Passive House BKLYN

Energy Revolution in a Brooklyn Townhome

Cellar Floor Slab – Water Drainage

The cellar floor and insulation system was a complex layering of elements designed to keep the water from infiltrating the cellar side walls or slab and to maintain a continuous air and thermal boundary.  The client plans to use the cellar as recreation and work space so even small amounts of moisture are of greater concern than it might otherwise have been.

The steps involved in the cellar are:
1) Remove old cracked slab.
2) Excavation about 12″ below existing slab height.  Attention was given to verify that the foundation wall footing was below 12″ at all points so as not to disturb the earth around it.
3) Dig three narrow trenches running parallel to the party walls about 10″ deeper than the existing excavation and gently slope the rest of the ground to be pitched toward the trenches.  Tamp the earth to reduce future settling.
4) Install recycled content HDPE percolating 6″ pipes into the trenches.  These pipes need to be gravity pitched to the sump pump in the front of the house. (See cellar 3D diagrams in the Building Envelope tab)
5) Cover the trenches with gravel and the pitched earth with 2-3″ of gravel.  This releases the hydrostatic pressure from the ground below the slab.
6) Tamp the gravel down to reduce settling.
7) Remove and reinstall gas lines, meters, and electrical panels off of the cellar front stone wall so that the insulation layer can be continuous behind.
8 ) Frame out the cellar walls with 2×3″ wood studs spaced 1 1/2″ off of the cellar walls and 6″ from the level line of the gravel below.  No bottom plate will be installed; instead we are temporarily bracing the wall with diagonal supports to keep the wall supported and level prior and during the insulation installation.
9) Install 2″ of spray foam insulation on top of the gravel floor and then 3″ of closed cell bio-based spray foam up the walls.  Because there is no bottom plate, this spray foam can be a continuous from the floor to walls.

10) Pour a 4″ concrete (w/ 20% fly ash replacing Portland cement) slab on top of the insulation layer which brings it to the level of the wood studs.


11) Remove the temporary supports now that the insulation has locked in the studs.

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2 responses to “Cellar Floor Slab – Water Drainage

  1. JJ Boudreau March 10, 2010 at 9:02 pm

    What a great idea!

    I think the planning that went into this was exceptional and that think this basement will be dry for the life of the home providing that all of the sub-slab piping was done correctly (it looks and sounds like it is.)

    I can see some columns made from brick. I did not see any closed-cell foam on these and was wondering how you made sure that these were waterproofed. I assume that each one sits on it’s own footing. Did you spray foam the footings and areas up to the height of the slab?

    • passivehousebklyn March 10, 2010 at 9:12 pm

      Good question. We did not spray around the columns other than the 2″ that was around the rest of the ground below. This was due to the fact that we didn’t feel in calculations that there was significant thermal bypassing from the 4 brick columns given that they were in the center of the house and the ground delta T in that area was not significant. Aesthetically we wanted the slab to touch the brick columns so a trade off was made. I will check with David White to see what the thermal bridge amounts to in terms of actual btu loss and add it to this comment next week.

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