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Energy Solutions

Building a ‘Layered House’

Paying attention to the various layers in your building envelope is important for ensuring airtightness and moisture management

A ceiling service cavity. Ducting for our HRV, electrical wiring, recessed lights, and plumbing fit into this access ceiling.
Image Credit: Alex Wilson
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A ceiling service cavity. Ducting for our HRV, electrical wiring, recessed lights, and plumbing fit into this access ceiling.
Image Credit: Alex Wilson
Recessed lights are mounted in the access ceiling panels. These will hold Cree CR6 LED lamps, which provide an attractive warm light. A ceiling panel made from three sections of shiplap pine. The access ceiling being installed The nearly finished ceiling at our house. We used Johns Manville Spider insulation (a spray fiberglass) to insulate the wall cavities, including the hard-to-reach band joist area, which is being insulated here. The air barrier is a layer of sheathing between the Spider insulation and a 6" layer of exterior insulation. The Spider insulation fills very well around the wires extending through the interior wall cavities.

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 barrier 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 1×10 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 sheathing 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 barrier); a rainscreen (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 insulation. Cellulose insulation (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 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.

Testing airtightness

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.


  1. David McNeely | | #1

    Suggest sealing the shiplap.
    If I was doing the finish work, I would seal all six sides of the shiplap to prevent cupping across the 10" width of wide-grained softwood. Splitting is also a likelihood since the pieces are so short, and I might use slots for the screws on the back to allow for movement across the grain. I would be especially careful to seal around the holes cut for recessed lighting—you will literally be cooking some of that wood, artificially making it expand and contract when you turn the lights on, then off. LEDs may be efficient, but they still give off a lot of heat.

    After all, the wood available to us these days is not like the old-growth you might find in some of the older homes.

  2. Gordon Taylor | | #2

    Great article, Alex.
    Many thanks for posting it.

  3. John Alden | | #3

    What about FEWER layers? (Combine Sheathing/Siding)
    Hello, frequent reader, infrequent commenter.
    I'm very much interested in building extremely sustainable but cost-effectively efficient homes.
    My general favorite wall assembly is a double wall dense packed cellulose with ply sheathing, rainscreen and hardi or other durable siding

    BUT, for a while I've been curious if anyone has considered with much thought a house/wall assembly of blown in cellulose or fiberglass in a double-wall assembly with taped plywood sheathing that ALSO acts as the board and batten siding with plywood battens covering (with caulking) all taped seams. Air sealing would obviously be improved by paint as well.

    Windows seems to be the most difficult weatherproofing detail, but doable.

    Pros - cheaper, less materials, and easily fixable waterproofing and sealing.
    Cons - no rainscreen or other secondary/back-up waterproofing protection

    Crazy? Genius? Other thoughts?

  4. GBA Editor
    Martin Holladay | | #4

    Response to John Alden
    Builders have been using plywood sheathing that doubles as siding for decades. It's called T-111.

  5. John Alden | | #5

    Martin Holladay
    Thanks Martin,

    I understand the grooved plywood product exists and is used in traditional construction, but have never hear the use of a combined sheathing/siding product on this site (in the context of green building). Is it an ineffective method from a waterproofing and air barrier perspective? More, equal, less, effective than rain screen / separate siding?

    Thanks for the additional attention to this question. I think it's an important one given the cost of siding a house.


  6. GBA Editor
    Martin Holladay | | #6

    Response to John Alden
    Over time, plywood tend to deteriorate when exposed to the weather. You'll often see signs of delamination after a few years. This is true for both ordinary CDX as well as T-111. It ends up looking cheap.

    Moreover, there are flashing issues that are hard to resolve, as you pointed out. It's hard to flash windows, and you end up needing metal Z-flashing at each horizontal joint, which looks cheap.

    Finally, this type of installation is hard to air seal.

    The bottom line: it's not a green technique.

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