More and more builders have realized the advantages of leaving stud bays empty and putting all of a home’s insulation outside of the wall and roof sheathing. If done correctly, exterior insulation can help produce a building that is almost airtight, very well insulated, and almost immune to water damage.
The construction method was first developed in the early 1960s by the National Research Council of Canada. In its purest form, the method is known as PERSIST — an acronym for Pressure-Equalized Rain-Screen Insulated Structure Technique.
Here’s how you build a PERSIST house:
- Frame the house with 2×4 walls. Even in a very cold climate, there is no need for 2×6 framing.
- The building should have no eave or rake overhangs. All walls and roofs meet as simple intersecting planes.
- After the walls and roof are sheathed with plywood or OSB, the sheathing is covered with a peel-and-stick rubberized membrane like Grace Ice & Water Shield. Because there are no overhangs at the eaves and rakes, the membrane can be folded down at the roof edges onto the walls.
- The walls and roof are then covered with at least two layers of rigid foam. The thickness of the foam varies depending on climate; while 2 inches of foam might be adequate in Florida, Alaskan builders are likely to install at least 6 or 8 inches of foam. Successive layers of foam should have staggered joints. The foam is mechanically attached to the sheathing.
- Vertical 1×3 or 1×4 strapping is installed on the walls to create a rainscreen, and siding is installed over the strapping. No housewrap or water-resistant membrane (WRB) is necessary, although one can be used if desired.
- The roof foam is topped with 2×4 sleepers, installed in the same position as the underlying rafters. The 2x4s are mechanically fastened through the foam to the rafters with long screws. The 2x4s are cantilevered at the eaves to create an overhang. Ladder-framing (outriggers) are used at the rakes to create a gable overhang.
- A second layer of plywood or OSB is installed on top of the roof sleepers, creating a ventilated cold roof. Roofing is installed normally.
“It going to rot!”
To some builders and building inspectors, PERSIST details seem counterintuitive or dangerous. One typical reaction is, “You can’t install peel-and-stick over your wall sheathing! It’s a wrong-side vapor barrier! The membrane will trap moisture! The walls can’t dry out!”
Actually, the peel-and-stick works perfectly. The membrane acts as a combined air barrier, vapor barrier, and water-resistant membrane (WRB). Because the membrane completely seals the walls and roof, it produces an unusually airtight envelope.
Since the membrane is on the warm-in-winter side of the insulation, it’s exactly where it belongs. All of the home’s framing and sheathing is on the conditioned side of the membrane, so these wood components are maintained at indoor conditions. That means they aren’t subject to swings in humidity or temperature; the framing stays stable and dry in all seasons, in all climates.
On the exterior side of the membrane, there aren’t any components which are likely to suffer any moisture damage. Since the system includes a rainscreen behind the siding, any water that gets past the siding drains quickly from the walls.
PERSIST has a few disadvantages. It costs more than conventional construction, because of the cost of the peel-and-stick membrane, the two layers of roof sheathing, and the labor required to build the roof overhangs separately from the main structure. Moreover, integrating windows into the system takes some head-scratching and attention to detail.
But the added cost of PERSIST buys many advantages:
- Air leakage rates are extremely low.
- The exterior foam limits thermal bridging through studs and rafters.
- Less expensive 2×4 studs can be used instead of 2x6s.
- The air barrier and vapor barrier are installed in a single step. Installing the air barrier outside of the sheathing is far simpler than creating an airtight layer of polyethylene or drywall, since there are no problems with intersecting partitions. Complicated roof shapes with multiple dormers, valleys, and cathedral ceilings are easily accommodated without hard-to-detail air-barrier seams.
- Since the structure and sheathing are insulated, they stay warm. Cold surfaces where condensation might form have been moved to a location exterior to the waterproof air barrier. If there are any imperfections in the membrane installation, resulting condensation simply drains away.
- Since the rubberized membrane is such an effective barrier, PERSIST works well for high-humidity rooms like indoor swimming pools or greenhouses, where other methods depend on meticulous vapor-barrier detailing.
- The system easily accommodates cathedral ceilings with exposed framing.
- The system has the well-known advantages of cold roof construction and rainscreen siding installation. Siding lasts longer, and damage from ice dams never occurs.
- Polyethylene under the drywall is eliminated, so drywall can be glued to the studs if desired.
- It’s easy to install wiring and plumbing in exterior walls, since the stud bays are empty of insulation. There is no need to worry about electrical outlets penetrating the air barrier.
- Plumbing and electrical rough-in can begin as soon as the self-adhering membrane is installed.
- The system provides usable conditioned attic space.
According to Chris Makepeace, a certified engineering technologist at Alberta Infrastructure in Edmonton, Alberta, and a proponent of PERSIST construction, PERSIST homes are often so tight that they are hard to test. “In one of the houses that we tested with a blower door, the place with the most air leakage was between the slab and the grade beam, where the air had to travel through three feet of soil,” said Makepeace.
Will it work In Florida?
Canadian builders aren’t the only ones interested in PERSIST. “Regardless of what climate zone it is in, this system will work,” Makepeace explained. “In Florida, the inside of the building is air conditioned, but you have conditions where at night the humidity is just out of sight. If that warm humid air contacts a cold surface, where will the condensation occur? With typical construction, it occurs in the stud cavity, and you can get mold growth in your buildings. In a PERSIST building, the condensation occurs outside of the membrane, and it won’t hurt the wall. Any condensation will drain down.”
Building scientist Joe Lstiburek agrees with Makepeace on this point. “It will work from Fairbanks to Miami,” he said.
PERSIST in Texas
By now, PERSIST details have spread far beyond Canada. One Texas convert is Austin builder Ray Moore. At a building science conference in 2000, Moore asked Makepeace for tips on air-sealing electrical boxes. “I got an answer that forever changed the way I build houses,” Moore later wrote. “He asked me in return why I was trying to stop the air movement at the level of the drywall when that was such a difficult location to seal with all the complex geometry in the interior of a typical custom home. The intersections of interior walls, electrical outlets, furr-downs, stairs on exterior walls, tubs on exterior walls, wire and pipe penetrations, and upper floor rim joist areas are all examples of detailing that is almost impossible to get airtight. Chris introduced me to PERSIST.”
Moore continued, “Since using this approach we have cut our cooling costs to less than half of our already low-consumption building methods. … Our homes have been blower-door tested at 0.76 ACH50. … The vast majority of structural rot and water damage is due to poor flashing design and poor or nonexistent application of flashings and drainage planes. All windows should be pan-flashed and head-flashed with end dams. All intersections of walls and extending structures should be properly flashed and water managed. I find this to be much easier to accomplish with a modified bitumen (rubberized membrane) than it ever was with felt paper or other housewraps.”
PERSIST with a twist
There are many PERSIST fans in Alaska, where builders have long understood the importance of airtight construction details that prevent condensation in building assemblies. In recent years, the Cold-Climate Housing Research Center (CCHRC) in Fairbanks has been promoting a modified version of PERSIST called REMOTE — an acronym for Residential Exterior Membrane Outside Insulation Technique. The REMOTE method marries PERSIST walls with a conventional unconditioned attic.
It’s almost always cheaper to build a REMOTE house than a PERSIST house. However, for the REMOTE system to work, builders must pay close attention to air barrier details at the top of exterior walls. The CCHRC has produced a useful manual on the technique, “REMOTE: A Manual”.
Alaskan builder Thorston Chlupp wrote a detailed and useful article on the REMOTE technique for the May 2009 issue of the Journal of Light Construction.
Alaskan builders are experimenting with changes to the PERSIST system. To save money, some have switched from peel-and-stick membrane to 6-mil poly or plastic housewrap.
Instead of leaving stud bays empty of insulation, some REMOTE builders fill them with fiberglass or cellulose. This practice is potentially dangerous, however. The more insulation in the wall, the greater the possibility that the wall sheathing will get cold enough to allow condensation to form. That’s why most PERSIST builders prefer to keep framing bays empty, with all of the home’s insulation outside of the rubberized membrane.
Last week’s blog: “High-Solar-Gain Glazing.”