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Porch / deck over conditioned space — insulation question

Geoffrey Cook | Posted in Energy Efficiency and Durability on

Ok and thanks in advance. I have a small deck/porch located directly over a bedroom on the first floor. The area is approx. 5’x12′ and it was uninsulated since the very early 1900’s when the addition was added. Here’s a little diagram. I’m wondering:

How do I insulate this area? I plan on replacing the worn out copper roof with a rubber membrane roof. The ceiling joists in this section are only 2×6 so I’m sure I would have to add some depth to accommodate anything close to code. Can it be done without venting? Putting insualtion on the top of the deck is not going to work so I’m wondering If just air sealing the whole area and pounding it with rock wool to R-50 will work?

Thanks…I’m in climate zone 6

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  1. GBA Editor
    Martin Holladay | | #1

    Most porches or decks are subject to wind-blown rain or snow, so you definitely have a challenge. Many buildings with a deck over living space have bad details and rot.

    First, you have to get the water-management details right. You need a carefully detailed roof over the porch deck. Pay particular attention to the flashing where the roofing meets the walls of the house, and the flashing at any penetrations (posts, etc.). Use the highest quality flashing you can afford.

    On top of this roofing, you will be building a deck. Usually, you'll need sleepers and an air space between the roofing and the new deck, to allow for drainage. If you're unfamiliar with these details, read up on this type of work before proceeding.

    Once this work is done, you can address insulation. You'll probably be doing the insulation work from below, so you'll probably need to demolish the first-floor ceiling. When insulating under a low-slope roof like this, you have two choices: either install thick rigid foam above the new roof sheathing -- and it doesn't sound like you have enough room for this approach -- or install spray foam between the rafters / floor joists. For more information on insulation, see Insulating Low-Slope Residential Roofs.

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