Attic living space insulation

[Dragonrider99]

Greetings. My question concerns adding insulation to an attic that will be used for a play area. The ceiling is made up of 2x8 rafters with a ridge vent and ventilated drip edge. There will be a knee wall installed approximately 2.5 feet high. I understand that if I install 8" insulation that I need to install air channels first. My question is: can I install 6" insulation and utilize the remaining area behind the insulation as the air channel? Also, should I insulate all the way down to the soffit, or should I stop at the knee wall and insulate the knee wall itself ? The floor is already insulated. Thanks for your help.

[little dude]

You need to use the air passage baffles from the knee wall up to the cieling section to maintain a constant space for airflow. Insulate only where wall board is hung or known as the heated space. Insulating to the soffitt from the knee wall does nothing for the room.

Is the floor above a heated space or a garage lacking climate control??

If the space under your floor is heated typically indicates the faced side or vapor barrier of the insulation will be towards the space under your floor for your attic addition. If this is the case, warm moist air from your heated attic addition will be trapped in the insulation which over time will cause problems.

Interior insulated walls are usually done with unfaced batts so moisture can pass through to not be trapped in the stud space. If faced insulation is unsed between interior walls should simply have cuts made in the facing to pass moisture.

[RyanVT]

Hey there Dragon,

I disagree with the dude about the knee wall condition. This space behind the knee wall should be part of the conditioned space. Your insulation and air sealing should occur prior to building the kneewall. You can see in the attachment that your air barrier should be continuous along the exterior shell.

If you rely on the insulation in your floor system (air barrier too?) you will have warm moist air entering this unconditioned space, causing problems.

As for the insulation.....

2x8's with an air space will not provide you with adequate R value in these cavities.....couple options in order of my preference:

1. damp spray cellulose, or Urythane spray foam. this should give you >R30 as well as an air barrier.
2. continuous vents from soffit to ridge, batt insulation, rigid insulation across rafters (taped seams). This option also negates any thermal bridging through your rafters. I love driving around vermont in the winters and seeing the snow melt lines on roofs from the rafters.......design opertunity....haha.
3. continuous vents, strapping on rafters to make deeper cavities, batt insulation. Still not happy with this one.

Hope this helps

[econguy]

hi ryanVT -

i had posted a similar thread ("insulating upstairs rooms"), and had gotten some good feedback from another member, although there is still the following question...

a portion of the ceiling is sloped, and is made up of the 2x6 roof rafters. i'm planning on installing R-21 batts in the rafter bays, then screwing 2" rigid board insulation to the underside of the rafters. in this case, should the fiberglass batts be faced or un-faced?

thanks

[David]

a portion of the ceiling is sloped, and is made up of the 2x6 roof rafters. i'm planning on installing R-21 batts in the rafter bays, then screwing 2" rigid board insulation to the underside of the rafters. in this case, should the fiberglass batts be faced or un-faced?

It's really important to use some form of plastic cap (nail) in your case. You'll have insulation weighing on foam board. You do not want this board ripping over a small nail/screw head. Use a wide surface head...aka, a plastic capped nail.

I don't think faced or unfaced matters. You have foam board on the face of it anyway.

[warthog]

Econ,

My home in alaska is built similar to what you described with the knee wall, sloped rafters and rafter tie ceiling. To get a vented (cold) roof, what I did is nail a 1 X 2 to each side of each rafter up against the sheathing. Then I put 2" blue board up against the 1x2's and nailed the blue board to the 1x2 with the plastic cap nails dfrette talked about. Now you have formed a 1.5" deep air flow channel between your insulation and the underside of your roof sheathing. Then I spray foamed under the blue board to fill out the rest of the rafter depth. I would recommend at least looking into the spray foam, it was not all that expensive relative to fiberglass, about $2600 for 1000 SF of roof at 5.5" thick. I have 2X10 rafters so we effectively got 7.5" of foam insulation for an R value of about 45. In mass you may be able to just put 2 layers of 2" blueboard and get about R28 that way without fiberglass. Dont know if thats enough for the east coast but here in alaska R45 was the minimum I was comfortable with. good luck however you do it.