Electric radiant heat mat for bathroom?
Robert Swinburne
| Posted in Mechanicals on
I am designing a well insulated double stud wall house – r35/r55 in New Hampshire. We would like to use solar hot water with some sort of boost/backup for not only hot water but radiant heat on the first floor. We will pour tubing in the basement slab for future use if we finish off the walk-out basement and decide we need it. We were thinking about no heat on the second floor except for electric radiant mats in the bathrooms under slate tile. It seems fairly efficient and cost effective especially if we are looking towards photovoltaics down the road. Does any one have any input or opinions?
-bob
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It sounds like you're designing your own home and are probably not a design or building professional, so I'll offer some general advice.
A house has to be designed as an integrated system, so make sure the structure is adequate for any future solar PV or thermal panels, that the roofing is appropriate for future add-ons, that there are mechanical chases for plumbing and wiring and dedicated spaces for storage tanks and other mechanical equipment that will become necessary.
Particularly since you seem to be headed toward zero net energy, I would suggest maximizing the thermal envelope as stage one. If you're going to the trouble and expense and resource use of a double-framed wall system, then I would aim for at least R-40/R-60 with plenty of foundation, sub-slab and slab edge insulation.
And a good strategy for attaining zero net energy is to reduce your electric load as much as possible consistent with the other design goals of the project. Electric resistance heat may not be the most appropriate use of limited energy resources. Mini-split heat pumps and geothermal heat pumps are far more efficient in their conversion of electrical energy to heat.
And, having built a number of double-stud wall houses, I would encourage you to investigate the Larsen Truss or modified Larsen Truss wall system, which uses fewer resources to create a more continuous thermal envelope and eliminates all the problems associated with double stud walls.
http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm
http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/History.htm
Bob,
You suggest using a solar thermal system and a PV array connected to electric resistance heaters as space heating systems in New Hampshire. You speculate that these systems are likely to be "fairly efficient and cost-effective."
They won't be. PV-generated electricity is far more expensive than grid power. Using a solar thermal system to provide heat in New Hampshire will not be cost-effective compared to many alternatives. The main reason: here in northern New England, we don't get much sun during the months when we need space heat. Moreover, the equipment required for solar thermal or PV systems is expensive. (I know, because I live with these systems.)
Robert Riversong gave you good advice. Invest as much as you can in your building envelope. Make your house as small as possible, and do a very good job air sealing. Provide much more insulation than the code requires, and save up your pennies to buy good triple-glazed windows.
Then put in a small heating system. Almost any option will be cheaper than the systems you have suggested (solar thermal or PV).
Last year I designed a house for my clients and we debated if we even needed to add a heating zone in the second floor. We thought with the passive solar design, thermal mass in the house (2" radiant conc. slabs), excellent insulation, tight enclosure, and a good HRV system, we might not need it. We chickened out at the last minute and added the zone. They report that it never comes on. (but they are known to use their fire place insert often).
On the other hand, more recently, we did a small house with the same concepts, and the same situation. We decided NOT to do a zone for the second floor, but we DID, stub up for a couple of radiant baseboard locations just in case. It was a good thing too, as our clients began to report an uncomfortable 6 degree temperature differential in the house between the two floors. One critical difference is the first house has an HRV system that my clients know how to use. They even know how to run it in recycle mode (designed actually as a defrost mode for the HRV) to just circulate the heat around. On the second house my clients were talked into a money saving "exhaust only" system that lacks this air mixing capability.
In summary, I think your plan in concept will work, but you'd be wise to be ready to add additional distribution just in case. Hope that helps.
Robert,
I am very interested in Larsen Truss framing. However, I don't see how it is it all that different that a double stud wall other than at the rim joist. I know that the rim joist area is important, but what are the other "problems" with double stud walls?
bob here,
actually this house is for a client, I have done super-insulated houses for many clients in all forms including SIPs, larsen truss, foam inside, foam outside, double stud wall and plain old 2 x 6. I may have just convinced my first client on another job to upgrade to triple glazed windows. - A first! Its is a hard sell when you compare dollar for dollar over 20 years. It s easier to sell the idea of comfort. Most of my jobs are super tight budget and I often have to do a sales job to both builder and homeowner to do anything beyond code minimum. Double stud is getting easier to sell to builders who think houses need to breath and therefor shouldn't be built too tight. - lots of those folks around here and as an architect, I have to deal with them. I have a set of details collected from 1/2 doz jobs that makes it almost a wash with the code min. 2 x 6. Splayed window openings are not possible at this budget level when clients aren't doing it themselves so I stick with a 9" double stud wall, sheetrock returns, lots of advanced framing techniques and if I can do the sales job well enough, 2" of closed cell foam inside the outside shell. The heat mat idea came from Rosenbaum who says to prepare for when photovoltaics become cheaper or utility owned. I investigated it for my own house, which has almost no insulation and a wood stove for heat, lights, water heating and cooking total about 700 kwh of electricity per month. the numbers were shockingly low in terms of $$. The house under discussion will have a full on ERV. which should prevent too cold bedrooms. Solar hot water is an easy sell and the $'s work fast. I have done a few houses where they ran radiant off a 120gal tank with a propane heating element for when solar isn't up to snuff. I like the simplicity of the system. - 1 tank - and you could add a huge water storage (tarm) tank later. I will specify stubbing for hyd. baseboards to second floor. -the plumbers will like that.
I'm also not such a good salesperson for small - the client freaked at the thought of a 12 x 12 room for her son. - don't get me started - I pushed as hard as I dared early on, the bank tells her what size her house has to be.(!) - on the other hand a 2500 s.f. house would take longer to lose heat than a 1500 s.f. house.
"BRETT MOYER: what are the other "problems" with double stud walls?"
Ambiguous load paths (inside "non-bearing" walls will take the 2nd floor and ceiling loads), excess materials (unless inner walls are 2x3), thermal bridging at sills and band joists (floors and ceiling), and difficulty in air sealing floor and ceiling platforms.
The modified Larsen Truss system eliminates all those problems.
All other things being equal, heat loss is directly proportional to surface area and a larger house has more surface area. Unless it's a high-mass house with proportionally more thermal mass than a similar smaller house, the primary determinant of heat load is size and the secondary determinant is shape (simpler and more cubic shapes are more efficient).
Then you must have a lot of sensible builders.
I remember when I was flabbergasted when an old timer, who was forced by code to install a ceiling vapor barrier, said that after the inspector leaves he slashes it with a knife because "a house has to breathe".
While neither he nor I (who thought of myself as an "advanced" builder) really understood the complex hygro-thermal issues, he was acting on common sense.
Common sense says that we can't live in a plastic box anymore than we would be comfortable very long with a plastic bag over our heads. Common sense says that our skin has to breathe, that we're most comfortable when our clothes can breathe, and that our homes - which are really our third skins - also have to breathe.
We more or less understand now that we need a regular supply of fresh air, and we manage that mechanically instead of by random leaks. But we don't yet understand that a house also has to exchange moisture, and that happens mostly as a byproduct of air exchange rather than by deliberate design. Yet we put nearly impossible demands on our HVAC systems when we build hermetically-sealed boxes and attempt to make their interior environments life-supporting.
Shelters built of hygroscopic materials that can buffer changes in interior humidity - just as thermal mass buffers changes in interior temperature - and that can move excess moisture in either direction as environmental conditions and seasons shift are far more life-sustaining than plastic boxes with artificial respiration equipment. And, if those materials are natural and, particularly, earthen, then the negative ion count remains high and healthy as well.
While common sense might not make it obvious what a breatheable house would entail, it's at least on the right track.
are you anti-passive house I wonder? The non-engineer part of me understands that even super tight, mechanically ventilated passive houses are only sealed up a third of the year here in VT and we have to also design for the rest of the year. I open my windows as early in the spring as I can bear to hear the birds outside and keep them open all summer even when it it very hot and humid and my floors warp. Keep in mind this is a house for some one with a low budget and no interest in do-it-yourselfing who would otherwise have bought a modular with an "enhanced insulation" package. As an architect I choose not to ignore people who are not the upper 5% who can "afford" an architect or those whose philosophies do not match my own. Of course that means that I live in a tiny and poorly insulated house that needs a new roof when I can save up enough $
As a designer/builder, I serve only those people who have few resources but an authentic need for decent housing, and a desire to live responsibly on the earth. That means I charge less for my time than an architect for my design/consulting services and less than most builders with 30 years of experience. And that means I also live in a very small 300 SF home for which I barter carpentry since I can't afford market rents.
You must either love fresh air or don't mind heating the outdoors, since the heating season lasts 9 months in Vermont.
Passive Houses, somewhat like the glass and steel monstrosities in most American cities, are designed to be hermetically sealed and mechanically-ventilated. In a mixed heating/cooling climate, that may mean keeping the windows closed all year (or all except the swing seasons). I suspect that many occupants of highly-efficient homes keep their windows closed much longer than people living in old farmhouses.
Our increasing isolation from the diurnal and seasonal changes of weather are a contributing factor to our general decline in health. We are not biologically adapted to an unchanging, controlled environment (viz, the increase in Seasonal Affective Disorder). So we respond technologically, by using full-spectrum lighting and negative ion generators, in lieu of going outside for some sun, wind and exercise.
Am I anti-PassiveHaus? I am pro-Nature and the dynamic homeostatic processes with which we've successfully evolved for millions of years. You can't fool Mother Nature (at least, not without penalty).
I admit that most if not all of my clients are "softer" than I am. My family burns wood October through April and in march/april oct/nov it is often only in the evening with the house open and airing out during the day. We sleep with a slightly open bedroom window year round but I admit we probably wouldn't if we had an HRV. My point is more that I feel I can have more affect by working with everyone and however far they are willing to go. If I contribute 1/2 doz. new houses and retrofits to the world every year that use significantly less energy than they would if I were not involved, I feel better than if I am only doing a couple super duper zero energy houses per year. As an architect II am overwhelmed by the shear volume of information about this and other issues that I have to wade through. This site is a great resource. Building science allows someone like me to benefit from the collective wisdom of thousands of builders instead of just relying on my own experience and that of the few local builders I work with around here.
Hi Bob,
Love the blog - I've been following yours and "Building Green in Vermont" for a while now. The following is just an opinion. If all you need is a little additional heat from time to time then electrical resistance heating is a simple, reliable and long-lasting option. Putting it under the bathroom floor wil create the most direct sensation of comfort, or indeed luxury, for the occupants. If there is an HRV in the mix then the heat will be distributed house-wide after it has warmed the toes.