insulating a flat roof


Old 03-29-02, 01:45 PM
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Question insulating a flat roof

Hi all:

First time I have used this board. Certainly nice of all the pro's to lend their knowledge and provide advice. So thanks in advance.

I live in hot/humid Central Florida and have a ranch style home with mostly a gable roof. However, there are 2 rooms on the back(South side of course) that have a flat roof. There is little or no insulation in this area - possibly a light layer of rock wool. I would love to insulate this roof to the best extent possible in the space provided by the 2x6 rafters. I know I could have cellulose blown into that space but am concerned with filling the spaces so that no air circulates and the natural air flow will not allow heat to exhaust heat from the area. What are my alternatives in terms of insulating the 16" spaces between the rafters over the flat roofed area? I think the spaces between rafters could be accessed from the attic side as well as the outside wall end.

Jim X
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Old 04-05-02, 11:46 PM
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Do you have underside access to the joist cavities? ie ripping out old ceiling and replacing after you insulate?

You could fit r -19 bats in that case ..but heres what i reccomend

Take a trip to your local home centers or yard and check out ask about a stuff called Reflectix.
It looks like aluminum foil with a plastic core sort of like bubble wrap. Its much more expensive than traditional insulation but works to Reflect up to 90 some percent of radiant heat. Perfect for a situation like yours where the hot roof would be transmitting heat into the room below.

I used this on an entire roof to meet r -35 standards coupled with fiberglass. The material is 3/8 thick and can be installed either between joists and simply staple or on the flat from the underside. The R value of the material is dependent on how you use it ie vertical horizontal.. but if memory serves its 14.9 R when used on cielings like that.

If you cant find the stuff in stores try doing a search online... Ive noticed a very cool cathedral ceiling in PA 90-100 degree summers the key is how it reflects heat not just slows transfer.

Hope this helps---Josh
Old 04-06-02, 08:08 AM
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Thanks for the info. Yes I have seen the foil and bubble insulation you refer to. Sounds great and glad to hear that it works as wellas advertised. I think I may try opening my eaves and try sliding something like Reflectix into the roof rafter cavities. I may have to be creative with how I do this, but I only have to span about 12-13 feet of flat roof - the other side is attic from gable roof. Possibly even an electrical fish tape from one side to other, attach 1"x2" to insulation and pull it through the cavities. Sound do-able?

Sounds like you have combined Reflectix with a fiberglass bat before. I have thought about the same using either fiberglass batt or possible a rigid insulation material. If I used a rigid board with Reflectix is there a best way to attach it? Staple vs adhesive?

Thanks again.
Old 04-06-02, 02:59 PM
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Up here in Toronto re:commercial construction it's now standard practice to install the flat roof,next comes 3" polystyrene foam and finally the gravel.Studies have shown the lighter coloured foam reflects the heat(3" gravel),adds insulation value and extends the life of the roof.
Woof :-)
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