Building a ‘Layered House’
Paying attention to the various layers in your building envelope is important for ensuring airtightness and moisture management
If you are building an energy-efficient house, you have to address air leakage and pay attention to the the integrity of the insulation layer. We can have the best of intentions and can install lots of insulation, but if we leave it leaky or include details that compromise the integrity of that insulation, then the home’s energy performance can be severely affected.
Take recessed ceiling lights, for example. From a design standpoint, they’re great, since the light source is roughly flush with the ceiling and all of the mechanism is hidden in the ceiling above (in recessed cans).
In a house with an unheated attic (insulation in the attic floor — which is the ceiling of the floor below) or with an insulated, sloped cathedral ceiling (roof), if we install recessed cans into that ceiling we’ve created a significant pathway for air flow and compromised the insulation. This is the case even with recessed lights rated for “insulation contact.” Those IC-rated fixtures are far better than older models that required a significant air space surrounding the lights, but they still result in significant air leakage.
Creating an access ceiling that looks good
One of the solutions to this problem is to create an access ceiling (or dropped ceiling) below the air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. of the insulated ceiling. Recessed lights can be installed in such a ceiling. Lest images of acoustic ceiling panels in commercial office buildings come to mind, rest assured that access ceilings can be done in a very attractive way.
Tedd Benson has been doing this for years with Bensonwood homes using his OpenBuilt platform, and our designer-builder, Eli Gould, has his own access ceiling detail that he’s using in our Dummerston home. He’s using this layered, access ceiling detail on both the first-floor ceiling (which is not insulated) and for a horizontal section of the second-floor ceiling, spanning between the insulated slopes and insulated rafters.
Eli builds roughly square panels out of painted 1x10 shiplap boards — three boards per panel. These boards drop in and can easily be lifted up to access the recessed lights (which in our house will house CR6 LED lamps from Cree). On the first-floor ceiling, these panels fit into tracks formed by added attractive beams that strengthen the ceiling joists.
We installed installing recessed lights in these ceiling panels, as wells as registers for our Zehnder heat-recovery ventilator. The ceiling cavity above the panels provides a space to run wiring, ventilation ducts, and — in some locations — plumbing. Future modifications to any of this can be made very easily.
Airtightness also depends on layers in walls
Our superinsulated wall system has seven layers: from the interior there is the layer of gypsum board; the wall cavity with fiber insulation; a taped and air-sealed sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. layer (using Huber’s Zip sheathing) that serves as the air barrier; a layer of exterior rigid insulation on the outside of the sheathing; a layer of waterproof but vapor-permeable housewrap (water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material.); a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. (vented air space) formed by vertical strapping; and finally, the factory-painted wooden clapboard siding.
By keeping the air barrier in the center of the wall — with cavity-fill fiber insulation on the interior — wires can be run through the that insulation without compromising the air barrier.
Effectively insulating a wall cavity with wires running through it should be done with something other than batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. . Cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. (dense-packed or damp-spray), fiberglass (dense-packed or spray), or spray polyurethane foam (closed-cell or open-cell) all fill well around wires. As I described in a blog a few weeks ago, for our house we used Johns Manville Spider spray fiberglass insulation, which has an acrylic binderGlue used in manufactured wood products, such as medium-density fiberboard (MDF), particleboard, and engineered lumber. Some binders are made with formaldehyde. See urea-formaldehyde binder and methyl diisocyanate (MDI) binder. to hold the insulation in place.
Wiring for wall outlets can also be contained in baseboard raceways. This is a detail that Benson uses with his OpenBuilt wall system — and one that Eli uses on some projects. It totally avoids running wires in the insulation, allowing easy modifications later, and it’s an ideal solution for panelized construction (in which wall panels are built in a factory and trucked to the job site). We considered such a system, but it would have added a lot of cost.
With our air barrier in the middle of the wall, the cavity-fill insulation can dry to the interior, and the exterior insulation can dry to the exterior. More and more building science experts seem to be recommending this approach. We’ll find out how it worked — or someone will — in 20 or 50 or 100 years when a totally dry wall system with no rot will be evidence of good moisture management.
We don’t yet know how good a job we have done with air sealing at our house. I’m hoping that we will end up with an air leakage rate as low as 1.0 air change per hour at 50 pascals of pressure difference (ach50) — as measured by a blower door. That will be far tighter than the average new home being built today, but still considerably leakier than a house built to the rigorous Passivhaus standard — which requires an air leakage rate of 0.6 ach50.
Even if the news is embarrassing and we don’t get to 1.0 ach50, I promise to report that here. If we don’t make it, it will likely be because some elements of our 200-year-old frame necessitated complex detailing with the sheathing layer or because we didn’t spend the money needed for the best Passivhaus windows and doors. But I’m optimistic.
Look for a future blog on the LED lights once they are installed.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
- Alex Wilson
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