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Insulation question: knee walls, roof, or attic floor?

Hello, I am working on a new construction Passive-Solar house in Northern VT. The house is under 2,000 SF, 2 floors with an additional unfinished (for now) walk-up Attic and full basement. I will be using the 10-20-40-60 rule for insulation (perhaps more). My question: should I insulate the roof of the attic and knee walls instead of the Attic floor since it eventually might be a finished space?

Also, I'm trying to find the best solution for HVAC..and probably over thinking it?? The house will have solar panels (electric?) but not sure what to tie it to? I think radiant floors will be overkill since the envelope will be tight. Gas is not accessible to the site, only propane. Perhaps a tank and a heat pump?? sigh
Thanks - Amy

Asked by Mary Ostberg
Posted Oct 29, 2010 9:49 AM ET


5 Answers

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It's your choice where you define the thermal boundary of your home. Insulating the attic floor is simpler, and usually results in a tighter envelope, but it's perfectly possible to insulate a sloped roof. If you decide to insulate your kneewalls, don't forget to include blocking between each joist, directly under the kneewall bottom plate. Each piece of blocking must be carefully air sealed. Pay close attention to air barrier continuity at all the jigs and jogs if you go this route.

What do you mean by "solar panels (electric?)" Are you confused about the options? Because, as I'm sure you know, there are also "solar panels (thermal)." Your question mark confuses me.

The answer to your question about what to tie the solar panels (electric) to is simple: They are connected to an inverter. If you have a grid-tied system, you need a grid-compatible inverter. If you have an off-grid system, you'll have an off-grid inverter -- as well as batteries and a charge controller.

If you want more information on heating a superinsulated house, you might want to read Heating a Tight, Well-Insulated House.

Answered by Martin Holladay
Posted Oct 29, 2010 10:02 AM ET


Thanks Martin, yes the question mark after the electric panels was because i was unsure if the panels were elect. or thermal. i understand it will connect to an inverter but then wasn;t sure what heat system to tie it too. I will read up on your suggestion..thank you! I also was just reading about the "Echo" solar panel system from PVT Solar.

Answered by Mary Ostberg
Posted Oct 29, 2010 10:12 AM ET



If the attic will become conditioned space, then the thermal and air boundaries have to be at the roofline (not at the kneewalls, as this interrupts the continuity), but that makes achieving R-60 without petrochemical foams a bit of a challenge. There are, however, ways to do that with either attic trusses or site-fabricated dropped cathedral ceiling below the rafters, and it's possible to do that and still ventilate the roof (which I believe is essential in your climate).

If the attic is planned to be finished living space, it's also important to consider fenestration for daylighting and cross-ventilation (as well as egress if it will be bedroom space), which can be challenging under a gable roof. Also, a 3-storey conditioned space will have significantly more positive pressure, from the stack effect, at the top of the thermal/air boundary and air-sealing becomes even more critical.

If you really think you'll need that extra space, it may be wiser to enlarge the footprint and area of the two primary storeys and limit the attic to occasional storage outside the thermal boundary. For passive solar design, it's critical to include adequate shading over each level of windows on the south side. One way to accomplish this is with a garrison-style home in which the second floor extends out beyond the first floor south wall to create both an overhang and additional living space upstairs without enlarging the foundation.

Given the paucity of sun available in northern VT, particularly in winter, heating with PV is not a good option, but PV can easily provide your electrical plug loads. Propane and wood are the two most appropriate heating fuels for this region (I'm in the Mad River Valley), and they complement each other well since wood can be either primary or a backup when the power is out.

I design passive solar super-insulated homes for this region (and teach those arts), and would be glad to talk with you about your project. You can reach me at HouseWright (at) Ponds-Edge (dot) net.

Answered by Riversong
Posted Oct 29, 2010 10:13 AM ET


Your wrote, "I was unsure if the panels were electric or thermal." So who's project is it? Are you working on someone else's project?

Because if this is your project, then you would know, wouldn't you? If you wanted to specify solar electric panels, that's what you would specify -- and if you wanted to specify solar thermal panels, that's what you would specify.

Answered by Martin Holladay
Posted Oct 29, 2010 10:26 AM ET



Efficiency Vermont has dedicated programs to assist in the construction of energy efficient new homes, including energy modeling assistance and technical support. It's well worth enrolling in their program: http://efficiencyvermont.com/pages/Residential/BuildingEfficiently/Vermo...

Answered by Jesse Thompson
Posted Oct 29, 2010 11:12 AM ET

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