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Community and Q&A

Radiant Heat for Room with Lots of Glazing

aztecrsf | Posted in Energy Efficiency and Durability on

We’re in the early stages of construction on a home in coastal San Diego with 10′ and 17′ of 8′ tall glass doors along a 40′ total width of the building. The doors are west-facing. I’m concerned that wall, which is our main living area, is going to be chilly.

I’ve been in a lot of homes here, and talked to countless homeowners. The common theme is that homes are cold, even newer construction, and you run the heat a LOT more than expected (a/c is rarely needed, save for a few weeks total in the late summer/fall, if at all).

We have an “inverted” floorplan, so that area is on the second floor, along with the master. The ground floor will actually rarely be used; it’s just secondary bedrooms, laundry, space for future owners with kids.

HVAC-wise, with state-mandated solar, we’re planning on heat pump forced air. DHW will be gas on-demand.

On to the point of this…. I never would have considered radiant heat until we encountered it in a rental we’ve been in for the last 18 months. It’s fantastic. Silent, comfortable, and it seems inexpensive to run via an ultra-high efficiency water heater. The rental house has a lot of glass, too. What a contrast to our previous rental, more recently constructed with less glass, using gas forced air, that was always much less comfortable even at higher temperatures.

So now I am wondering whether it makes sense to add radiant heat to the great room area only, as our principal heat source. We’d likely only use the forced air as a separate zone for the master and another zone downstairs when guests stay with us. But one thing (besides cost) that gives me pause. A lot of days some of that glass will be open, no heating needed…until sundown, at which point we almost invariably need/want heat. I’m concerned radiant’s slow response time will then take too long to heat the space up, only making it comfy hours later, and until mid-morning when it shuts off.

Any ideas on what I can do to ensure that glass doesn’t make for a chilly experience? I’ve spoken to HVAC guys and they don’t seem anywhere near as detail oriented as the posters on this site. Their answers have ranged from “you’ll be fine” to “add a register there then.” And of course, if I talk to radiant guys, they think it’s a great idea!


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  1. DC_Contrarian | | #1

    If you're not modeling, you're guessing.

    There are no hard and fast rules, every house is different. If you do an energy model like a Manual J or BeOpt you should get a pretty good idea of what the performance of the house is going to be. Manual J is focused more on making sure you're comfortable in the extreme cases while BeOpt is more about overall energy usage.

    As for the slow response time: there are systems that are designed for better responsiveness. Warmboard is one of them if you want a name to Google. Basically you want your floor to have as little heat capacity as possible (it used to be thought that you wanted a lot of heat capacity and you may still read that in places, early experimenters mistook heat conduction for heat capacity. Warmboard gives high conduction with low capacity.) Your house will have a fair bit of heat capacity as well, if you close the windows and turn on the heat when the temperature starts to drop the heat should be kicking out before the house cools.

    The usual knock against radiant is that if you have air conditioning too, in most climates a heat pump sized to satisfy the cooling need also provides the heat you need. But HVAC is all about comfort. If you find the radiant makes you more comfortable, well, it's your money.

  2. paul_wiedefeld | | #2

    Confusingly, gas boilers and tankless water heaters aren’t efficient at all, but are labeled as such for some reason.

    Hydronics can be responsive if you use panel radiators, baseboard, or non-concrete radiant floor. You can even do radiant walls and ceilings if you find the right installer.

    The tradeoff here is that glass is terrible at insulation but the views are often worth the discomfort. Sunrooms cause comfort issues in all 50 states.

    1. aztecrsf | | #3

      Tankless aren't efficient? Confusing indeed. I'll guess that it's because they waste a lot of energy when trying to heat water quickly. I'm not sure how that changes when you consider recirculating hot water, that you use only a couple times per day.

      Re: glass/views, there are a lot of pretty much all-glass walls along the coast. That's gotta be cold living.

      1. paul_wiedefeld | | #4

        Gas appliances just aren’t efficient when compared to today’s heat pumps and today’s gas combined cycles. You get more heat per unit of gas turning it into electricity. If a minimum efficiency gas appliance is low 80s % efficient, a “high” efficient gas unit is mid 90s yet a bargain bin heat pump gets 1.5 units heat from the same 1 unit has. So the “high” to low gas delta is maybe 10% while the delta between “high” gas and worst heat pump is nearly 60%.

        1. DC_Contrarian | | #7

          It's a little more complicated than that. When gas is converted to electricity, the highest efficiency is around 60%. Another five percent is typically lost in transmission. So resistive appliances are much less efficient than appliances that burn gas directly, you need a heat pump with a COP of at least 1.5 to be competitive with even the least efficient direct-burn appliances.

          Note that natural gas also has distribution losses. An advantage of electricity is that the distribution losses are just heat, whereas the hydrocarbons in gas are potent global warming gases.

          1. paul_wiedefeld | | #8

            Like I said, with todays heat pumps and combined cycle generators it’s not that close. In San Diego? Forget it, gas can’t compete efficiency wise.

          2. DC_Contrarian | | #9

            According to Wikipedia combined cycle plants have a maximum efficiency of 64%.

            Gas appliances with efficiencies in the low 90's are not uncommon. To match that end-to-end with an electric appliance you need a COP of at least 1.5 and probably more like 2.0.

  3. user-6623302 | | #5

    They sell in-floor convectors for just this application.

    1. DC_Contrarian | | #6

      Yeah, or you could run ductwork in the floor since it's a second floor and have long, skinny registers in a line along the windows.

  4. Expert Member
    Akos | | #10

    If you look carefully in most design magazines, places with large glazing will have a narrow linear vent either above the glass or in the floor. In either case, these can be hooked up to your central air and provide heat and cooling exactly where you need it. No need for the complexity and cost of floor heat. You can get linear vents that are mudded right into the drywall for a clean look, almost invisible.

    If you must have floor heat, there are lot of resistance matts that can provide supplemental floor heat right by the window while the bulk of the space heat for the place is provided by a central heat pump.

    West facing glass is very hard to shade properly without trees or major outdoor structure. This means it will be a large cooling load, so having vents by the glass is a good thing. Putting this on an separate zone might not be a bad idea as only this part of the house will need heat and cooling depending on the sun and time time of the year. You don't want the rest of the house to freeze while trying to keep the living space temperature reasonable.

  5. aztecrsf | | #11


    "West facing glass is very hard to shade properly without trees or major outdoor structure. This means it will be a large cooling load..."

    This I am not worried about. Glare off the ocean can be substantial, but it's almost always <70F at that time, and any sun hitting that glass (if the doors are closed) is usually welcome. In a previous rental, we did get a lot of late afternoon heat, but here that's a plus. Mostly. Your point is still valid, though, as both heat AND cooling could be useful there. It would have to be in the floor (vaulted ceiling), which lucky for us will have joists running in the right direction to make that possible. And I like the idea of separate zone.

    1. Expert Member
      Akos | | #12

      I would get somebody competent (not an HVAC tech but an HVAC engineer) to run the cooling numbers on that. Rough back of the envelope calculation says about 2.5 tons of heat gain from the just the glass. No matter how nice it is outside, inevitably you'll want to close the glass say during fire season, which point you'll be baking in there. Either size the cooling to handle it or figure out better shading arrangement.

  6. nynick | | #13

    I am building something similar but 12x24, all glass. We are definitely putting this on a separate heat pump zone, but I like the idea of resistant mats on the perimeter or narrow linear vents.

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