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Insulating the ceiling of a basement to mitigate cold floor

Dpcvt | Posted in General Questions on

I am building a house, while living in it. I re-used and extended an existing poured foundation and full basement. I insulated the walls with 2″ XPS rigid board glued all the way around the walls. I spray foamed the bar joists and the spray foam completely encapsulated the top of the foundation wall and lands on the top edge of the XPS board. The basement seems very air tight, and remains a steady 55 degrees. It has also been quite dry since I redid my curtain drains.

There is plumbing in the basement and a heat pump water heater, but no boiler or other mechanicals generating heat. I do have a rinnai heater down there to use when I want to use the space as a workshop, but generally it remains unheated.

The main floor of the house is a bit chilly. The house is very easy to heat, and the upstairs is very comfortable. I can feel a definite radiant cooling effect from the basement being cool. The slab is uninsulated, and I believe the cold slab keeps the basement at its steady temp.

I enjoyed the DIY spray foam process for the bar joists and I am thinking about spraying about 2″ between the floor joists under the advantech subfloor to mitigate the radiant cooling effect on the main floor. I realize this will cool the basement temperatures some, but I cant imagine the temps ever causing a freezing condition down there.

I would like to do spray foam since it seems like the quickest way to work around all the plumbing and mechanical items in the floor joist cavities.

Is there any downside to using closed cell foam to do this? Is my thinking flawed?

My purpose would not be to save overall energy, but to increase comfort in the house.

I am located in Central Vermont.

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  1. Expert Member
    Dana Dorsett | | #1

    The closed cell foam would have to be either painted with intumescent paint or covered with a timed thermal barriers such as half-inch gypsum board to meet code in my area. The XPS on the wall needs it too, though they would often give a pass on leaving the foam on the band joists exposed.

    A single layer of 2" XPS doesn't meet code minimums in VT- it needs to be 3" to meet the letter of code, and 3.5" to meet the actual performance levels over the lifecycle of a house.

    DIY closed cell foam kits are the opposite of "green" (as is XPS), primarily due to the HFC blowing agents used, but also the amount of polymer per R. R15 rock wool batts snugged up to the subfloor would deliver higher performance for less money, even at box-store pricing.

    If you insulate between the basement and first floor the basement will be colder. If instead you insulated the slab it will be warmer, perhaps warm enough to address the first floor comfort issues. Given the high expense of DIY closed cell foam, consider putting couple inches of EPS above the slab instead. The 55F basement temp now is about half-way between your deep subsoil temperature (~45F in central VT) and the temperature of the subfloor (~65F when the first floor is 70F.) With an insulated slab the basement temperature will likely rise to at least 60F, maybe higher.

    How are you heating the first floor? Could it be used to heat the floor itself?

  2. Dpcvt | | #2

    The first floor is heated with a wood stove primarily, with air to air heat pumps as secondary and a single, 36000 BTU Rinnai as backup.

    I dont have any plan for the generation of hydronic water.

    I think adding board to the floor at this stage would be quite labor intensive, given what is currently installed in the basement.

    Rockwool between the joists would be something worth considering. I don't really care if my basement gets colder. The original house that stood here was very cold, the basement was leaky and uninsulated and the floor had fiberglass in it, and I never had a pipe freeze down there.

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