New Build, In Slab Heating vs. Crawl Space

Hello!
We are in the planning stages for our new home build this year and have been considering which GC to use.
Our plans were engineered for a slab foundation, as we would like to take advantage of our South facing windows for passive solar gain. We also want to do radiant floors for our main heat source. Since we’re doing a slab, we felt that doing in slab hydronic made the most sense for efficiency.
However, our GC candidate has (last minute) told us we should do a crawl space for installing the hydronic heat and have the warm board act as the subfloor for ease of installation/maintenance. This would require re-engineering our plans as well as considering the benefits/costs of doing it this way.
Does it make sense to do a crawl space for hydronic heating instead? For reference, our property is at 9k feet elevation and can get pretty cold in winters, but still plenty of sun.
We’re definitely leaning towards still doing in-slab heating with the required slab insulation below it, but wanted to hear your thoughts.
Thanks!
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Replies
Personally speaking, I'd prefer a crawlspace (or better yet, a full basement) for general mechanicals, maintenance, accidents & storage. The warmboard would perform way better and be more responsive than a slab. But if I the goal was to save money, radiant embedded into a concrete slab on grade is a cheaper option.
Interesting. I've read the radiant slab is more efficient due to thermal mass, is that not the case? We're also hesitant of a crawl space due to mold issues, but I appreciate the feedback on it.
Completely agree with the other two posters. It won't be cheaper, but the conditioned/encapsulated crawlspace is a superior option. If it makes sense in your location (where are you?), consider a pier and beam foundation with a curtain wall and rat slab encapsulating the crawlspace area under the beams. The benefit with pier and beam is that you don't have to worry about the slab shifting or cracking due to expansive soils since the house rests on the piers drilled much deeper than the clay layers. Think about future remodels and how easy it will be to have access to all of your plumbing and electrical from the crawlspace. Also, it will be more enjoyable to walk on since the subfloor will have more give than hard concrete. And like DC said, thermal mass is not generally a good idea; you should prefer quick-responding surfaces and Warmboard or similar products can deliver this. A slab takes a long time to change temperatures so you end up overheating in the afternoon and underheating in the early morning hours.
Can this issue be remedied by having an in-slab thermostat? I've heard about these issues before but I would think there would be a remedy of some kind?
I guess it depends on where that temp sensor would be located. Usually, by the time you see the heat at the thermostat, it's probably too late. You would have to have a smart algorithm that proactively predicts what the heating or load of the home is in advance, and pre heat the slab only to that point without going over. This has a slightly beneficial effect of shifting the runtime of the heat pump or boiler which could be nice if you have time-of-use electricity plans, but it's hard to do accurately.
An in-slab thermostat doesn't help with problems when the heating load changes quickly.
The heat output of a heated surface is entirely determined by the difference between the surface temperature and the air temperature. If the heating load changes quickly -- say if the sun comes out from behind a cloud -- the heat output can only change as quickly as the temperature of the surface can change. And the only way that the surface temperature can change, even if you've cut off the source of heat, is by releasing heat into the room. If you have a heated surface with a lot of heat capacity that's a lot of heat and will take a long time. The room will overheat.
The same thing happens when heating load grows quickly, say when the sun goes down at the end of a sunny day. Until the heated surface gets up to temperature the heat output won't match the heating load and the room will get cold.
Concrete is a terrible material for residential floors. The Warmboard will be expensive but it will give much better comfort, it will be much more responsive to changes in the heating load.
"Thermal mass" is pseudoscience.
Why is thermal mass pseudoscience?
There's no science behind it. There's not even agreement among proponents as to what the property of matter that they're talking about is. Nor what the units are, nor how to measure it, nor how to model it.
An important detail here is your climate zone and if you need cooling.
In places that need cooling, floor heat is not the best option you essentially need two complete HVAC systems.
I love radiant heat...but...in an modern very well insulated house with high performance windows, the floors do not tend to be cold.
Boilers are expensive
Radiant heat does not provide AC
When you can heat a [for instance] 1000 square foot space with one 2000 dollar heat pump why spend a bunch of money on hydronics?
If cold climate ducted heat pumps had been where they are now 15 years ago, I would have not spent 15 grand on a boiler.
We don't need ac where we are. Aren't there cheaper boilers for radiant floor systems now than 15k? I'm also still concerned with mold in a crawl space....
A conditioned crawlspace, as well as conditioned attics, are required by IRC R408.3 to have 1 CFM per 50 sq. ft. (or 20 CFM per 1000 sq. ft.) of ventilation. So you are right to worry about mold, the code is worried about it as well! But I believe the supply or return CFM of a dehumidifier will count for this requirement so just pop one down there and rest easy.
https://codes.iccsafe.org/s/IRC2018P4/chapter-4-foundations/IRC2018P4-Ch04-SecR408.3
Moldy crawlsapce comes from bad old details. Stuff like exposed dirt without a vapor barrier is asking for trouble. Any crawl built properly (poly under the slab, insulated stem walls, sealed and insulated rim joist and conditioned) will stay pretty close to house conditions, so the RH and humidity will never be high enough.
About the only way you'll get a radiant system in there for under $15k is if you DIY. Regular ducted HVAC won't be all that much cheaper but now you have proper air filtering, fresh air distribution for free (you'll need this even with radiant) plus a choice of fuel sources. Around me heat pump is about 1/3 the cost of oil, about 1/2 the cost of propane and about the same cost as gas.