Flash and Fill + XPS (no OSB)
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| Posted in Energy, Heating & Insulation on
I am planning a new construction in zone 7 (cold). I will be using 2 x 6, 24″ oc advanced framing with 2″ closed cell spray foam and cellulose fill – as described in a recent issue of Fine Homebuilding. Could this method not overcome thermal bridging issues by swapping in 1″ of rigid foam XPS for the traditional OSB sheathing? Can I spray the foam directly on the XPS? Do any of you pros see any problems with this approach? Thanks.
Replies
If you are in an area the doesn't have Tornados, Hurricanes, or Earthquakes, then going without OSB might be a smart thing. Very few places are like that though.
Why would you use XPS, when PolyIso (the same stuff that is the "flash") is such a supperior product? And while we are there, if you don't go with OSB, why would you "flash" to begin with? Rigid foam board doesn't crack and isn't liable to poor installation from a rookie sprays tech. I'll bets it's cheaper to, and the seams are taped for the air sealing you were looking for with the spray foam.
Thanks for the sound advise.
OSB or not?: Of couse I intend the structure to be structurally sound, so maybe I need to go with full OSB. I guess I was thinking that hurricane ties + bracing or OSB in the corners only would do the trick. Anyone have thoughts on how much the structure would suffer with corner sheathing/bracing only vs whole house OSB?
XPS vs Polyiso: I thought XPS would be based based on its stiffness/rigidity (assuming no OSB) plus water resistance. But perhaps Polyiso is the best bet... Anyone else have an opinon on the best rigid insulation to use if not using OSB?
Why "flash" at all?: I was looking to get 45-50% or better of my total wall R value in the exterior foam board and flash coat. The logic is to keep water from condensing inside the wall. If I try and get that much R value in just the foamboad I'd need >2" which is a pain in the a$# (so I'm told, I have no experience here...). I also thought the spray foam would do a better job at air sealing than just the foam board. Comments appreciated.
Re OSB, you really may not need it, you may just need metal crossbracing. Your engineer will tell you.
Bah! I got my Expanded and Extruded backwards. I was thinking EPS - the white balls, instead of XPS, the harder board with almost the R value of PolyIso.
As far as differences, it's more about cost. R5 for XPS, R6.5 for PolyIso.
Yes, adding 2" or more can be a pain in the butt. But you also gain a heck of a lot of value. Replacing siding happens rarely in a houses life, so now is the right time to figure out how you want the house to be for the next 20 to 50 years.
Adding foam board on the outside commonly requires redoing the windows as you need to bring them out to the outside plane of the house. You need to built out nailers and framing for the windows, as well as applying nailers for siding. It's not hard, but it does require an eye for detail and is more work that just slapping everything right back up as before.
This is a good time to get the online subscription to FHB or JLC (or both) and look up things like exterior foam and deep energy retrofitting.
Unless of course you think energy is gonna be CHEAPER in the future :)
5n
Take a look at a Mooney Wall for aid in separating thermal bridging.
flash and fill
I don't think you need to worry about not having sturctural sheathing (OSB) on your framing. I framed 100's of houses in the 70's with celotex sheathing and let-in diagonal 1x4 bracing to keep the walls plumb and from racking. I think the question you need to ask, of the spray foam contractor, is if there is any negative reaction to spraying the back of the XPS sheets. My guess is the foam is most often sprayed against the back of OSB or plywood. If you don't get an answer from a spray foam contractor, you could always ask the spray foam manufacture.
... I framed 100's of houses in the 70's...
In 40 years we've learned alot, specificly about preventing wind and seismic damage.
My house is still standing - built in 1960 - with just 1x6 let in bracing on every wall, even non-structural walls... but then again, we still haven't yet had "The Big One".
Lots of structures were still standing just fine in Northridge... then the Northridge earthquake happened.
Let in bracing has its place, in areas not constrained by seismic or high wind events.
No reason I can see
to add "flash" foam to the back of rigid foam. Flashing is for air-sealing and possibly for dewpoint control if you make it thick enough. In your case I would simply add the correct thickness of rigid foam to the outside of your frame and then dense-pack the stud bays with cellulose.
Why not put the rigid foam over the OSB? My brother i having a cairrage house built and they put 6" of rigid foam over OSB sheathing.
From everything I've read so far, you need at least 50% of the R-value in foam to prevent condensation.
the perfect wall - ...again
Thank you for all the constructive comments.
Let's assume I am going with 2x6 construction and no fancy deep wall or moody wall and I keep the full osb sheathing to be structurally on the safe side (strong).
Lets assume some spray foam in the cavity for great air sealing and high R (full fill if the bank account holds out, flash and fill with cellulose more likely).
Let's assume I want rigid foam on the exterior for thermal bridging but max 1.5" in order to stay out of trouble with tricky details around window, doors,and with long fasterns etc.
If I go this way I've basically got a Joe L 'perfect wall' http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall many are familiar with. BUT... I don't have the necessary R value on the exterior of the osb for zone 7 - I would need 3" rigid foam or so. If I don't use that much exterior rigid foam will I run into trouble with condesation on the OSB? Or does the interior spray foam save the day?
Target R35 with no unusual methods used - ie easy for trades not to screw up.
Thanks!
OK, in that case, I suggest spinning the insulation layer/thermal break/vapor barrier around.
Outside to in:
Siding
Air barrier (Tyvec or similar)
OSB
2x6 walls filled with cellulose
Inside face foam "flashed" too...
2" ridgid foam board
Taped and sealed seams
Nailers
Drywall
The exterior gets built as normal, except the OSB doesn't get put on until the foam is flashed. The inside is normal in that drywall is hung on the nailers. Outlet boxes are attached to the nailers instead of the studs, wire is run in the rigid foam instead of through holes in the studs.
The taped and sealed INTERIOR face of the foam board keeps water vapor out. The exterior face of the studd assembly lets any water vapor inside travel back out to the outside.
Yes, the trades will be able to do this.
I wouldn't risk it
If I don't use that much exterior rigid foam will I run into trouble with condesation on the OSB? Or does the interior spray foam save the day?
Look up the Green Building Advisor article on calculating the thickness of foam sheathing.
Personally I wouldn't risk a foam sandwich like that. Just figure out a way to install the 3" you need on the exterior. It's not that big of a deal.
Thanks davidmeiland. A little dose of common sense I guess was all I needed. Cellulose fill, OSB and thick foam. Simple. Smartest thing I've heard so far.
Thanks!