Thursday, December 9, 2010

Passive House Basement Foundation Detail

Here is a basement foundation section I came up with recently.  I'm excited about this configuration for three reasons:

1.  It uses very little rigid foam, which constitutes a cost saving, good environmental stewardship, and leads to good indoor air quality.  the rigid foam may even be able to be eliminated altogether, depending on the thermal analysis.

2.  The construction process will prove fairly straightforward and simple, with walls generally being able to be built in the flat and tilted up.  The slab is the only concrete in the system, which also keeps the environmental footprint down.  It is easy to build.  The elevator pit can be incorporated into the wooden floor system, rather than needing to be formed into the concrete.

3. The exterior wall can be made with concrete, if a client is really too nervous about the wood, though the insulation value is reduced.

Other features, both good and bad, of this design include:

  1. Simplified plumbing, since most of it can be done above the slab.
  2. Possible issues with finished space - the building department may see the basement as finished space and charge more for the permit and subsequent property taxes.
  3. Pressure treated lumber - You'll see that the exterior shell suggests a very thin shield against the soil - I'm thinking 5/16" fiber cement siding, with asphaltic damproofing, then the HDPE dimpled membrane.  This does away with the poured concrete wall - which takes a lot of space - say 8 or 10inches, which is a major financial and enviro cost, and gives nothing to insulation - and proposes instead a very thin shell, held up with lumber.  The fiber cement shell is provided with the above moisture protection, but also with an additional drainage layer behind.  This can prove an effective measure to ventilate the thick, insulated basement foundation wall, as well as to keep that outer shell very dry.  The question, after all this, is - do we use PT lumber on the outer frame, or regular lumber?  We all know the PT lumber is very hard on the environment - so my preference is to do away with it.  Use regular lumber.  The battens to which the FC siding is attached can be plastic-wood. - I'm still trying to figure that out - but the issue with all this is the buliding department - will they accept this untreated-lumber below grade construction?  This will be a very interesting issue.
Here is the scan of my sketch:


  1. Interesting detail, but what do you do for frost protection under the footing?