Cross-strapped walls – R value?
JohnWalker
| Posted in Energy, Heating & Insulation on
Hi
I am curious about the R value of a 2×4 and/or 2×6 wall (16 & 24″ oc) with interior 2×2 cross-strapping and wet blown cellulose insulation.
Somewhere I was able to download ASHRAE ratings for typical 2×6 walls etc but did not download or find cross-strapped.
Can anyone direct me to such an R value study?
I am sure the reduction in thermal bridging is more significant than the extra 1.5″ of insulation. (And before anyone suggests exterior XPS I want to use the sheathing for seismic bracing, need the exterior to be vapor permeable due to the wet winters we have, and would like to take advantage of the blown insulation contractor onsite – i.e. one shot deal.)
Thanks
John
Edited 9/14/2009 9:04 pm by JohnWalker
Replies
i've done a lot of it... but with dens-pak cells instead of wet blown...
all my calcs say the net wall R-valu is R-19 for 2x4 with 2x2 strapping
this amounts to 5" framing thickness
What is "dens-pak" cells and what is their R / inch value?
Thanks
John
You could sheath the inside walls instead.
Tu stultus es
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
http://www.buildingscience.com/documents/information-sheets/high-r-value-wall-assemblies/high-r-wall-03-interior-strapping-construction/
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Rob
Great stuff on that site.
Curiously their 2x6 @ 24oc with 2x3 internal cross strapping is slightly BETTER than their 2x6 @ 24oc with 1" external extruded PS. I presume there are more losses for window framing, rim joists etc with cross strapped(?)
John
very slightly... R1 for whole wall value. and unless you are filling the 2x3 cavity with spray foam or something, your air leakage control isn't as good as the exterior rigid. plus the exterior rigid can just increase foam thickness. we are doing 1.5" for example, and now it's better than the interior strapping method for R value AND air control.lots of things involved there though; builders changed siding choices and won't let me go thicker on the external EPS :D-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
I'll need tapped tyvek etc to meet blower door testing requirement so am not relying on XPS. Must be difficult to frame windows etc with 1.5" of external XPS?
Thanks
John
Tyvek is not a very good way to seal up a house. taped seams on XPS would do a better job. there are leaky tyvek'ed houses all over the place.the builders don't seem particularly concerned with framing the windows. They are, incidentally, using "zipwall" under the EPS... structural requirements on a commercial building require the sheathing layer.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
we've done a couple repair jobs on houses with 1" xps on the outside
they used no blocking, hung the windows on the xps ..
naturally everything moved..
what a messMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
sounds like fun! I assume a continuous sheathing product like zipwall would allay any concern there?-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Rob-
What kind of setup do you guys use to hang siding over thick exterior foam. I'm assuming furring strips. If so, what about around window openings? How about your flashing details? I was going to put 1.5" polyiso on the exterior of my house (I already had a bunch of it) over the OSB, but I've decided to just strap the interior instead. My hardiplank will be a lot easier to hang now.
Edited 9/22/2009 4:14 am ET by bk24
we're using a vinyl siding that will warrantee when nailed through 1.5" of foam, and our external sheathing... under the foam.. is continuous as well so we're not just nailing to studs.I'm not sure what the window/flashing details are at this time,.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
I've been watching a few houses go up here. One in particular is a two story with those fake 18" roofs attached to the sheathing along the second floor rim joist elevation. They didn't put any housewrap etc on the sheathing first. Our local code requires 2" holes in the sheathing so its useless for air sealing.
On the weekend I watched a crew of 6 putting on what appeared to be scraps of typar using a case of tape. Also no SAM on window frames etc.
Our climate is rainforest.
Awesome job guys.
John
Edited 9/21/2009 9:07 pm by JohnWalker
Edited 9/21/2009 9:08 pm by JohnWalker