Northern climate: is a heated slab necessary for comfort?
Emel
| Posted in General Questions on
We are in a northern climate near the Canadian border. We plan to double insulate the slab floor and use both a wood burning stove and mini split for our heat source. Our contractor is recommending infloor radiant heat but I’m wondering if the expense is really worth it since we are already insulating the slab and have two other heat sources.
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Replies
Absolutely not
I love radiant heat, but once you build a house that does not have so many air leaks that cold air puddles on the floor, the floor is not cold.
I ran tube over the concrete and under hardwood floors in my downstairs and after delays I never bothered connecting it to the boiler. The floor is virtually the same temperature as the walls
Wood cork etc floors rather than concrete and tile.
Have the contractor show you the LoopCAD drawing, or whatever they use to model the heat transfer. At most you need PEX at the periphery and the bathrooms. We installed Warmboard in our house and the majority of heat comes from the bathrooms
In a well-insulated, well-sealed house the surface of the slab will be at most a degree or two colder than the rest of the house.
The reason a concrete slab feels colder than other flooring materials at the same temperature is that it has high conductivity, feet or other body parts in contact with it lose heat quickly through conduction. Even a thin layer of an insulating material over the concrete will address that.
A basement built to modern standards will have very low heating loads. Heated floors barely kick on under those conditions.
Double insulate the slab by all means, but I'd agree 100% that radiant is an unneccesary added expense. Depending on the wall system and exterior/interior insulation details, you'll want to ensure the detail at the edge of the slab includes a thermal break: https://buildingscience.com/sites/default/files/migrate/jpg/BSI059_Figure_01_web.jpg
Some more ideas here: https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.larimer.org%2Fsites%2Fdefault%2Ffiles%2Fuploads%2F2021%2F06_foundation_and_slab_insulation_-_final.pdf&psig=AOvVaw2pZRWwtb93_59ihbMipfXn&ust=1721222171312000&source=images&cd=vfe&opi=89978449&ved=0CBUQ3YkBahcKEwjYiZK70quHAxUAAAAAHQAAAAAQFA
A heated slab is great when you cover it with a drafty poorly insulated house and can get lots of cheap fossil fuel to burn in the boiler. If you are on this site, it seems unlikely you will build that house.
The heated slab will add 50-70% to your HVAC budget and almost forces you into burning fossil fuel on site to heat the boiler and more like 150% if you get AC and a heat pump to make the hot water for the floor.
Just how warm will the slab be? If you build a compact tight well insulated home the math says it will be much cooler than your skin so it will not feel warm.
Walta
Just to put some numbers on this, I just had a room-by-room Manual J done. The house has a finished bedroom in the basement, it's 220 square feet. Heating load is 1300 BTU/hr, or about 6 BTU/sq foot. The rule is 2 BTU/hr per square foot per degree of difference between floor and air, so that would mean a floor temperature 3F above air temperature.
I did some measurements with an IR thermometer, and in an insulated basement the floor is about 2F below the room temperature. So that would mean increasing the floor temperature by about 5F, from 68F to 73F if the room is at 70F. It would still be well below body temp so heat would flow out of your body when in contact with the floor.
Complete waste of money and resources. With the addition of a minisplit heat pump, my in-floor heat has not run in years. It's nice to have in the bathroom, but for that small area you're better off using wires instead of hydronics.