Foundation Design
To achieve the passive house standard, major changes are needed in the foundations details we typically see in Nova Scotia. Although we can build a passive house with a basement, it is more energy efficient to build a slab on grade foundation. This along creates a large thermal mass to help storage the heat for the solar gains and is much easier to insulate.
A key principle in Passive House is thermal bridge free construction. This means completely wrapping the house is a thick layer of insulation including the foundation. A typical foundation in Nova Scotia is built on a footer. Either the basement wall or frost wall sits on the footer and the exterior framing walls are built on these walls.
It is very difficult to completely wrap a footer and foundation wall in thick insulation. – too many joints and lots of surfaces to cover. So, we move to an engineered slab, which is also load bearing and therefore requires the stamp of a Professional Engineer.
Thickened edge engineered slabs are commonly used in garages and sometimes residential but the thickened edge is also hard to insulate and labour intensive to install. For the Hawkins House foundation we are borrowing an idea from one of the US passive house projects, nicknamed the hat top foundation.
6” thick slabs of EPS foam are laid under the concrete with custom edge pieces that provide the perimeter frost protection and insulation for the edge of the slab. This is a tidy solution on paper but we are looking forward to how easy it is to install. In asking questions of my foundation guy Ed Weaver, his answer was “ I don’t know, let me think back to the last time I installed 6” of foam under a slab….never”
I give him full marks for his willing to break from standard methods and work with me on this new solution.
6 Comments for this entry
Natalie
Hi Rob: Yes, a 2 or 3 story with a simple shape is more efficient. With Passive House, the thermal mass is less important to the efficient than in an only passive solar design, so you are always better to have a lower envelope area to volume ratio for efficiency. And yes, it is cheaper to build up with the high insulation levels under the slab and in the ceiling. I designed this house to also meet the UK Lifetime for Homes guidelines for accessibility and marketed to the folks looking for this type of one level living so I paid the cost of additional insulation to meet the standard!
Natalie
John O'Brien
Natalie,
Do you spec a custom insulation package for the insulated raft foundation or did you go with Legalett up in Quebec.
I’m at the design phase now and haven’t found any other Canadian suppliers of this type of foundation. Lots of options over in europe…
Natalie
Hi John: We had the foam insulation custom cut by a local EPS manufacturer. Trufoam. They are very responsive to custom designs and I know they ship to other areas of Canada and the US.
Sorry for the long response time, we are crazy busy with the finishing of the project.
Natalie
John
Thanks Natalie,
I actually know one of the Truefoam reps for New Brunswick quite well. Not sure why I never thought to ask him. Would you mind me asking who did your structural engineering work on the foundation?
Natalie
Sure, I work with Pat Griggs in Halifax.
Rob
Fascinating! We look forward to more blogging as this proceeds.Now we want to build one.
Would a 2 or 3 story be more efficient, having a smaller envelope, or less efficient, having a smaller surface area to the thermall mass underneath? It would likely be cheaper to build, no?