2×6 Sill and Top Plates w/ 2×4 Studs
sgaynair
| Posted in Energy Efficiency and Durability on
For an affordable townhouse project, we are considering 2×6 sill and top plates with 2×4 studs so we can put 2″ of EPS in the wall. There will also be a 2×6 around windows and doors.
My question is about condensation inside the wall between the EPS and OSB. Should I use 1-1/2″ EPS so there is a gap between the OSB and EPS? If anyone has done this, is it more efficient than a 2×6 wall with batt? I understand the EPS will make it a little more expensive.
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Replies
Unless you are bracing some other way I don’t think you will be able to do that. The reason zip r does is they have done testing for their specific product.
sgaynair,
The wall will perform much as though you used Zip-R as the sheathing. 2" of EPS will keep moisture from condensing on the inside face of the foam. Unlike exterior foam, you stack-up will keep the sheathing cold. The vinyl siding will help it dry. If you use other type of cladding I'd include a rain-screen gap.
A gap between the sheathing and foam doesn't help at all, and practically - how would you fasten the sheathing to your studs while still maintaining it?
Does using 2"x6" plates buy you anything you wouldn't have with simply building a 2"x4" wall and covering it all with foam?
Thank you for your response. EPS on the exterior of OSB is great for EIFS and brick exteriors. I have heard that real stucco and siding over EPS can sag over time since they are essentially cantilevered off the framing. I thought a direct connection to the OSB would be better.
With Zip-R as an option, it comes down to the cost of this detail with less expensive parts vs the cost of Zip-R over 2x4 framing.
sgaynair,
I think your home-made version of Zip-R may make sense.
Deleted
Your plan sounds very similar to extended plate and beam framing. If you haven't seen this report, it might provide some useful information. There is a chart comparing energy efficiency to other wall types on page 7: https://www.homeinnovation.com/-/media/Files/Reports/Extended_Plate_and_Beam_Construction_Guide.pdf
This is a great document - thank you for sharing!
I think the comparison chart is on page 13:
I don't think you'll have a problem here in terms of the insulation and moisture, and I agree with Malcolm that the gap is more likely to introduce the potential for moisture problems than to solve anything.
I would be concerned with the structural aspect of this assembly though. Zip-R has been through engineering tests, so if you follow Huber's nailing schedule, the panels can act as structural sheathing and provide shear/racking resistance for the wall. Your site built assembly won't have those engineering tests, so it won't pass muster in this regard. There are other ways to deal with the structural bracing though, such as let-in bracing. I'd have an engineer look at your project and suggest a bracing design that will work for you.
Bill
Zip-R with 2" foam is only rated for shear strength if the nails are 4" long, which few nail guns can provide, and those that do are too powerful to set the heads consistently flush with the surface. It's also not rated for high-wind areas. Your idea could work structurally if you incorporate another form of lateral bracing, such as let-in 1x4s or crossed metal straps.
That's what I did on my house - had a panelized manufacturer/erector add let-in metal bracing (cross straps) to a 2X4 exterior wall panel (no exterior sheathing)
I then covered the outside with a 2" Dow "Wallmate" EPS product (R10) with slotted grooves to accept 2X3 strapping (24" oc) that would lay flat (for a complete thermal break) - I installed these by screwing them into the studs (so horizontally on exterior walls). I then strapped the exterior wall vertically to create a rainscreen & hung my siding from that...
I then sprayed 2-3 " closed cell foam (R16) before adding sheet rock (I worked for a spray foam contractor so just paid for materials)