GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Is it worthwhile to insulate a hydronic heat floor to avoid loss to a unused basement?

Mike Legge | Posted in General Questions on

My house is ICF walls,SIP roof,and 4″EPS under the basement slab. The basement is about 2/3rds the volume of the of the main floor.It is absolutely tight(less than .8ACH) with a vent fan if needed.It is painted a white throughout.It’s 9/10ths below grade.Only one door and no windows. The main floor is manufactured “I” beams which are exposed in the basement ceiling.. Measuring air temperature, the basement is always 1degreeC below the main floor.I am told due 97% to irradiation from the white plywood under the 2″ concrete slab of the main floor.
1). Is it worthwhile to insulate the basement ceiling?
2). How best to do this for the least cost?
3). Is radiation the main culprit?
Thanking you all as always.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Keith Gustafson | | #1

    Right now you have a radiant ceiling in the basement

    any standard insulation [cellulose foam etc] would be better than nothing.

    I know that even 2 inches of XPS was effective to make the radiant floor in my last house 'work' during construction

  2. Aj Builder, Upstate NY Zone 6a | | #2

    Insulating the 1st floor will make the cellar cooler. If you like the cellar temp then forget it.

  3. Paul Brazelton | | #3

    Design guidance for hydronic floors is to always insulate beneath, even if the floor is above a heated space. The idea is to only heat the floors, not the ceilings below (as Keith indicated). If the basement is unused, you're basically sacrificing a significant portion of the main floor's heat to keep an unused space warm.

    The easiest and cheapest way to stop this is to put fiberglass batts between the beams, beneath the main floor. You'll probably have to use an insulation support wire. Make sure you get full width (16") batts, as the I-joists create a larger cavity than the typical 15" batt can fill.

    When you're done, your basement will become substantially cooler, but you'll be saving probably a significant amount of energy, and will likely have a more responsive radiant floor in the part of the house you live in.

  4. Dick Russell | | #4

    With a house that tight, you undoubtedly have an HRV or ERV to provide fresh air, and perhaps a thermostat or other control for intermittent fan operation, to circulate air within the house so many minutes out of each hour. If you have this in the house, and the basement air is included in the periodic air circulation, then you'll likely find that even with insulating under the heated floor the brief introduction of heat via the air circulated from the heated part of the house is sufficient to keep the basement within perhaps 5-6 degrees of the heated living space, perhaps a bit more in the harshest parts of winter. A mostly sub-terranean basement that is well insulated all around doesn't lose much heat, given the mild temperatures all around it.

    In our new house (superinsulated), we have forced warm air heat with intermittent fan operation set for 10 minutes out of each our. The lower level is unoccupied most of the time, so we have left the thermostat off. Similarly, the bedroom end of the upper level is on a separate zone, and we prefer cooler temperatures for sleeping, so we have left that tstat off also. Both the lower level and bedroom zones have stayed within five degrees of the heated part of the upper level, while night time temperatures have dropped into the upper twenties (F). I expect that as winter comes on and bitter weather sets in we will have to turn on at least the bedroom zone tstat or add a second blanket to the beds.

  5. Mike Legge | | #5

    Thank you all for you advice. I still wonder that if the heat loss is mainly radiant, shouldn't a radiant barrier be included in the insulation.The basement is not ventilated. The house ventilation is via a "solar chimney" which opens depending on the wind direction and speed.It uses Bernoulli effect to cause air extraction. If no wind, the computer turns on extractor fans.There are no thermostats, only multiple thermistors in the floor and airflow sensors to adjust the needed ventilation.All rooms are connected by ducts from an air intake.There is no HRV. The control is on a tablet which has a pictorial floor plan that is colour coded by moving a "slider" on each room and uses an algorithm connecting temp, ventilation and humidity.I am still unsure of the most effective way to insulate and do value your expert opinion.

  6. Keith Gustafson | | #6

    radiant barrier' is mostly con man code. insulate with regular old fashioned insulation. Foil faced iso if that works for you

  7. Aj Builder, Upstate NY Zone 6a | | #7

    Mike, your home is super high tech. Seems like your builder's team of experts should be consulted. Have you? Local eyes are to me the direction you need to go.

  8. Aj Builder, Upstate NY Zone 6a | | #8

    Keith, 1 degree C. is Mike's radiant temp facing cellar. The cellar is very well insulated. Insulation added would most likely not save a $100 dollar bill annually though I would rather state that after a visit to the home.

    1 degree ..... Is not 10-20 degrees that is the normal temp above room temp of radiant floors as measured on the topside.

  9. David Meiland | | #9

    I degree C is the radiant temp facing the cellar AFTER the heating system has heated it. The heating system is probably almost as effective at heating the cellar as it is heating the first floor... it's just a radiant panel, under foot in one case, over head in the other.

    It isn't easy to talk about cost/benefit in something like this without an energy model. Was one prepared for the building of the house? Maybe the energy consultant who did that could throw some floor insulation into the model and answer the question in a few minutes.

  10. Keith Gustafson | | #10

    Aside from the heating the basement[and losing the heat through basement walls] a radiant system [well setup] is pretty low margin. If it is designed to only heat upstairs, then it will need hotter water when trying to heat downstairs or it will not keep up on cold days. running hotter water is less efficient and can be less comfortable if it were extreme[unlikely here]

    I am somewhat surprised that this setup gives no trouble, although if it is smart enough to balance room temp with only floor sensors, it may be self correcting invisibly, but wastefully nonetheless

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |