Burning (thermal) Bridges

Our final shipment of panels has arrived!  The small city lot means we really have to make efficient use of available space, so even the mound within the garage footprint has become a staging area for panels.

We quickly constructed a ramp and double-staircase to aid in moving the heavy panels.

We quickly constructed a ramp and double-staircase to aid in moving the 10-foot panels.

The panels are a bit heavier than they look.  Two people can handle a panel on flat surfaces, but it’s much easier (and safer!) with four people when going any distance or changing elevations.  The bridge in the above picture was a big help for logistics, but it certainly isn’t of the “thermal” type, so I’ll get to the point.

All materials have insulating value, but they’re not all created equal. When compared to metals or concrete, wood is not a bad insulator.  However it is considered a weak spot in a wall assembly otherwise filled with insulating materials such as mineral-wool, fiberglass batts or EPS (Expanded Polystyrene).  Its high solids content contribute to its “conductive” characteristics and this is the reason a thermal imaging camera will clearly show where the wood studs are if there is enough of a temperature differential between indoors and out. This thermal image is of an insulated wall; the darker the colour, the cooler the temperature (Photo credit: http://www.massinfrared.com):

Thermal image of an insulated wall.

Thermal image of an insulated wall.

Thermal bridges are a problem in any building, so we go through a great deal of trouble to minimize their effects in a Passive House.  Complete elimination would be ideal, but drastic reduction is the reality.

The exterior walls in Kingston Passive House are constructed of Structural Insulating Panels (SIPs).  This type of construction dramatically reduces the quantity of solid wood in the walls, by enabling the “skins” (plywood or OSB) to be load-bearing.  There are some places in a SIP assembly where solid wood is non-negotiable: the bottom and top plates for example.  We did however work with the SIP manufacturers’ Engineer to think a bit outside the box and further reduce thermal bridging.  The panels we are using are a full 12.25 inches thick… designed specifically to accommodate a 2×12 around window and door openings and at corner junctions. With the exception of the foundation (where back-fill pressure must be considered), a 2×12 is more “structure” than needed for a residential two-story building.  We are replacing many of the 2×12 members with a pair of 2×4 members, spaced apart as to reduce thermal bridging… also known as creating a thermal break:

SIP showing a thermally-broken spline at the corner junction.

SIP showing a thermally-broken spline at the corner junction

At corner junctions, one panel simply butts up against the side of the adjacent panel. Normally a single 2×12 would be used here, but a pair of 2×4’s seprated by four inches of insulation, creates a thermal break in the assembly.  Similarly, we are double-framing around window and door openings:

Thermally-broken SIP window "buck".

Thermally-broken SIP window “buck”.

Missing from the above picture, is more (thermally-broken!) wood at the end of the SIP lintel.  The wood is necessary to transfer loads down through the jack studs, but again a pair of 2×4 supports is plenty strong and will outperform a single 2×12 in terms of resistance to heat-conductivity.  We will get even more creative on the second floor, where ledgers are attached to the house for supporting the porch roof, but you’ll have to wait a bit for those pictures.

I’ll leave you with one last picture… taken today (July 30th) just after the stone-masons finished our garage foundation wall:

Garage Foundation Complete!

Garage Foundation Complete!

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