Big Green: Effective R-value of rigid insulation with z-furring

[Meg Needle]

We usually design similar walls with the Densglas between the metal studs and the rigid insulation.  You can still dampproof the Densglass and then install the insulation with small button fasteners instead of the z-strips.  We usually have masonry veneer so the masonry anchors can be the type of system to secure the insulation to the sheathing.  However, in your case, the rigid insulation can be speced t&g so with the Densglas backing you can probably find a screw type button fastener for the insulation.  We have had good luck fastening the Hardi through the foam to the studs or possibly even the Densdeck with the right screw fasteners and spacing.  By eliminating the z-channels, you'll minimize your thermal bridging and, most likely be able to use the full R-value of the foam.  You'll need to make sure your dew point is not in the stud cavity though.  With some evaluation, this might prove a good alternate.
 
Meg
Lord Aeck Sargent Architecture
 

________________________________

From: biggreen-bounces at lists.biggreen.org on behalf of Kristian Kicinski
Sent: Tue 1/8/2008 1:24 PM
To: biggreen at lists.biggreen.org
Subject: Big Green: Effective R-value of rigid insulation with z-furring


Hello BigGreen:
I am looking for information on how to calculate the effective r-value of rigid insulation when it is installed between metal z-furring channels.
 
Our wall assembly is as follows:
6" metal studs with R-21 fiberglass batts
1" vertical z-furring attached at each stud
R-5 polyiso rigid insulation installed between z-furring
1/2" Exterior gypsum sheathing (dens-glass gold)
Fluid-applied air barrier (Henry's)
Cement board siding (Hardi)
 
During design we intended the rigid to be installed over the studs with no z-furring in order to prevent thermal bridging in the assembly, and for the gypsum sheathing to then be fastened through the rigid.  However, the contractor has already begun installing the z-channels, so now I am trying to determine the effect this will have on our energy performance, since the z-channels create a thermal bridge through all the insulation.
 
Thanks,
Kristian Kicinski
 

Kristian Kicinski AIA
LEED Accredited Professional

Bassetti Architects
71 Columbia St. #500 | Seattle, WA 98104
P (206)340-9500 | F (206)340-9519

kkicinski at bassettiarch.com <mailto:kkicinski at bassettiarch.com>