Insulation design
Bbrown3381
| Posted in GBA Pro Help on
Climate zone 2a…..south east Georgia coastal island. Bought a 1962 block one story house, The interior walls are painted block….we like the beach look associated with this concept. So, all insulation must be on the exterior of the house….which now us striped to bare block. Interested in the correct approach for water management and insulation. Exterior will eventually be cedar shingles.. Sort of thinking….spray closed cell between furring strips with ridged foam sheeting over the spray foam and strips. Then plywood covered with rain drainage screen topped with shingles.
Thanks for the help
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Forget spraying foam over the wall, you will not get a flat surface to allow installing further layers.
This could work: Starting at the exterior block surface, add 2" polyiso insulation (foil facing out), then 1x4 vertical furring strips attached with 3.75" or 4" Tapcon screws driven into the block. Then 1/2" plywood sheathing, asphalt paper and shingles. You need to allow venting of the space between the insulation and the plywood, so that requires a slight gap in the siding across the top and bottom of the wall, with drip edge flashing and a bug screen. There are details for that on this website. For greater R-value you could use 3" polyiso and 5" Tapcon screws.
and don't forget to tape the seams or this can happen...
http://www.buildingscience.com/@@search?SearchableText=foam+shrinks
http://foursevenfive.com/foam-fails-reason-5-excessive-shrinkage/
Building Science Corporation... exterior spray foam assembly
http://www.buildingscience.com/documents/insights/bsi-048-exterior-spray-foam
Thanks...very helpful! A couple associated questions. Any need for PT furring strips and. Exactly where would you place your flashings.
Thanks
Bob, Please read the link I posted from Building Science Corporation. Your answers are right there with drawings.
The mounting of the windows & window flashing may alter what makes the most sense here. (The "innie/outie" issue.) If you aren't re-mounting and re-flashing the windows, the exterior of the block is your drain-plane, and putting rigid foam directly against the block may inhibit drying of the bulk water incursions too much. (Not that you EVER get wind-driven rain beating on the windows in a GA coastal location. :-) )
In that case replacing/reflashing the windows a liquid-applied weather barrier on the exterior of the block wall would air seal it as well as lower the vapor permenace to acceptable levels. (Under 10 perms is good, under 5 perms is better- it may take more than one coat to get it under 5 perms.) Then, 2-3" of rigid rock wool held in place by furring similar to a rigid foam approach would provide maximum drainage & drying should work pretty well. You would still use mesh underlayment for the shingles, but you'd still need #15 felt or housewrap between the shingles & plwood.
See: https://www.greenbuildingadvisor.com/blogs/dept/musings/housewrap-can-liquid-applied-wrbs
In a US climate zone 2 location even 2" (~R8) of rigid rock wool would be better than IRC 2012 code-min (since it's a "mass-wall" but 3" (~R12)would still likely be cost effective over the long term.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
See table 2, p.10 of this document about financial rationale fro going higher than code-min (they're suggesting even R15 continuous insulation may have a rationale, but that depends on how much energy and insulation actually costs in your neighborhood.):
http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones