Insulating a low slope roof


Old 08-18-02, 05:21 AM
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Insulating a low slope roof

I'm planning to add a new room to my house. My house has steep roofs (12 pitch). Due to the layout of the existing roof, adding a high pitch new roof over the addition will be costly and perhaps tricky from the standpoint of guarding against water leaks at the roof intersections. So, I'm opting for a low slope (1 pitch) shed type roof built just underneath the eaves of the existing roof. The new rafters and ceiling joists will be 2x8 members, so the space I have for insulation at the eaves will only be about 7 inches, and about 16 inches where the rafters and ceiling joists join the wall of the existing house. What are some good ways to insulate this new roof?
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Old 08-18-02, 12:32 PM
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You are building a potential energy monster,but, it can be saved. Lucky it's new construction. I'm guessing you'll have three outer walls and a crawl space or slab. Thesse need to be insulated too.

Your biggest heat gain/loss will be thru the rafters since they are in direct contact with the roofing material and ceil'g drywall.

Best way to insulate is with a type 3 ( 2 layer)radiant barrier (RB) between the rafters, a single sheet RB attached to the bottom of the rafters (like a vapor barrier), 7/8" steel furring strips across the bottom of the rafters and then drywall. This is the only effective thermal break I know of for these types of ceilings.
Bulk insualtions are a disaster for these types of ceil'gs.

Use the type 3 RB for the walls too.

If you are on slab I have a bubble foil that is both an insulator and vapor barrier for under the slab.

If you are over crawl space there is an effictve way to use RBs there too.

My experience is that if you insulate as I have out lined then you will probably use no more energy than before the addtion. You will most likely not have to upsize the furnace or a/c unit either.

Thank you for considering my opinion.
Old 08-18-02, 04:38 PM
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Thanks for the information. Yes I'll have a crawl space. Need details for using RB there. Also I was planning to have 2x6 walls, but with the RB system can I use 2x4 walls?

Old 08-19-02, 09:37 AM
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You can use 2x4 construction. A two layer RB will take up about 1/2 that space.

The crawl space is done as follows. Keep in mind that the crawl space has to be as dry as possible, this may require a sump pump. Your area codes may require vents. If so, you can close and seal them after the inspection.

Install 4 mil plastic to the sides of the bottom plate and let hang down and across the floor. Overlapping at least 6" cover the rest of the earth. Install a second 4 mil over the first, cossways, but not up the wall. Install a single layer RB, 48" wide to the
side of the bottom plate over the 4 Mil. Let it hang down. The bubble foil you see in lumber yards, etc. is too expensive for this application, the thin RB material is just as efficient and costs about 20 cents/sf + UPS. The 2 layer 22 cents/sf + UPS All come in 500sf rolls.

Cut RB panels with enough flange area so that you can install between the joists at the rim board.

Because of the large wall surface to floor area ratio I install a perforated single layer material on the out side besides the two layer material in the wall cavity. The perforations allow the moisture to exscape but insulates the studs from the outside. Run the material over the rim board too. For maximum results install 1x2 strips over the osb, over the stud lines. Install the RB on top of the 1x2s. This helps isolate the wall system against conduction. If you are planning to use siding, use steel siding. Besides having superior performance characteristics, it also act as a RB.

You will need a 1/2 crown air stapler to install, or a commercial electric stapler. If you have buy one check Sears. 3/8" long staples.

If you want to see some samples let me know at my e-mail.

Knowing what part of the country you live in would help inform you of any additional requirements.

Using the RBs, your addition will probably be the most comfortable room in the house.

Thank you for considering my opinion.
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