Don’t call it a comeback…
I’ve written about exterior foam insulation as a wall concept before, but thought I’d touch on a few of the specific systems out there: the PERSIST and REMOTE approaches. These are two incredibly well insulated, air tight approaches to creating an efficient, green house.
What is PERSIST?
The PERSIST (you may have noticed that building science has a dire shortage of acronyms) wall system stands for: Pressure Equalized Rain Screen Insulated Structure Technique. Catchy. This wall assembly is basically what you get when building scientists blow up the standard wall concept and start from scratch.
Where would you locate the vapor barrier and how would you vent it (or would it be vented)? How would you frame the walls and what about eave overhangs that exist mostly to give homeowners a place to store their ice dams?
The PERSIST system tackles all this stuff. The system locates most, if not all the insulation on the exterior, ensuring the wall cavity stays warm.
The features include:
– The vapor barrier is typically a rubberized asphalt peel and stick membrane. This is installed directly on the exterior plywood or OSB sheath, beneath the exterior foam.
– No eaves or gable overhangs. This gets toward the ‘simple is easier’ concept. By removing the kinds of complicated framing elements like one finds in McMansions or Colonials, you create a perfect continuous impermeable barrier. When the peel and stick membrane is applied, it can be wrapped down over the roof edges.
– Exterior foam board insulation, typically 6+ inches of foil-faced polyisocyanurate. This wraps over the entire house, roof and walls.
– The framing walls are left uninsulated. The exterior foam board is doing that job. The walls can be built with 2 x 4 studs as there’s no need for deep wall cavities to accommodate insulation. The empty wall cavities are used exclusively for electrical and plumbing services.
– Siding is attached to the exterior with vertical strapping. The strapping allows for a back vented drainage of the exterior.
– A vented roof is created by essentially building a standard vent roof outside. 2 x 4 are installed on edge, fastened to the rafters through the roofing foam. OSB or plywood sheathing is installed on the 2 x 4s, creating an exterior vented roof.
So that’s a PERSIST wall system – what are the ups and downs?
R-Value – Let’s get the obvious one out of the way. PERSIST systems can be built with much, much higher R-values than a standard framed building. If built with 2 x 4 framing with no cavity insulation and 6 inches of exterior polyiso foam board, the R-value would likely exceed R-30 with no thermal bridging.
Thermal Bridging – The PERSIST approach entirely eliminates wall and roof thermal bridging. Where standard stud walls can have their R-value compromised up to 20%, the PERSIST exterior foam approach would work very close to the advertised R-value.
Ease of Installation Part 1 – This is split up, as there are aspects of PERSIST much more challenging than standard framing. But there are some simpler aspects like that the 2 x 4 framing is easier and less expensive.
Also, the empty cavities allow easy access for service installations and the drywall doesn’t need to be airtight (the peel and stick layer does the job). Instead of an easily perforated poly layer or detail work on an airtight drywall system, the drywall is just installed. This leads to:
Air Leakage – The PERSIST approach builds very tight houses. So tight that mechanical ventilation is required and it routinely meets any CFM50 housing standard.
Moisture – The PERSIST approach is very good at dealing with moisture, though this runs counter to standard building thought. The vapor barrier in cold climates is located on the ‘inside.’ Traditionally, this means a layer of poly plastic stapled to the studs.
With PERSIST, we locate the peel and stick membrane on the exterior of the wood sheathing, but inside the insulating foam. So, the vapor barrier is still on the ‘warm’ side of the insulation, preventing dew point issues and condensation. It just happens to be outside.
Because the peel and stick membrane acts as the water resistant barrier as well, it combined with the rain screen to drain and dry the exterior wall. The only downside to all this is the requisite attention to flashing and water control.
Green – We’ll give the system a tentative green thumbs up. It is largely replacing one material for another (for example, cavity fiberglass for exterior foam board) and it is much more energy efficient. Using 2 x 4 studs and polyisocyanurate foam, both are greener options than typical 2 x 6’s or XPS foam.
Getting right to it …
Cost – PERSIST costs a good deal more than standard framing. Between the peel and stick membrane, the extra polyiso insulation (especially if the cavities are still insulated) and the added roof structure, all add a considerable chunk to the building expense. This is leavened somewhat by using 2 x 4 framing and no cavity insulation.
Material – Ties in with both the green and cost aspects but PERSIST framed buildings use a lot more material than a standard framed house. Yes, we’re downsizing from 2 x 6, but we’re adding 6 inches or more of polyisocyanurate. And on the roof, you’re adding an entire additional structure with eave and gable overhangs on top of the foam.
Ease of Installation Part 2 – Yes, the wall cavities may be easier for service installations. But from no other angle does it require less effort. The roof structure requires extra material and effort, all the foam board must be carefully staggered and taped, all the windows and doors need to be boxed and flashed with close attention to water management details. Altogether more effort.