# Insulation quandary (knee walls with photo)



## kevabar (Feb 12, 2012)

I am having trouble piecing together my insulation plan for a 2nd story renovation (2 story cape cod). 

Existing:
2x4 exterior and knee walls
2x6 rafters
Some extremely inefficient insulation from 1952.

My plan


1) 2x6 glued and screwed to face of rafters to increase cavity to 11".
2) R30 batt insulation for a depth of 9.5" with ventilation channels under roof boards
3) 2x4 glued and screwed to exterior and knee wall framing.
4) R21 batt insulation (5.5" depth batts)
5) Use 2" DOW Super Tuff blue/pink foam board on framing facing interior of room and then tuck-taping and Great Stuff foam to provide a vapor barrier.
6) 1/2" drywall for fire protection.
7) Batt insulation will run in joist cavity from vent channels (at soffit vents) to approximately 6 inches under the subfloor. (I might use pink foam board block caulk/foamed at interior end of joist cavity to block air flow between ).
8) There are 2 rooms on either side of the stair landing on the 2nd floor. The stair case is finished living space and the 2 rooms were designed so that the home-owner could finish in the future (which is now me). The first side we are planning to finish will be the master bedroom. The other side will hopefully be a master bath down the road. The bedroom side will be connected to our gas HVAC system. We had an HVAC guy inspect our system and said the existing system will adequately handle the additional finished attic space.

Some assumptions:
-The area behind the knee wall will be unconditioned and will vent from existing soffits to existing ridge vent.

My questions
1) Do I have to worry that the backside of the foam board will not have any fire protection other than approx 5" of batt insulation?
2) Do I have any other vapor barrier issues?
3) If the weight of added 2x6 framing on the existing rafters is too much, I could reduce it to 2x4 (or 2x3 if really needed).
4) Am I missing anything here? Or am I oversimplifying this?

Am I on the right track? Thanks for reading my wordy post!


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## Windows on Wash (Aug 30, 2011)

Where is the home?

Ever consider just spray foaming the whole thing and make it a hot roof deck? While it might be a bit more expensive, the impact on ceiling height and hassle would be greatly reduced.


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## kevabar (Feb 12, 2012)

I am in south Jersey directly east of Philly; zone 4. I do not want to spray foam because I merely do not have the funds for that. Plus, I do not want it directly on the decking.


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## Gary in WA (Mar 11, 2009)

1 plywood gussets with nails

2 use rigid f.b.against f.b. spacer strips for no convective heat loss at the baffles; http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

5 f.b. on the attic side with/without ignition barrier, check locally; http://dow-styrofoam.custhelp.com/a...sion/L3RpbWUvMTMxODYwMjg4MS9zaWQvKkNueDl5R2s=

*4.4.1-2: * http://www.icc-es.org/reports/pdf_files/ICC-ES/ESR-2670.pdf

6 ADA drywall; http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Gary
PS caulk the sill plate/floor joint.


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## kevabar (Feb 12, 2012)

Thanks for the input, guys! that is some great info.


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## Gary in WA (Mar 11, 2009)

You're welcome, that's why we are here!

Gary


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