Use PERSIST and put the WRB inboard of the sheathing OR use PUR nailbase panels and put the WRB outboard?
I’m insulating my house on the exterior – the design currently call for 9″ of Neopor with the WRB against (Prosoco – spray/roll-on ) the sheathing. The Neopor is outboard of the sheathing. 9″ is a lot – but gives me the R-40, without having to use any insulation on the interior. I’m getting a great price on the Neopor.
Just yesterday someone suggested PUR Nailbase panels instead of Neopor. PUR is a (polyurethane) panel, and would reduce the size of the overall wall thickness, is vapor impermeable, and fire resistant -higher embodied energy though .
If I go with the PUR nailbase I can go two ways….
1) I could buy one panel with the OSB already attached on one side and put that OSB to the exterior, while the second panel which would not have any OSB attached would go against the sheathing (inboard) that has the WRB on it.
OR
2) I could buy both panels with OSB, eliminate the sheathing (add racking to the frame), and put one nailbase panel against the framing and add the second sheet outboard of that. The WRB would goo on the outside of the outboard sheathing. This would make detailing around windows far easier, reduce the incidence of air infiltration via the nailbase seams, due to the offset panels. I’m using 4’L x 16’L panels either way, that go from footing to eave.
This situation # 2 though seems as though it could cause a problem if (when) moisture gets behind the WRB. With the PUR being vapor impermeable, the incidence of moisture driving into the insulation to meet up with the dew point is minimal, but even so – the OSB on the outside won’t let it dry right? The nailbase seems to be an “easier” install – especially #2 where the WRB detailing would be Faaaaar easier on the outside.
Here are two drawings depicting where the WRB could go and the two different wall make-ups.
One PERSIST type – and the other Nailbase scenario # 2.
Dana, Martin – anyone else???????
Thoughts or concerns
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Replies
Brad,
You forgot to tell us your climate zone (unless I missed it).
If you live in a cold climate, you need to consider the fact that polyisocyanurate foam performs poorly at cold temperatures, while the performance of Neopor (a type of EPS) actually improves at cold temperatures.
For more information on this issue, see In Cold Climates, R-5 Foam Beats R-6.
Martin,
I'm in climate zone 6A. Central NH. I'm not using the PIR (polyisocyanurate) foam panels which I know thermally degrades as it gets colder. I'm considering PUR (polyurethane) foam panels by Green Mountain Panel (formerly Vantem Panel). They don't degrade, are fire proof, and have a high R-value. They are vapor permeable though, so I'm trying to consider where the best place for my WRB. See the diagrams...
An issue with closed cell 2lb. polyurethane at higher-R is that it is usually blown with HFC245fa, which has a lifecycle global warming potential of ~1000x CO2, whereas polyiso is blown with pentane at only 7x CO2.
In a zone 6 climate only the outer ~20-25% of the total R would take a performance hit if it were polyiso,and the middle third would usually beat the labeled performance. If you want to be nice to the planet, using EPS or polyiso for the bulk of the R, safe the PUR only for the exterior-most R. With an intermediate layer of OSB you could use fiber-faced roofing polyiso, which is only a class-II vapor retarder.
There are water-blown 2lb. PUR foams, but most run about R5/inch instead of R6/inch. There are also HFO1234(....) blowing agents suitable for blowing 2lb foam with a GWP less than 4x CO2, but few manufacturers have moved over to those processes yet. If Green Mountain Panel will stipulate that they only use low GWP blowing agents for the foam it makes a real difference on the lifecycle impact of the product.
Dana,
Thanks. The GWP is definitely a major concern and I will follow up. Thanks for giving me the info to ask the right questions. Can you elaborate on the detail I talked about?
I'm not planning to sandwich OSB between the two layers of panels, but if I have two nailbase panels , with seams offset 2' and the outboard OSB has WRB on it - is there a concern that the OSB can't dry to the interior?
Brad,
Thanks for the clarification. I confused polyiso and PUR.
Martin,
Any comment or advice on the question posted?
Brad,
If you install two layers of nailbase panels with the OSB sides facing out, you are assembling site-built SIPs. I see no reason why this wouldn't work -- but of course another option is just buying SIPs.
I suppose the advantage of two layers of nailbase panels (compared to SIPs) is that you can stagger the seams.
If you go this route, you should probably follow Lstiburek's advice for SIPs -- namely, make sure to include vertical furring strips between the OSB and the siding, to reduce the risk of OSB rot.
Martin,
Thanks. My experience with SIPs is that the weak point is the seams. Regardless of the type of insulation I have considered I have always designed the offset panel into the design. Taping SIP seams has always seemed a weak and unsustainable element. I've always wished that SIP manufacturers would create a offset panel......