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RISurfer 09-29-2009 12:03 PM

How to insulate attic crawl space
I am adding insulation to a salt box cape. I am located in Rhode Island. I have 2 seperate attic areas. The first area is above the bedroom which is easy to insulate. The second area is an area through one of the bedroom and runs the full length front side of the house. This crawl space is insulated with R19. The insulation is along the rafters. During winter this space is cold and leaks into the bedroom. The knee wall door is insulated to R30. I would like to improve insulation in this area. My question is should I insulate the knee walls in this space and or floor with unfaced insulation? If I do insulate this area will I run into ventilation problems? Any help would be appreciated. Thanks.

PerpetuallyRepairing 09-29-2009 08:24 PM

Knee walls are fun. :jester: In my mind this is all attic space which should be R30 whether the insulation is horizontal or vertical. Air leakage is a big issue and if you have the money, spray the whole thing with open cell foam.

You can use fiberglass insulation if you are scrupulous with air sealing. Seal the edges of the drywall wearever you can with caulk. Also the pipe and wire penetrations. Also caulk wherever wood boards come together. To address air leakage underneath the knee walls, take a plastic bag, fill it with insulation and shove it in the holes between the floor rafters. Foam the edge of the bags with canned spray foam. You can also buy one of those DIY foam kits such as "Foam it Green". It is faster than caulking. Cover everythign with R30 fiberglass, being careful to cut it to fit.

Maintain the air channels whereever the finished space reaches the roof. I like to cover the faces of the insulation that are open to the kneewall with Tyvek. Tape the tyvek together with "Tyvek Tape" and tape it to the wood around the edges. Fold the Tyvek under the styrofoam that makes the air channels next to the roof.

Make sure all access doors latch well and have weather stripping. Air sealing is the key.

RISurfer 09-29-2009 09:08 PM

1 Attachment(s)
Thanks for the reply. So I should insulate the knee wall and the floor with R30 unfaced and the rafters with R19 faced? Attached is a photo that replicates my attic. Thanks.

RISurfer 09-29-2009 09:11 PM

Forgot to mention this attic area has duct work in the floor.

PerpetuallyRepairing 09-30-2009 08:17 AM

For Rhode Island I would do the whole thing with R30 faced. You want to be sure the duct work joints are sealed with metal foil tape and the perimeter of the duct is sealed to the drywall below it and the knee wall above it where it goes under the knee wall.

mpepin 09-30-2009 09:41 PM

I agree totally with Perpetually. Air sealing and R30 are the way to, spray foam if you can afford it will accomplish both. Here's a possible problem with your situation, the duct work is in the attic space and the insulation is in the rafters, which means, this is technically conditioned space. (You mentioned the kneewall door is R30, just the door?) It should not be cold in there, it should be the same temp. as your bedroom. Inaddition to the rafters, the gable end walls should be insulated too. Typically you would insulate the rafters and gable ends OR the kneewall and floor joists. If you properly insulate the kneewall and floor joists, your ducts will be running through unconditioned space meaning they will need to be insulated themselves. You may be better off re-insulating the rafters and gable ends properly OR insulate the kneewall then blow in insulation on the floor, up to 30" deep, so the ducts will be at the bottom of the blown-in insulation.

RISurfer 10-01-2009 09:56 AM

I think I will insulate the knee wall and floor with R30 and possibly add an additional R30 unfaced. I probably will keep the R19 in the rafters and add an additonal vapor barrier basicly because if I place R30 in the raftes,I will have to compress the insulation to fit. Wouldn't this decrease the efficiency of R30? Thanks for the help.

Scuba_Dave 10-01-2009 11:57 AM

Yes, compressing insulation reduces the Rvalue greatly

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