Any case studies available about in-floor radiant heat in PGH?
Good day GBA!
I am at the point in my PGH design that I need to decide whether to include hydronic tubing in the walkout basement slab. For the record I’m located about 3.5 hours north of Duluth, MN in NW Ontario. My home with have an approx 1800 sq ft footprint with a main floor and walkout basement. I plan to have the slab detailed as the finished floor. We have paid special attention to passive solar. We plan to put 8” of foam under the slab, 3” around the slab perimeter, r40 basement walls, r40 main floor walls and r100 in the attic.
In-floor hydronic heating has been beaten dead on GBA, however, most of what I read is rather anecdotal. This is our forever home and obviously it’s impossible to go back and install tubing after the slab is poured.
Can anyone point me in the direction of some actual studies comparing real world comfort of in floor hydronic heating vs none in a “Pretty good house”?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
How do you want to create the heat? A heatpump will benefit more from the lower supply temperature compared to a gas boiler. With 8" foam under the slab you will have no real comfort issues anyway.
I trust you have done the motions to decide that 8" is proper under the slab - compared to for example less under the slab and more on the side of it. (or wing insulation)
Heat/ac will be via heat pump. Air sourced and ducted most likely. Ground sourced is also possible as I have digging equipment and lots of space. No gas available at the property, nor will I have propane. There will be a woodstove in the basement. Combo air to air and water units seem to becoming slightly more available/affordable.
Backup heat (as we dip below -30C for about 1 week per winter) will likely be a combination of the woodstove and heat strips/resistance coil in the HRV.
The sub-slab insulation will likely come from Iso-Slab (Quebec based company). It’s a complete system for frost protected shallow foundations. Essentially a foam tub.
I’ve read numerous comment on various threads from experienced builders that even with decent amounts of sub slab insulation in PGH type builds that slabs can still be only around 60F. Stratification plays a big role.
There are some here that have advocated for selective radiant heat. DC has that in his house with the bathroom floor heating first until the heat load increases and his FCUs pick up the slack.
The "problem" is basically that a PGH has a low heat load. Your 1800sqft basement might only have a heat load of 12k BTU. So on the design temp day, your basement floor is only 76*. I have a 100sqft section of my kitchen floor I heat, and 75+ is where it feels nice to me. The problem is that the rest of the year, your floor will only be a few degrees warmer than the room, not noticable.
And then the other big issue with concrete, is the response time. 1800sqft at 4" thick is almost 100k Btus at 75*F. If the sun comes out, you might start to overheat the house.
My intent is to heat a small area, say living room or kitchen. I don't know how this would work with concrete embedded PEX however.
This is the exact reason why I’m looking for some controlled studies.
I understand that with better than PGH insulation levels, to floor will likely never feel warm to the touch. However on bare concrete a 70 degree floor must feel better than a 60 degree floor.
In engineering you use models that were derived from studies.
The normal model used in buildings is to assume that heat loss is linearly proportional to insulation level and temperature difference. If you have a temperature difference, there is going to be a heat flow. A 60F floor is going to result in a significant heat flow out of the room. The usual model is to assume there is a layer of air along every surface of the room, that layer has some insulating value, the heat flow through that layer of air allows the room to be at a different temperature than the surface. The standard assumption is that layer of air has an insulating value of R0.5.
So in order for the floor to be at 60F in a room that's at 72F, there has to be 24 BTU/hr per square foot going down into the floor. And all the heat flows into an assembly have to balance, that 24 BTU/hr has to be going somewhere. With a basement floor, the only place it can go is into the soil below. If you have, say, R8 of insulation between the top of the floor and the soil below, to get a flow of 24 BTU/hr through that R8 you have to have a temperature difference of 192F, your soil has to be at -132F.
More likely your soil is at about 45F, you get about 3.2 BTU/hr through the floor and the floor is about 1.6F below room temperature.
I fully understand that most of the programs and experts say it’s “not necessary” and won’t create a perceivable difference in comfort.
I wish I had screenshot info that was provided by a PGH builder about real world floor temps in a PGH. They were in the neighbourhood of 60-65F.
That sounds like he might have forgotten some insulation or had a nasty thermal bridge.
Out of curiosity, I'm going to measure my uninsulated basement floor when I get home.
JMRtbay,
Right, but what DC laid out is a model that shows a predicted difference in the temperature between the slab and the air in the room for an insulation level of R-8. You are proposing somewhere north of R-24 under the slab. There simply is no path for you to end up with a difference of 5 degrees with that much insulation (or even with a lot less). Physics doesn't lie.
On an unrelated note: I would reconsider the deck framing you have shown on the elevations so that the posts don't fall directly in front of windows. There are also several which can be eliminated altogether.
I checked my uninsulated floor in my basement. 60*. And my basement is a bit cold in general, air temp is probably around 65 down there, not many vents in the basement. So if that PGH has 60-65* floors, they missed a big chunk of insulation somewhere. If you use Iso-Slab, you won't have that issue. It's possible they accidentally coupled the slab to the footings or something along that route.
I just used an IR thermometer to read surface temperature on the floor and on an interior wall at a height of 48". The difference was 2F. I have zero insulation under my concrete, just a layer of Marmoleum over it.
I used an IR thermometer, and not a particularly good or new one. Its readings tend to be affected by the reflectiveness of the surface. To make sure that the readings were comparable between the drywall and the Marmoleum, I took a reading from the drywall adjacent to the floor, it was the same as the floor.
The readings I got were 68F for the wall and 66F for the floor. A digital thermometer on the wall next to where I took the reading said 72F. So I would guess the floor is about 70F, I really need to get a better thermometer.
My house isn't PGH, but it's pretty good. I scored 1.4 ACH50 on the blower door test and most of the house is 2x6 plus R9 of continuous exterior.
The heat output of a floor is determined entirely by its surface temperature and the room temperature. The design rule is assume 2 BTU/hr per square foot for each degree F difference between the floor surface and the room temperature. If you've done a Manual J with room-by-room heat losses you can easily figure out what your floor temperature needs to be for each room.
I would expect a PGH to have a design-day heat loss in the ballpark of 10 BTU/hour per square foot. That would mean a floor temperature about 5F above room temperature. In my experience floor temperatures below about 80F are imperceptible from room temperature. Keep in mind that's the design day, most days in winter are considerably warmer than the design day so most of the time the floor isn't going to be even that warm.
Thanks for your thoughtful insight and information. I keep leaning towards leaving the tubing out, but then get pulled back in. The FOMO is real. (Fear of missing out)
I’d really like to stay out of the hydronics game.
I love radiant heat. Grew up in a house with [uninsulated concrete slab] radiant heat.
I installed radiant heat in my last house.
I ran tubing in the entire downstairs and in the baths of my current home during a major remodel.
It is a huge waste of money and effort in a well insulated modern house.
I have never hooked up the radiant in this house
Once you insulate the floor, it is no longer 'cold'
If, however, you are going to heat hydronic-ly, radiant is efficient and effective.
If you want to feel warm floors it should be possible in a well insulated house.
In a 'regular' 'old school' [IOW 1990 code] house it is a struggle to get enough tube in the floor to heat the house at design temp while keeping the floor temp low enough. My last house had 1/2 od tube on 8 inch centers on the entire floor with IIRC 128 degree water. Or maybe it was 118, been a while. I think the max recommended for under wood floor was maybe 140, again, by memory.
The key would be to lower the amount of tube in the floor and place it only where you want it. The output of the floor is not really defined by square foot of floor, but foot of tube and temp of the water in it. Put it where you want it to feel warm
But don't, once you insulate the floor, in a house with no drafts, no cold air flowing down the windows onto the floor, insulation under the slab, you just won't miss it.
I don't
For the amount you’ll be spending on the home, it’s pretty cheap to add the loops and decide after a winter if you want heated floors.
But the cost of hydronics isn't in the tubing, it's in everything else. And if the house isn't going to use hydronics except for the basement floor, there's a considerable cost in getting that tubing hooked up. Obviously if the hydronics are already there and it's just a matter of adding a zone that's different, but that doesn't seem to be the plan.
Agreed but that decision can be made after 1 winter with the tubing but without the rest of the system. It’s a relatively cheap experiment and then it’s not random people like us deciding, it’s the person actually living there. If the tubing gets abandoned in place, not that big of a deal.
Paul,
I'd add the caveat that it isn't just putting in the tubing, the system needs to be designed, and the zones worked out too.
A lot of people throw tubing into their slabs around here just in case, and if they do decide to hook it up, wish they had put a lot more thought into it.
For sure. But the big bucks don’t come until later. Seems pretty harmless to put them in the slab.
I appreciate the insight from everyone!
If it were me I would just add resistant electric heat to floor where you want a heated floor and use standard heat pumps to do majority of heating. If you want a warm floor certain times just turn them on…. You will almost certainly never regain the initial cost of hydronic install via energy efficiency.
One other thing to look out for is the wood stove. Make sure you get one that can be choked down, otherwise you'll be too hot.
"real world comfort of in floor hydronic heating"
I'm just another anecdote but I've been living with radian heat floors since 1999 and I would never forego it. In 3-4 years I expect to build myself a new home and I'll be doing a slab on grade with radiant heat. The only thing I'm considering doing differently is putting the piping in a topping slab over the structural slab, or alternatively using Warmboard over the slab. I'll be doing a cost comparison when the time comes. I'm leaning toward Warmboard because it may be a better fit with a 3/8" engineered hardwood floor, which I would prefer instead of using the topping slab as a finish.
Whatever the naysayers say, I hope you do the radiant.
What’s your building envelope like? Code built or better?
The first house in 1999 was a bit better than code, but not like PGH now.
The second house, which I'm in now, is an old house on a concrete slab. I put down tile in the living and dining area with electric floor heat. The house is well below current code.
So my experience is not directly relevant to floor heat in a PGH. But I wanted to share it nonetheless.
I am 2 hours south of Duluth in zone 6A, we have Warmboard on 1st and 2nd floors. Have air sealed most of the house to 1.25 ACH50. Most of the heat comes from the bathroom and foyers (tile areas). When it gets cold, stage 2, the zone controls turn on. This keeps the house very comfortable.
Nothing wrong with radiant heat providing you engineer the system. Generally you want to do a room by room heat loss then make sure you balance the heat loss with heat gain, i.e., enough PEX to heat the room. You also need to remove excessive humidity and cool the house during the summer. You can use a hydronic air handler for cooling.
So yes, you do spend more money with a hydronic system. But they can do heating, cooling and DHW with a heat pump. How much more is based on the number of zones you need.
I would suggest reading John Siegenthaler’s (Siggy) articles to get an understanding. His books provide the basic equations and concepts.
As a reference, just a bit above freezing here. Floors are 2C above house air temp, the bathroom is closer to 6C. The bathroom floor does feel warmish, the rest can't tell.
The slow response of an 8" slab is a real issue. In only have 1.5" overpour and even with that it takes about 4h for house temperature to change from a thermostat setpoint change. The system also undershoots setpoint in the evening on a colder but sunny day when solar heat gain kept the heat off most of the day.
Target radiant is the way to go. If I could go back in time when I built the place, I would skip 80% of the radiant and the multisplit with wall mounts and install a ducted air handler. Would have been simpler, and way cheaper and cost less to run. The only radiant you really need is anything tiled, anything cantilevered with exposed floors and under larger windows. All those can be done with resistance mat for the fraction of the cost.
I find that the reason floors feel cold is not so much the temperature as the conductivity. Something like tile or concrete feels cold on bare feet at room temperature because it can draw the heat away so much more quickly, while a wooden floor or rug at the same temperature doesn't feel nearly as cold. What's tough about basements is you always have to keep in the back of your mind that it might flood, so you don't want to put down anything permeable.
This is an older article from the Journal of Light Construction that talks about hydronic heating for low-load homes, but they do make some good observations between hydronics (radiators) and in-floor radiant tubing installations.
https://www.jlconline.com/how-to/hvac/hydronic-heating-for-low-load-houses_o#:~:text=A%20properly%20designed%20hydronic%20system,of%20similar%20heat%2Dtransport%20capacity.