The Little Details – Sealing the Sill

When building a super-insulated energy efficient house, many of the little details make a huge difference.  This is especially so when it comes to creating an effective air-barrier.  The air-barrier (different from and not to be confused with moisture barrier) is really important.  It makes sense – if air is leaking into your house through gaps, you are bound to lose heat.  More importantly though, the cold air meets up with warm air – generally in the middle of your wall space – and condensates.  This can result in mold and mildew.
The most critical areas are the sill between the foundation and framed-wall and around the windows.  There are several products that can be used to seal the sill (where the framed wall meets your foundation).  These range from foam sill gaskets, acoustical sealant or caulk, spray foam, etc.  Some folks in the green buidling world, however, are of the view that the cheap foam sill gaskets sold at Home Depot etc., do not work very well.  These folks feel that they do not fill voids well, that they allow air leakage and may not stand up to the test of time.  See  and
Because the jury seems to be out on the cheap foam materials with respect to whether it works as stated, we’re going to use an EPDM rubber sill gasket on the outer 2×4 wall.  This gasket gets stapled to the bottom of the wall and when compressed fills all gaps and voids as a result of the two plyable compression gasket-ridges.  It costs about 10X as much as the cheap stuff (due to shipping), but I think this is $100 well-spent because now I don’t have to worry about it.
Related information on air-sealing:
~ by John
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