Anatomy of a Super-Insulated Passive House Wall Assembly

I should start this post with a clarification… There are many wall assemblies that will help a building meet the Passive House standard.  The materials you choose (and the quantity) will depend on many factors, including the climate, aesthetics, availability and cost.  Once your materials and construction type have been selected, it’s a matter of adjusting the wall thickness in PHPP (the software tool used for energy-modelling a Passive House) until you meet the standard.  The exterior walls are only part of the equation – PHPP also takes the windows, floor assembly, air-tightness and much more into account.  We chose Structural Insulated Panels (SIPs) for the wall construction due to the minimal thermal bridging, high insulating value per unit of thickness and speed of assembly.

In addition to the SIP panels, we have chosen to install a service cavity on the inside perimeter.  This is essentially a 2×4 stud wall partition put in place to aid with the installation of electrical and plumbing along outside walls.  The sub-trades can drill holes through those studs without compromising the air-tightness and insulation of the SIP panels, both of which are critical in a Passive House.  The interior seams of the SIP panels will be taped with a product designed for that purpose, not the red tapes used for detailing vapour barriers and house-wrap.

To reduce thermal bridging and greatly minimize potential for air leakage, we are hanging the floors on the insides of the wall assemblies.  In typical construction, the floor assemblies are built on top of the walls.  This creates a thermal bridge at the end of every joist (engineered or otherwise) and makes proper insulation and vapour barrier installation very difficult.  Metal hangers attached to the SIP cap-plate will support the floor joists and greatly simplify our quest for a super-insulated, air-tight building envelope.

Wall Assembly for the Kingston Passive House

Note the (almost) continuous insulation layer… it is interrupted only by the top, bottom and cap plates required per the SIP manufacturer’s specification.  Also note the unusually high truss design;  These raised-heel trusses ensure we’ll have room for the full depth of insulation even at the eaves where a typical house would see a significant reduction.  We still need a solution for the sump pit – it’s an obvious thermal bridge in this diagram, so it will need to be dropped down flush with the top of the gravel and we’ll fabricate an insulated lid to seal things up.

Up next:  Windows (brace yourself!)

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Solar Gains

When people hear “Passive House” they commonly associate it with a Passive Solar home.  There is a similarity between the two:  They both take advantage of solar gains during the heating season.  The differences are in the building envelope.  A passive house does not rely on thermal mass to slowly release stored heat (although it could).  It relies on quality triple-pane windows and a super-insulated, air-tight building envelope to keep the heat from escaping.  We want the free heat from the sun, but we need to be careful about how we utilize this resource.  There’s a delicate balance between collecting heat in the winter and overheating in the summer.  Shading is also critical in this equation, and I’ll go into greater detail on that topic in another post.

We started with a detailed look at the property.  It is a city lot, with partial Southern exposure.  I say partial because it’s a corner lot and the lot boundaries adjacent to the streets run Southwest and Northwest.  There is also an existing house next door (to the Southeast).  The lot survey will tell you all you need to know – the property line trajectories are marked down to the nearest degree.  The Southwest lot line is 42 degrees from South and the lot is not large enough to rotate the home for optimal exposure.  Now it’s a question of moving the house as far forward as possible to lessen the shading effects from the neighboring house, and playing with window placements and sizes to optimize gains, losses, daylight and of course – curb appeal.

We used SketchUp to design the house.  It’s a free 3D rendering tool from Trimble (formerly Google) and is surprisingly powerful for its price.  Not only is SketchUp capable of very accurately generating a virtual model of your home, it can also geo-locate and subsequently model how it behaves from a sunlight/shading perspective any time of day, any day of the year.  For more info on SketchUp:

http://www.sketchup.com

Design… where do you start?

While the design of any home is a challenging exercise, it takes on new meaning in a PH (Passive House).  Meeting the world’s most demanding energy standard for buildings is no small feat – the buildings must be designed from the footing up with PH in mind.  There is also the temptation to make it look like a PH…  OK, there’s something wrong with that statement.  A PH does not have to look any different from a code-built home.  It is a remarkable home though, and it’s tough to ignore that while in the design stage; Should it look different because it is a high-performance home?

The software tool used to model energy usage (PHPP) takes everything into account (more on that in another post).  The PH standard is based on energy use per square meter of floor area, so it makes sense that a building shape that is simple (corners are notorious for creating thermal bridges in construction) and a footprint with a favourable A/V (area to volume) ratio makes the standard easier to reach. As you can imagine, a long, stretched-out bungalow will have more outside wall area per square meter of floor space than a 2-storey building in the shape of a cube.  The problem of course, is that it’s tough to make a cube look interesting without making it really stand out …  The neighborhood is well established, and while it does not present a particular theme or home style, the houses are what you might consider to be normal/standard given their age.

We’ve settled on a simple rectangular shape, influenced mostly by the dimensions of the city lot it is to be built on.  I’ll go into more details of the design in future posts.  In the mean time, here’s a sneak peek:

Welcome…

As the title would suggest, this blog is a means of documenting the design, construction and challenges associated with Kingston’s (in Ontario, Canada) first Passive House.

What is “Passive House” ?

It is the world’s most comprehensive and ambitious energy-modelling standard.  Although relatively new to Canada, the Passive House Standard (PassivHaus in Germany) dates back over 20 years, has become very popular in Europe and is gaining traction all over the world.  Passive House is non-prescriptive, meaning it does not require you to use specific materials or building systems.  Stick-frame, ICF, SIPS, and even straw-bale can be used.  At the heart of this standard is the ‘Passive House Planning Package’ (PHPP) which is a very thorough energy modelling software tool. PHPP gathers every detail of the house (orientation, areas, volume, windows sizes / placement, all wall/ceiling/floor assemblies) and computes energy consumption, taking local climate data into account.   Although there are many requirements for meeting the standard, the most widely referred to are:

  • Air-tightness – 0.6 ACH50 (air changes per hour at 50 Pascals pressure – positive and negative) is the maximum allowable.  Passive houses frequently go well below this number, but for comparison’s sake, an R2000 home allows 1.5 ACH50 and modern code-built homes are normally in the 4-8 range.
  • Ventilation – A home this tight requires active, controlled ventilation to remove indoor air polutants and bring fresh air in, while recovering over 80% of the heat (or cooled) energy from the outgoing stale air. The HRV or ERV runs continuously and Passive Houses are very healthy to live in for this reason.
  • Annual Space Heating and Cooling demand – A passive house is designed to use no more than 15 kWh/(m2a)… that’s 15 kilowatt-hours per square meter of floor area, per year.  Essentially this gets heating down to a level where the house can be heated without a conventional furnace or boiler.  The house still requires a heat source but it’s almost negligible in that it can be delivered to the rooms by post-heating the fresh air from the HRV/ERV.
  • Windows – They must have total (frame, spacer/glazing and installation)  U-values of 0.8 W/m2K which translates to R-values of 12.5 or better.  Common double-pane windows are roughly R4 so this marks a dramatic improvement in window performance – justified by the fact that windows are the single largest source of heat-loss in a typical home. Passive House windows are triple-pane with one or more low-E coatings and must also have a high Solar Heat Gain coefficient (SHG of 50% or more solar transmittance) to take full advantage of solar gains in the heating season.

More details on the standard will pop up as the build progresses.  For now it’s all a numbers game… taking an optimized (for Passive House) floor plan and adjusting glazing, wall, roof and floor assemblies in PHPP in order to meet the standard.  These numbers will of course be verified prior to certification.

Stay tuned!