are foiled on one side and about 2" thick. It seems to be made of compressed fiberglass insulation. Could it be used as a combination radiant barrier and insulation, too? I would nail it to the underside of the rafters with the foil side toward the roof. That would allow the full 3.5 air flow from the soffit to the ridge vent. And, the 2" thickness would give some sort of insulation during winter. Just thinking outloud, SE. Thanks again. Gene

    Bookmark   March 19, 2011 at 6:40PM
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Sounds like that would be fine as long as there are no gaps and you have enough material to do the whole lid. The purpose is to create an envelope and not disturb the airflow in each rafter space. Each rafter space would have it's venting from the eaves up to the ridgeventing so that the heated air is trapped between the foil and the roof sheathing pushed up and out the ridgevent.

    Bookmark   March 19, 2011 at 7:03PM
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