When looking for the essential nature of construction, the architect Louis Kahn famously asked: “What do you want, Brick?”. Today, in looking at the nature of high-performance building enclosures, we shift the question and ask: What does a building want?
Our answer is that a building wants a vented rainscreen. We argue that this approach works in pretty much the entire USA (and Canada). Rainscreens offer the following key benefits:
- They keep the majority of rain off the waterproofing membrane.
- They keep the sun off the weather resistant membrane (WRB).
- They provide optimum drying potential of the enclosure when it is cold.
- They help maximize the performance and life expectancy of the waterproofing and wind/air barrier.
The forces that accomplish this are quite easy to understand. You wonder why we build any other way?!
The first task at hand for a building enclosure is to keep the water out (see the ground rules in an earlier blogpost). A rainscreen does this beautifully. The weathering (wood, hardi-) boards on the exterior protect the wall from bulk water and wind impacts, thus protecting the actual windbarrier/WRB from the toughest elements of nature. The WRB is protected from driving rain and can shed water effectively under much less stress, sheltered by the rainscreen. The SOLITEX Mento WRB by Pro Clima is waterproof, airtight and vapor open at 38 perms (Sd-value of 0,05m). Connected to window and door frames with CONTEGA EXO adhesive tape, a complete waterproof, airtight and vapor open WRB can be realized.
Next we must make the building airtight. As noted above, the WRB (SOLITEX Mento) behind the rainscreen provides a windtight/airtight layer, and protects the fibrous natural insulation. Wind-washing of insulation, especially at corners, can reduce the insulating properties of materials by nearly a factor of 5. Ideally an air barrier is also placed at the interior side of the insulation – best done with an intelligent vapor retarder (INTELLO Plus) or taped plywood on the warm side of the insulation. An interior service cavity will help ensure a successful airtight result.
When fibrous insulation is protected by a windtight/airtight layer on the outside and an airtight layer on the inside, it is optimized for maximum thermal performance.
Next in importance is moisture/vapor control. A rainscreen is a huge asset in addressing solar diffusion and vapor drive. Shaded by the rainscreen, the membrane is not exposed to direct radiation and solar heat. The rainscreen itself is also able to dry quickly after a rainfall. By minimizing both the rain water and direct solar radiation on the (vapor open) membrane, solar driven diffusion into the insulation layer on sunny days is also minimized. (See this ORNL article for more info on solar driven diffusion benefits of vented rainscreens.)
The second part of moisture/vapor control is to maximize the drying potential by letting more moisture out than enters the building enclosure. For this to happen, we first want to make sure that the exterior is as vapor open as possible, which will allow the wall to dry outwards in the winter. As noted above, SOLITEX Mento 1000 (and Mento Plus) have perm ratings of 38 and therefore provide maximum drying potential in the cold season – while also completely windtight and waterproof.
Together with insulation layers such as blown-in cellulose between the studs, a wall with any desired R-value is readily achieved – and a wall design that protects both the interior spaces and the enclosure assembly against unhealthy elements such as foam, rot and mold.
Finally, a rainscreen provides physical protection from brute force actions. Protected from physical abuse, the water, air and thermal control layers can perform optimally for maximum life expectancy.
Orient Point Passive House by Ryall Porter Sheridan Architects
Passive House Consultant: Right Environments