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!

Progress Report!

The Kingston Passive House team has had a busy (and very toasty!) 3 weeks.  There was not as much soil as we were hoping for, so we opted to hammer through the top 14″ layer of limestone (subsequent layers are much harder) and resigned ourselves to the fact that Kingston Passive House will just have to sit a bit higher on the lot.  One benefit of the increased elevation is the basement floor will be higher than the sewer and storm laterals – no sewage-ejector required for the basement bathroom rough-in!

Starting the SIP foundation

Starting the SIP foundation

Since the house sits on bedrock, we also reduced the footing and pad sizes;  we used less concrete and lowered the house by another couple of inches as a result.  The “blue” you see on the footings is a capillary break. Although the Permanent Wood Foundation (PWF) SIPs are designed to function in a moist environment, we still want to keep them as dry as possible.  The stepped footing in the back is for the garage; no need to pound through rock for that.  Here you see the first four SIP sections are in place… the beginning of a very well insulated home.  The water-filled hole is for the sump pump and tank.  If you look closely (click on the image for a better view) you can make out the six PVC conduit sections through the footing at the back.  These are for the three loops of 3/4″ PEX that will make up our Subsoil Heat Exchanger (SHX).  The pipes will exit the house, and travel around the perimeter of both the garage and house before coming back in.  You might also notice the footing extensions; A SIP can not handle a concentrated load (such as the end of a beam) so you can either carve out insulation and embed a post in the wall, or place the posts right beside the wall and avoid creating a thermal bridge.

This next photo shows our hemlock posts in place.

Foundation walls up, Posts in place.

Foundation walls up, Posts in place.

They sit on a galvanized steel saddle to keep them secure. The saddles also act as a capillary break between the hemlock and the concrete footing.  Why hemlock?  Placing 9 hollow steel posts through the basement floor and sub-slab insulation would represent a significant thermal bridge.  The wood posts will not conduct heat energy into the ground to the same extent as steel.  You could also argue that wood posts have a lot more character than steel.  Even the offcuts were destined for a higher purpose; inspiring the crew to create “Hemlockhenge”.

The piles of recycled EPS are only half of what is required for sub-slab insulation. There will be two layers (ten inches total) below the concrete slab.  Radiant heat would be overkill in this house, so the only thing in the slab will be some reinforcing steel.

This final picture shows the open web floor joists.  They span greater distances than dimensional lumber and will allow us to run services in any direction within the floor cavity.  Being an engineered product, they are also more consistent in terms of being straight and do not have to be overlapped at the beams, making subfloor installation that much easier:

Open Web Engineered Joists

Open Web Engineered Joists

Earth, Air and Passive House

We don’t often associate “earth” with a fresh air supply, yet earth and air do make a great team in a high-performance home.  The concept of passing air through earth as a means of tempering it is not new; the temperature of the earth is relatively constant once you get down 6+ feet below grade.  When the ground temperature is warmer than the outside air in the winter, or cooler than the inside air during summer, we have an opportunity to harness some cheap geothermal energy!

During the heating season, fresh air is essential but enters the home at a price. In a leaky home fresh air comes in at outdoor temperatures;  this is why we need to get the air-sealing right… If we control where the air comes in, we can pass it through the HRV and harvest heat energy from the stale outgoing air.  I’ve talked about HRVs in a previous post, and mentioned the importance of their efficiency, but the fact of the matter is, the colder the outside air, the cooler it will be even after going through the HRV heat exchanger.

We need a constant supply of fresh air, so we can’t turn off the HRV during cold spells; we can however, improve HRV efficiency. When building a new home, we typically have to excavate for the footings and foundation, so why not plan ahead and run some pipe beside the footings before back-fill?  In a Passive House, these pipes are known as a Subsoil-Heat Exchanger or SHX for short.  Earlier SHX implementations used earth tubes – essentially a large diameter intake for the HRV installed below grade.  Although effective, the Passive House Institute no longer recommends earth tubes.  If not perfectly installed, they become a trap for moisture.  Where there’s stagnant water for long periods of time, there is mold and mildew;  certainly not an environment suitable for transporting your fresh air supply.   I’m a big fan of simple, and having no moving parts makes an earth tube very simple indeed; yet in the interest of guaranteeing healthy indoor air quality, perhaps a bit of complexity isn’t so bad:  The alternative to moving air through the earth to pick up some of its heat, is moving a liquid.  The large diameter earth tubes are replaced with 3/4″ PEX tubing and a highly efficient circulation pump moves food-grade propylene-glycol (protection from freezing) through the pipes and then through a liquid-to-air heat exchanger.  This way, the fresh incoming air is warmed by traveling through the heat exchanger as opposed to the ground.  The circulation pump runs on a thermostat so it is activated only when the incoming air is colder than ground temperature.

In the summer, the SHX is set to run when it gets a bit warm inside.  A separate heat exchanger is installed after the HRV, and subsequently has a cooling effect on the fresh air distributed to the house.  This heat exchanger requires a condensate pan to catch any moisture extracted from the humid summer air.

The soil is also important for SHX efficiency; if the PEX tubing is in a porous soil or (even worse) gravel, there is little surface contact and performance will suffer.  In a dense packed soil such as clay, there is much better contact.  From a SHX perspective, we have ideal conditions at Kingston Passive House… clay on top of limestone bedrock.  I’ll post pictures as the SHX components are installed.