Posted By Bob I on 15 Jun 2016 08:21 AM
Go to the source! You need to talk to the good folks at https://foursevenfive.com. Send them a section of your wall and ask for their advice on the best and most robust methods for air sealing.

You are right that plywood will not stop the frame's movement. Plywood (or any boards), however, will not crack & fall apart like 1" foam will. EPS is a very fragile product, and it sounds like a very flimsy wall overall.

Are you saying that plywood will make the difference structurally between a very flimsy wall and a solid one in this case, Or are you referring to the air-sealing aspect when you say flimsy? I don't want the air sealing to be flimsy and would address that as necessary. Please clarify. I'll admit I have a hard time regarding the assembly as structrally flimsy. Structurally, the building as a timberframe is of the highest integrity. The insulation frame is soundly bolted to the concrete slab, as well as to the timberframe. With nailers at 2' centers throughout as well as let-in diagonal bracing it's perplexing to me how this non load bearing assembly could be regarded as structurally flimsy.

I designed, cut and built timber frames for many years so I know they are (or can be) structurally solid, but have you ever even held a 4x8 sheet of 1" EPS? Pick one up, move it around, bend it, work with it & you'll understand what I mean. You could push your finger thru your 2 layers of EPS without getting scratched.

Thank you for sharing the information.

I'd like to revisit this topic, having several unanswered questions and quickly approaching the completion of the exterior light frame.

I got advice here to wrap the outside of the studs with an Intello product. My understanding was that it was installed against the interior face of the studs and the cellulose was blown in behind. Is it recommended to install both an interior and exterior layer?

I firmly hold that my wall detail is, structurally speaking, very sound. Research I've done as well as my engineer confirm that, in my circumstance, plywwod or osb isn't necessary, and foam sheathing directly to the studs will function well. Is the consensus that this is a bad idea? I'm not trying to save the 1000 bucks I can sheathe the house in plywood with, but if I don't need it, I can reduce the "footprint"of my house even more.

I greatly appreciate the responses I've gotten. Having worked in construction all my life, this site's willingness to address the highest quality building practices is a breath of fresh air. Thanks

I see no problem with wood paneling/taped Intello/rigid frame+cellulose/taped 2" XPS/wrap/siding.

Standard Insulation can be best option.

Posted By Bob I on 15 Jun 2016 08:21 AM
Go to the source! You need to talk to the good folks at https://foursevenfive.com. Send them a section of your wall and ask for their advice on the best and most robust methods for air sealing.

You are right that plywood will not stop the frame's movement. Plywood (or any boards), however, will not crack & fall apart like 1" foam will. EPS is a very fragile product, and it sounds like a very flimsy wall overall.

EPS comes in many forms and densities. You will not put your finger through 60 psi EPS. http://www.insultech-eps.com/insulation.html

I like plywood sheeting for the sound attenuation and XPS, EPS or polyiso Thermax on the exterior. Despite what many here have reported, an exterior sheet of insulation will overcome the significant heat loss of thermal bridging and is one of the easiest assemblies to inspect for initial quality.

http://thehtrc.com/2013/common-sense-building-exterior-wall-sheathing

https://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh

This is a pretty old thread to be reviving but...

Even 1.5lb density Type-II EPS (typically rated 25psi) isn't something you'll be poking your finger through (unless you're a Kung Fu master :-) ), and is rugged enough to be the go-to material for insulated concrete forms, or walk-able under torch down asphalt or membrane roofs.

At lower density than Type-II EPS is more fragile, sure! Poke a finger through it? Yes, unless it comes with polyester or polyethylene facers.

2.5" of Type-II EPS literally doubles the thermal performance of a 2x4 / R15 studwall.

A mere 1.5" of Type-II (or even Type-I) EPS on a 2x4/R13 wall outperforms a 2x6 / R23 studwall.

The increased resilience of having sufficient foam-R for dew point control at the sheathing far exceeds that offered by magic-membrane interior side vapor retarders like Intello Plus or MemBrain.

Posted By Dana1 on 11 Oct 2016 05:24 PM
This is a pretty old thread to be reviving but... The increased resilience of having sufficient foam-R for dew point control at the sheathing far exceeds that offered by magic-membrane interior side vapor retarders like Intello Plus or MemBrain.

Well, you had to give me more to think about, didn't you. At the risk of betraying more ignorance than I'd like... in a perfect world would the correct thickness of exterior foam installed with all joints and penetrations completely sealed against any infiltration reduce or negate the need for an interior vapor retarder?

You have to pick one. I have done both but not in the same all stack-up. In new construction the exterior plan is most common.

Posted By dfvellone on 11 Oct 2016 06:01 PM

Posted By Dana1 on 11 Oct 2016 05:24 PM
This is a pretty old thread to be reviving but... The increased resilience of having sufficient foam-R for dew point control at the sheathing far exceeds that offered by magic-membrane interior side vapor retarders like Intello Plus or MemBrain.

Well, you had to give me more to think about, didn't you. At the risk of betraying more ignorance than I'd like... in a perfect world would the correct thickness of exterior foam installed with all joints and penetrations completely sealed against any infiltration reduce or negate the need for an interior vapor retarder?

Even in an imperfect world with less than perfect air sealing, having sufficient R-value on the exterior eliminates the need for interior vapor retarders (which are nearly never installed sufficiently air tight everywhere to be risk-free.)



The IRC recommendations in Table R701.7.1 in Chapter 7 based on framing size are minimums- it's safer to pad it a bit.



http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%207.html

I'd been planning on using intello on the interior that would double as the netting for dense pack cellulose. With sufficient r value on my exterior is it an unecessary redundancy that I can replace with the standard insulweb that the cellulose contractor recommended, or is it worth the added expense for added protection?

Different purposes - Insulweb is designed to let air pass through so the cellulose fills the bay without air bubbles; Intello is an air barrier. You can use it for your purposes; you need to slice holes in the fabric and tape them afterward.

If you are installing sufficient insulating sheathing to have dew point control, Intello Plus doesn't provide any significant additional resilience, and there are cheaper ways to air seal a wall. Investing the difference in cost between Intelllo and Insulmesh and applying it to thicker exterior foam would be a better investment.

But it's fine to blow behind Intello using a few slits and taping them when our done, as Bob points out.

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