Is PERSIST really practical in coastal Maine?
Hello, all! We are in the planning stages for building a ~1300sft home in coastal Maine (6A, I believe), and are trying to strike a balance between performance, low-maintenance and cost. We’ve found Persist to be an elegant solution, and we really like the look of having the exposed framing on the interior – Matt Risinger’s work in Austin comes to mind.
https://architizer.com/projects/perfect-wall-house/
For us to reach the desired R values of the walls, there seems to be good details and best practices out there for fastening thick amounts of foam. We are aiming to use 8 inches of EPS with a rainscreen and metal siding. For the roof, however, we are more concerned… If we lay 12 inches of foam over the OSB/peel and stick, then lay sleepers down, OSB and metal roof on top of that, how on earth are our contractors going to be able to drive screws in and actually hit the rafters without poking a hundred holes in our airtight envelope? Are we asking for trouble?
There don’t seem to be many examples of people pulling this off, or maybe I’m not looking in the right places. Is this why REMOTE was developed? Does Persist simply make more sense in Virginia or Texas than in Maine? Are we off our rockers, trying for Persist on the roof?
Also, are there good replacements for Grace Ice and Water Shield for wrapping the house? Looks like SIGA’s Majvest 500 SA might be a good no-VOC alternative?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
User-700 etc.,
First of all, can you tell us your name?
GBA does not advise people to use a strict PERSIST approach, mostly because it costs more than simpler approaches.
For more information on walls, see this article: How to Design a Wall.
When it comes to roof assemblies, the best approach is to install only enough rigid foam above your roof sheathing to keep your sheathing above the dew point -- in Climate Zone 6, that would be R-25 of rigid foam -- and then install the rest of the insulation between your rafters. For more information, see Combining Exterior Rigid Foam With Fluffy Insulation.
If you insist on keeping all of your roof insulation on the exterior side of the roof sheathing, you may want to install SIPs (structural insulated panels) or nailbase on your roof.
Apologies - I updated my profile screenname, but it doesn't appear to be "taking"... My name is Geir.
Thank you for your response - it looks like if we ran 8 inches of EPS all around the outside, then blown in cellulose or perhaps roxul batts between rafters, we'd be golden.
That's far less distance to be driving a screw to hit the rafters...
Geir,
The apparent simplicity of that very appealing house Matt Risinger built in Austin is the result of a lot more hidden complexity and planning than I would want to attempt. Having exposed framing and the sheathing being the interior finished surface means a completely different sequence of construction.
Hitting the rafters and wall studs is difficult in regular Persist construction, but doubly so when the consequences of missing results in marring your finished interior surfaces. Similarly, the planning and execution of plumbing and electrical services becomes much more complex than it is in typical construction. You also have the added task of creating continuity between the interior structure and any porches or roofs surrounding the house.
As Matt's house shows, it's very doable, but not for the faint of heart. If you do go this route please consider blogging your build for GBA. I've love to see it.
Would you consider glueing your insulation layers down? That could save a lot of potential shiners.
I personally like it, it's easier work and less thermal bridges. I used 3"+3" polyiso on my recent flat roof. you can get thicker slabs of foam to make the install more efficient, I'm sure you can get 4'x8'x6" sheets and glue two layers down. In fact, I'm about to order 4" thick EPS for my sub slab insulation
that doesn't help for your sleepers, But at least at that point you're screwing a lot less and minimize potential for shiners. having a good jig/system in place goes a long way.
Btw, Do your sleepers go under or over the osb? If my memory serves me from Last time I watched Matt's video I don't think he had osb on the roof, I believe the point being to have good ventilation under the roof decking. Just like in a rainscreen wall Assembly.
Wow, that Risinger home in Austin is gorgeous. It brings back memories of endless hours of being a kid watching my dad sort lumber. That was fifty years ago; where do you get wood like that today?
One thought on SIPs, if you decide to go that way.
Murus offers a SIP with factory applied tongue and groove face: http://www.murus.com/specs/clad-2100eps Could work for you depending on how you plan to orient and support the panels as well as the detail for the eaves of your home (field built versus an extension of the SIPs).
Hi Geir, good to see you posting here. A few ideas to consider:
Use TJI rafters. The top flange is a wider target to hit. They are a bit of a pain to install and cost more, but are lighter and straighter than dimensional lumber.
Use Neopor or polyiso foam, at R-5 and R-5.6/in respectively, rather than regular EPS.
Make or buy a jig for installing screws perpendicular to the roof plane. Most skilled tradespeople have a good eye for this sort of thing, but not all do, and you're right that 1 1/2" is a small target.
Install the foam (or nailbase, or stress skin) before insulating the interior, so you can spot any shiners.
Use a separate roofing underlayment (WRB for the roof) so a few holes in the airtight layer don't matter as much.
Use structural SIPS instead of PERSIST. Not my favorite approach because SIPs are problem-prone, but properly detailed they should work fine. You could add fake interior framing if you want the look.
At our building science discussion group in Portland we just discussed self-adhered membranes. Siga's is brand new but looks to be a good product. Cosella-Dorken/Delta has one that is robust (Steve Thomas from This Old House brought a sample, recommended personally by Matt Risinger and Joe Lstiburek, but somewhat more expensive than the alternatives). Henry Blueskin VP100 is the one most of us are familiar with. PERSIST was developed before these low- and no-VOC, vapor-permeable products were available.
Side note: if you're interested in joining the discussion group mailing list, contact them here: http://performancebuildingsupply.com/.