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Q&A Spotlight

Should I Skip the Radiant Floor Heat?

Consultants say that a radiant-floor system will be overkill, but the owner doesn’t want to learn too late that the advice was wrong

Before the pour: Tubing that will carry hot water is placed over a layer of rigid insulation prior to placing the concrete floor. Once the concrete has been poured, there's no going back to install the tubing if, in fact, it's actually needed for heat.
Image Credit: Flickr

With an R-90 roof and R-60 walls, Jenz Yoder’s new off-grid house will be well insulated. Yoder’s quandary, outlined at Green Building Advisor’s Q&A forum, is whether radiant-floor heat is a good idea.

“I had two consultants tell me that I will not need radiant floor heat, [that] it will be too much,” Yoder writes. “We will have a whole-house air circulation system and a gas fireplace. I am worried about not putting in the pipes in the floor and then being wrong.”

One option for this Climate Zone 4 house would be to install radiant-floor tubing only in those rooms far from the fireplace, Yoder adds. Or, adding a heating element to a whole-house ventilation system.

What should Yoder do? That’s the topic for this Q&A Spotlight.

That’s way too much insulation

Charlie Sullivan’s suggestion is to start with a heat loss calculation, but before those results are in hand he suggests that Yoder will be disappointed if the expectation is to have warm floors during the winter with a radiant-heat system.

“If you want radiant floors because you think it will be nice to walk on a warm surface in the winter, you are out of luck,” Sullivan says. “If you make the floors that warm, the house will get way too hot and you’ll need all the windows open.”

Installing the tubing in a small area might be an option, he adds, but with that much insulation there probably won’t be that much variation in temperature in different parts of the house.

Dana Dorsett, however, suggests the insulation Yoder plans for exterior walls is “crazy high,” and that a gas fireplace “is likely to roast you out of the house… even when it’s below 20 degrees F.”

Referring Yoder to information published by the Building Science Corporation, Dorsett says that a quick check suggests R-25 walls and an R-60 attic with R-7.5 under the slab is about right, meaning Yoder’s wall R-values are about twice as high as they need be, and the attic insulation is half again more than necessary.

“Spending the ‘extra’ insulation money on one or two small air-source heat pumps and rooftop photovoltaics (PV) is probably a more financially rational investment,” Dorsett says. “In 20 years, when they are nearing end of life, the replacement equipment or PV will be both cheaper and higher efficiency than they are currently.”

Even if all of the extra insulation is very low, he continues, it’s “still not necessarily ever going to be cheap enough on a lifecycle basis.

“In an R-30-walled home, occupant behavior makes a much larger difference in energy use than another R-30 of insulation ever could,” he says. “(Half of nearly-nothing is even less than nearly-nothing.)”

The insulation was very cheap

Ordinarily, Yoder replies, Dorsett’s advice would make sense. But in this case, the insulation was very inexpensive — Yoder was able to pick up enough used polyiso from a commercial roofing job to put a 9-inch layer of it on the house for $1,400.

Yoder also adds some details about the project: “I am doing double stud walls with a complete thermal break between them,” Yoder says. “Kind of like doing SIPs. I agree that compared to the walls, the under-slab insulation is weak. This insulation is over a 5.5-inch slab of concrete that is designed for unstable soils. That makes my floor 11 inches thick, so I decided to to stay with 2 inches [of foam].

“Also,” Yoder adds, “in the center of the house is a 8-foot by 13-foot concrete vault/storm shelter with 8-inch thick walls and roof. That should help keep temperatures even.

“The property is already off grid with PV and wind. Our elevation ensures that no matter what the daytime high temperatures are we always dip below 70 degrees at night. By using a whole-house ventilation system we can cool the house at night and should need very little cooling.”

So, says Richard McGrath, maybe the two consultants who have told Yoder to skip the radiant floor heating aren’t so smart after all.

“A house like yours could very well require [water] temperatures in the sub 90-degree range to heat at a design of 20 below zero, which would mean they would be lower yet for greater than 85% of the season,” McGrath says. “What nobody ever mentions is that you can also use that same tubing to cool, at least handle the sensible load with warmer mediums requiring less energy. And, no, they will not condense since they are a bit higher in temperature than the dew point.”

Hydronic heating in an off-grid home?

Dorsett questions whether hydronic heating and air conditioning are wise in an off-grid home, suggestion it wouldn’t be worth the extra battery costs.

But here, too, Yoder adds a twist. The building site is windy, and the extra electricity he gets from the wind turbines heats water that could be circulated through the floor. There’s plenty of room on the site to put underground lines for cooling, Yoder adds, and because he’s doing the work himself, “I can afford to do things differently.”

“I installed my own PV system, which has been successfully running our living quarters in the barn for six months,” Yoder adds. “We average wind here year round that l gives night charging capability. The cost of running electric to our house would be $20,000 so we decided to go off-grid and so far have only spent $14,000.”

The choice of a water heater is key

On balance, McGrath thinks radiant-floor heat would be a good choice. Overheating shouldn’t be a concern, and Yoder should be able to heat and cool water in a number of ways.

Whatever he does, however, McGrath recommends that the stored water be maintained at a temperature of at least 140 degrees F., in order to head off potential health concerns. It can be tempered with cold water to a safer 120 degrees before it is delivered to the point of use.

“It is my opinion that an off-grid house should have the luxury of several choices to heat and cool,” McGrath says. “Water can be heated and cooled in many ways; how many anyone would like to install to hedge against any number of things is his choice.”

But the idea still doesn’t make sense to GBA senior editor Martin Holladay. “I have lived in an off-grid house for 40 years,” Holladay writes. “With a big enough battery system and a large enough wind turbine, you can generate as much electricity as your bank account allows (except, of course, on those dark windless days during the winter). I’m not in favor of fireplaces or complicated heating systems that depend on electricity for an off-grid house. Keep it simple.”

Holladay says that he knows of several owners of off-grid houses who installed hydronic heating systems that included pumps. The owners later abandoned the systems because the pumps depleted their batteries.

“It is possible for a smart engineer to design a hydronic system to use very little electricity, and I don’t doubt that you [McGrath] can design such a system,” he says. “The problem is that very few plumbers or heating contractors know how to do this. Moreover, designers of boilers, fuel pumps, and oil burners don’t care about electrical use.

“Most hydronic systems have oversized and inefficient pumps,” Holladay writes. “Is the problem solvable? Of course. But there are many opportunities for off-grid homeowners to be surprised and disappointed when they tell their heating contractor, ‘Install a hydronic heating system. I’ll just run it off my batteries.'”

Our expert’s opinion:

For this Spotlight, GBA technical director Peter Yost has called on Mark Sevier, whom he describes as a “top-notch mechanical engineer” and a former colleague at Building Science Corporation and now a project engineer at an electric utility. Sevier owns a net-positive energy home in the Boston area and constantly tinkers with its radiant system. Here’s Sevier’s response:

I’ve used the same logic that putting tubing in a slab is cheap (especially if self-installed), and therefore worth doing if you have the least bit of a passing thought that you might want to heat the slab or use it for thermal source/sink in the future. I put tubing in both upper and lower slabs in my garage, notably using cheaper PE tubing that can’t tolerate higher temperatures/pressures since I intend to be sending lower water temperatures.

I’m also assuming that Yoder’s project isn’t primarily cost or speed driven, where the tubing may be self-installed and/or the modest tubing cost wouldn’t be a show stopper. It seems to me that projects that come up against questions like this are partly experiments, and not spec-builds, so my attitude is that you should incorporate every interesting idea and feature you can think of going in, since retrofitting can be difficult.

Most likely the gas fireplace will heat the space without difficulty as long as the spaces are open to one another. With good insulation and airtightness and some degree of temperature fluctuation tolerance, they may only need to run the fireplace for a few hours a night/morning; building materials store heat, so exactly matching rated heating output to actual heat loss isn’t as critical as people seem to think it is.

For what it’s worth, it has been my experience that insulated but unheated indoor tiled or exposed concrete slabs on grade tend to still be cold on the feet due to their large ability to absorb heat and only being heated by radiation from above, so tempering the concrete just to room temperature (i.e. not delivering heat to the room) would make the floor more thermally comfortable. Wood flooring, rugs, and/or slippers could also work, depending on the finish decisions and expectations.

To get a low wattage hydronic system, I think one would need to leave no electrical decisions up to a contractor. There are ECM hydronic circulators at this point that use few watts, since they undo the oversizing that comes with contractors who don’t care about kWh and don’t ever want to get a callback. It doesn’t necessarily take many watts to circulate a water loop, but no one designs such things, and electricity is not a common concern (although utility-sponsored energy efficiency programs like the one I work for are working on this).

I have recently been working on a complex mechanical system, and have come to see that while it’s fun for a mechanical engineer to toy with, having a simple back-up system should be a consideration for a less technical spouse or significant other in case the in-house engineer gets hit by a bus unexpectedly.

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