Electric radiant floors in a mobile home
I’m planning on building a tiny mobile home a little less than 300 square feet total. I live in Northern WI and want better insulation and better building materials and designs than the manufactured homes. I also need some of common building chemicals out of my home.
I seem to do best with electric heat or radiators, but building on an 8 x 32 foot trailer has limited floor space that I don’t want to be taken up with baseboard heaters and find that water radiators would probably be out of the question (or would they?). I wonder whether electric in-floor heating will lose too much heat below (even with a high R-value insulation) and result in high energy costs. I live in an apartment now and do well with electric heat, but I’m on the 2nd floor, so I’m not sure what would happen to efficiency with cold air below me. I could use a vented propane heater for back-up, but would like to rely on in-floor heating if possible. I prefer not to have forced air heat, even though this seems to be the popular way to heat mobile homes.
I will design the tiny home for good passive solar, but I may not always be able to depend on it, depending on where I am parked.
Also, given I would not have heat vents, I’m curious as to whether I would need a heat exchanger to keep the air turnover high and how I would do that, or if turning on a bathroom van or range exhaust a couple times a day would suffice in addition to doors opening in the winter. (I’m not concerned in the summer as I keep windows open most of the time.
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Replies
Kim,
Although electricity is an expensive fuel, your planned house is quite small, so your energy bills are likely to be low, even if you heat with electricity.
As you guessed, including plenty of insulation and paying attention to airtightness are important if you want low energy bills. Lots of manufacturers make electric resistance heating cables that can be installed in your floor. Just be sure that you don't install the cables under your kitchen cabinets or refrigerator.
Ventilation is an entirely separate question from space heating. Your idea of using a bath exhaust fan to provide ventilation will work fine in such a small house. Choose a quiet model like one from Panasonic, and put it on a timer; you won't need to run it for 24 hours a day.
Thank you for your feedback! And thanks for the lead on the Panasonic bath fan. I didn't realize that the can be put on a timer, which would help.
Kim,
Here's a link to a good timer to use to program a bathroom exhaust fan used for ventilation:
Tamarack AireTrak Advantage control
Thank you!
I've been looking at designing a cottage for one with 500-700 sq ft and keep coming to the conclusion that electric heat makes the most sense for very small and very well insulated.
Using a mini split heat pump gives better efficiency, but might be too much $ and capacity for under 300 sq ft.
Could you use a 8 x32' SIPS panel (10-12") as the floor over the trailer frame?
Hi Richard,
I've never seen a mini split heat pump before. I'll look into it a little more. Here's my experience, I recently lived in a 300 square foot cottage (unfortunately not properly insulated and in the North). It had forced air gas heat and the air exchange came in by the floor and blew heat out the top. It was horrible. The floor was always ice cold and one was constantly putting on layers when the blower was off and peeling them off when it came on. I've been much happier in small spaces with radiant heat that can be set on low and with a sense of more constant temperatures. I'm sure the amount of insulation also helps. There is something to having warmer floors though, which I have now due to the fact that I live above someone else. In the winter, I can even turn the heat off on a sunny day and just use passive solar. I don't think I would be able to do that without a sense of some warmth in the floor. (This is just my experience without the technical expertise though.)
I have not thought about the SIPS panel for the floor. I was planning on using soy foam. I would be interested in hearing from someone about what would be more appropriate on a trailer frame as I've never lived in or built a trailer home before. I think if I use radiant floor heat I would shoot for a tile floor if that affects anything. Otherwise, I would use wood. I would just want to make sure the floor has both a high R-value and is strong to avoid sagging. Since it's a custom build without manufacturer limitations, I can over-engineer it a bit to make sure these things happen.
Hi Richard,
Just another thought regarding the mini-split heat pump. When designing a tiny space you run out of space on the walls and for furnaces or water heaters very quickly. You may not have as much trouble, when going to 500 - 700 square feet, but it still takes some planning on knowing exactly where tables, closets, cabinets, etc will take up space. Good storage and a place for all the essentials is paramount in the design of small spaces.
The split heat pump does take up wall space, which puts it at a disadvantage for me with my low square footage.
Your right about the mini split taking up wall space which is at a premium in 300 sq ft.
Also a mini split requires an exterior condensor and line set which is a problem if your mobile home is truely mobile. As I said, a mini-split is probably overkill in 300 sq ft. both in cost and capacity. A very well insulated 300 sq ft will be quite comfortable with electric radiant floors.
I suppose this is a question for another thread, but assuming that the fan on a timer needs to be cycled on and off frequently (for example, every other 5min interval) during occupancy, to compensate for lack of low-enough cfm settings, how significant of an effect on power draw do those frequent start-ups have?
? Perhaps we should start another thread to see if anyone has a clue?
Minneapolis,
I can't imagine that there is much of an energy penalty, if any, that occurs when a Panasonic fan starts up. If I wanted the fan to operate for 50% of the time, I'd probably run it for 20 minutes on, 20 minutes off.
I posted another thread. I'm also wondering how much it really needs to be on. 50%? 10%? Twice a day for 20 minutes? I'd rather it not be on very much, but how do I know what is needed and if there is a better way to make sure air exchange is adequate (other than opening windows and losing heat.)
Kim,
You want to follow the ASHRAE 62.2 recommendations, explained here: Designing a Good Ventilation System.
Here's the formula: 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. So a 300 square foot house with one occupant needs 10.5 cfm of ventilation.
So, if you bought an exhaust fan that moved 42 cfm, you would operate the fan for 25% of the time -- say, 15 minutes every hour.
Thank you! This is very helpful and much more than I would have thought. I'm hoping the panasonic bath fan is much quieter than what I currently have or it would drive me crazy. Do you think it is the best solution for ventilation in a tiny space that doesn't have heat ducts? (Energy-wise and sound-level-wise.)
Kim,
Yes.