I have a shed roof over a screened-in deck. The underside of the roof is open to the deck – exposed rafters (2×10″) and roof sheathing.
Even though the deck is open/screened on three sides it gets pretty hot in the summer sun from the heat radiating down from the roof. well over ambient air temp in fact.
What’s the best approach to insulating this? It would go hand-in-hand with applying some sort of paneling to the interior side of the roof to hide the insulation and provide a bit more of a finished look. A pair of ceiling fans is also being considered.
One consideration is that by attaching a ceiling surface to the rafters the only ventilation to the insulation would be the continuous soffit vents.
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My brother just built a shed roof over a sunny patio area in CO. The first time they sat under it on a sunny day (meaning most days), they felt the heat radiating down at them. Same as you are experiencing.
What you need is a low-emissivity covering over the bottoms of the rafters. Anything white or light-colored will do. Do a search on soffit materials for porches. Certainteed and others make various products for this. Besides being white, for very low emissivity, it would dress up the appearance greatly.
If you use something like that, you can leave a couple of inches of space at the lower and upper ends of the rafters, or perhaps fill those spaces with matching soffit vent strips, to allow air to move freely up between the covering and the roof deck, to vent it. The heated air in the cavity will move out laterally at the top, where the roof meets the wall of the house. I hope there aren't any windows right above where the roof edges meet the house wall.
A pair of ceiling fans is also being considered.
That's your best solution and other benefits will be added.
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thanks for the input. In a nutshell - skip the insulation, wire it up for fans, and put up some sort of internal ceiling. I'll do the wiring and boxes for the fans and see what sort of ceiling or soffit materials I can hunt down locally. It's ~400sqft of ceiling to tackle.And for the comment about windows above the roof.... "yep". another gotcha we discovered. If the windows over the roof are open the heated air rolls right off the roof and in the windows. Kind of a trade-off - the deck had been getting full south+west exposure and was borderline useless on hot days. The roof is now shading a lot of first floor window footage too so there is some benefit.