Recently I was asked to project manage a PassivHaus build. This house, called SmartHome by the owner, was already framed and had the roof on. Most of the windows and doors were in, as well as most of the interior framing. There was no mechanical system, exterior insulation or moisture/vapour barrier. Above is a picture of the house at it currently looks (March 25, 2015).
We had many discussions about how to heat this 4000 + foot house and the guiding rules were that there was no need for natural gas onsite and the that we would allow for solar thermal and PV.
The heat and cooling loads were both around the 7.5kw (25.5mbtu) mark so we felt that an Air Source Heat Pump would work. A ground source HP was dismissed due to cost vs output. To get 7.5kw from an ASHP unit at -20C, we needed a unit with a nominal capacity of more than double that, 15 to 20kw, depending on the maker. But more about that later.
After sorting out interior framing changes and defining the layout for the kitchen, we added the last windows and set out to finalize the insulation levels. As the goal is to meet the 10w/m2, we will need to put R55 on the outside walls and R80 in/on the flat roof. In our case, we have issues of clearance to the neighbours house. Because of this, we have to use non combustible products within a specific distance from the house. Roxul will be used as a non combustible outer layer. The wall will consist of an impermeable moisture barrier (Blueskin WB) applied to the plywood sheathing. The Blueskin is necessary to act as a vapour barrier and to seal any penetrations and screw holes that hold the insulation on. On top of it will be 6″ of ISO foam and then 3″ of Roxul comfortboard. A rainscreen will give separation of the Roxul from the sheathing so that proper drainage can occur. All the connections have to be metal.
The foundation wall is made up of Durisol wood/cement blocks which go together in a similar way to an ICF block and filled with concrete. These blocks are very porous so must be well sealed on the outside right down to the footing.
The options for heating came down to ductless split heat pumps in key areas or floor heating throughout. Even though the floor heating in gypsum cement is more expensive, it has the benefit of fire protection, sound deadening which is a bonus in a large open concept house and very even heating. The negative aspect is that cooling has to be accomplished by different means. Another reason for the centrally located HP is a great reduction in complexity. There is only one outdoor unit similar to the one below and one set of electrical breakers and piping.
To make a comparison with gas heat, I like to say that if you spend $1 on electrical resistance heat, you will spend approximately $.30 on high efficiency gas or about the same on a heat produced by a modern heat pump. A2W (air to water) heat pumps are common in Europe but quite rare in North America because the manufacturers believe there is not enough market to support them. They are, however ideal for very low energy houses, especially when used with solar thermal for hot water.
The heating system will be augmented by 4 solar thermal panels which will act as an awning over the south facing main floor windows. (photos to come). There will also be drainwater heat recovery on the two main stacks tied directly into the showers.
The basement is now poured with 4″ of low carbon concrete, approximately 1/2 the embodied energy of regular concrete. There is a standard radiant tubing system in the concrete. The goal is to heat the house with water temps of no more than 30C and most of the time it will be around 24-27C.
The insulation levels are quite high in the floor, 6″ of XPS insulation or R30. Much of the insulation is reclaimed from roofing contractors but due to supply issues, we had to buy some new material. There is a lot of discussion around the use of XPS and high density EPS foam. The EPS is a lot better than XPS in terms of embodied energy as well as a lot less expensive, however R value is lower so you need a bit more of it.
The air tightness goal with a PassivHaus design is 0.6 air changes per hour @50 Pascals (ACH50). The Bluskin will seal all small penetrations and bridge gaps in the sheathing. Foams of different types are filling up the main gaps. Many ideas were entertained to accomplish the tight envelope and in the end the Blueskin won out as the most efficient and fool proof way to do it.
As of April 6th, the blueskin is almost complete on 2 sides and we expect to have it finished within a week, in time for the blower door test.
Blueskin in the front of the house is up. We have to do the garage as well because it is part of the building envelope.
Well, it is February 2017 and it has been a slow project. We are now at the finishing stages with almost all the insulation and siding installed, kitchen is nearly done and the heating is finished. For the past 2 winters, we relied on an electric furnace just blowing hot air on the main floor and, as of the end of last winter, we had the floor heating working powered by 9kw of electric elements in a tank. The 9kw was necessary due to the unfinished exterior insulation and unfinished styrofoam XPS on top of the inverted roof.
The Nordic A2W heat pump has been running for a few weeks now and has been performing nicely, coming on perhaps once an hour in the coldest days for a few minutes. It is heating up the 300L storage tank. More photos to come.