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Old 01-25-2014, 09:58 PM   #1
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Insulation of a story and a half home


Hi everyone, long time lurker, not a big poster. This website has been a great resource in the past, and I'm hoping that a few of you could help point me in the right direction regarding insulation choices for a home that I just purchased.

image 1: http://imgur.com/b6rE2GC
image 2: http://imgur.com/PxaQ9FI
image 3: http://imgur.com/oUFN4QS

Home details:
- 2500 square feet
- story and a half construction
- located in zone 5
- vaulted ceilings in the upstairs
- ductwork and plumbing vents run in knee wall
- closets built into the knee wall upstairs
- 2x6 exterior walls
- 2x6 rafters

I'm leaning towards insulating the roof deck from the back side of the knee wall, down to the soffit with a spray foam for ease of installation, along with it's air sealing qualities. I've got a large cold air return line running through that area and it definitely fits the description of a "devil's triangle." From the knee wall up to the roof peak, I'm leaning toward using a dense pack cellulose along with dense pack in the exterior 2x6 walls. I know that I'll need to fur out the ceiling joists to get a proper depth for the required insulation and will have to account for room for vent baffles if I go with a vented insulation design.

I've read that it's tough to properly build a non vented ceiling, so I'm very cautious about going that direction. I don't want to have to do this a second time . Here's what I've come up with:

1. r-30 - r-38 for zone 5 cathedral ceilings:
a. install baffles against the roof deck from soffit up to ridge vent.
b. scab another 2x6 onto the existing 2x6 for a total depth of 11".
c. drywall the ceiling, caulking and taping all seams.
d. fill 10" cavity with dense pack cellulose @ r-3.4/inch
---
total r-34

2. r-38 - r-60 for zone 5 standard ceilings:
a. install baffles against the roof deck from soffit up to ridge vent.
b. scab another 2x6 onto the existing 2x6 for a total depth of 11".
c. install 2" of xps foam board, caulking and taping all seams. (r-10)
d. drywall the ceiling, caulking and taping all seams.
e. fill 10" cavity with dense pack cellulose @ r-3.4/inch
---
total r-44

3. r-13 - r-15 for zone 5 exterior walls:
a. dense pack cellulose @ 5.5" = r-18.7
---
total r-18.7

Hopefully this all makes sense.

Thank you!

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Last edited by 794613; 01-25-2014 at 10:02 PM.
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Old 01-26-2014, 10:16 AM   #2
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Insulation of a story and a half home


I would treat the kneewall areas as conditioned. Easier and makes for better performance.

Here is a great read to demonstrate your options.

http://www.finehomebuilding.com/how-...f-venting.aspx

I prefer a combination of rigid foam, with a vent space, scabbed down rafters for effective depths, and insulated with a combination of rigid foam and/batts.

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Old 01-28-2014, 02:37 PM   #3
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Insulation of a story and a half home


Since all 3 options state cellulose, sounds like you are going that route.... keep in mind to use 1/2" unfaced foam board as the air baffles because the foam baffles may collapse under the dense-pack (IMO, 3.5#pcf minimum for the air stoppage/water-moisture wicking abilities) and the plastic ones are air/moisture impermeable- just what you don't want with cellulose. It could only dry to the inside then. Add cavity sides and a center foam board standoff (on edge) for your 1-1/2" air space to help keep the air space intact. IMO, the biggest problem you will have is making sure the framing at the dormers (for continuous air-flow; soffit/ridge) is vented near the sheathing before filling the rafter cavity with DP. Use house wrap to keep the DP in.

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Last edited by Gary in WA; 01-28-2014 at 03:03 PM. Reason: sp
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Old 01-29-2014, 11:18 AM   #4
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Insulation of a story and a half home


Forgot the links, must be getting dense.....;lol; http://www.buildingscience.com/docum...-dont-be-dense

http://www.google.com/url?sa=t&rct=j...59930103,d.cGU

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Old 01-31-2014, 08:51 AM   #5
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Insulation of a story and a half home


Thanks for the recommendations, I'm definitely planning on bringing the "attic" into the living space by insulating the roof of the home. I really like the idea of spray foam (open cell) but I'm nervous about potential damage to the structure if the foam were to allow condensation to get into it. Also, on paper the open cell foam is only good for r-4 vs cellulose at r-3.6. So in zone 5 it'd be hard to get the r value into the r-49 to r-60 (that a normal attic would have). If I were to go with an unvented roof assembly, I could fur out the ceiling with a 2x4 attached to the 2x6 through the 1.5" thick side for a total depth of 9 inches. 9 inches of open cell foam would net me r-36. What are your thoughts on this? Am I wrong to be nervous about using open cell foam for the roof joists in an unvented application?
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