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Community and Q&A

Choosing an electric heat source for a small freestanding office/library

user-7066239 | Posted in Mechanicals on

We’re building a 240 sqft office/study/library in the backyard for my partner to work in on a regular basis. The building has a gable roof with sloped ceilings (7′-9′). It will be highly insulated — spray foam on ceiling (R-32) and walls (R-20) with an insulated floor, a few windows and a glass-front door. We live on the border between 5A&6A in New York State.

Also (and this is different from most case studies I read about on the forums), we will be storing a large number of books on bookshelves in the space (hence the few windows), and want to avoid large temperature fluctuations for this reason. Some fluctuation is OK, but we don’t want to turn the heater completely off at night, for example…We never want the temp to get near freezing.

Of course we want the heaters to be comfortable, energy efficient as possible, and as inexpensive to operate as possible. We also want to maximize wall space for, again, the books (and tables and desks).

I’ve been reading everything I can find, but am torn between:
1) Two cove heaters
Q: Assuming we mounted these high on the wall with the recommended amount of clearance around them, but surrounded by shelves of books just below and on either side, would the heat from the heaters be hot enough to damage the paper over time? (I don’t have a good sense of how hot the units themselves are, and how they impact their immediate surroundings.

2) Electric radiant wall/ceiling panel (about 600W) or multiple radiant wall/ceiling panels.
Q1: How do you choose between mounting these on either wall or ceiling? (I’ve read of it done both ways, but no clear discussion of pros and cons.) Is it even effective to install them on a sloped ceiling?
Q2: What is a recommended vendor for these? I’ve seen the Amaze Heaters (some reviews complain of a toxic smell that won’t go away, and no one answers when I call their customer service line) and Deelat (can’t find any reviews for these). Are there other vendors I should be aware of?

3) Two Envi heaters (convection)
These seem to take up a lot of room (both for clearance and the fact that they stick out from the wall), but they’re cool to the touch and seem user friendly.

Going on our guts alone, we like the idea of the electric panel on the wall. Seems like it would take up the least space, and blend into the background the best, and provide the least negative impact to the books. If we were able to ceiling mount, all the better. But I can’t find much discussion of people actually using them, or the pros and cons.

Would love any discussion and input you may have! Thanks.

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  1. user-7066239 | | #1

    Hello All,
    I forgot to fill in my user name before posting. I'm Elias.

  2. ethant | | #2

    Elias. A couple of things. First of all, your levels of insulation aren't really that high, though maybe they'd be considered so for such a small space. Code minimum (2009 IECC) for Zone 5 is R-38 in ceiling, R-20 walls. So you are below code minimum on the ceiling and just meeting it for the walls.

    That being said... having worked out of a small, uninsulated travel trailer for a few years, I can assure you that you'll have no trouble heating such a small space.

    My larger concern for you is the possibility of toxic off-gassing from the foam, and assuring that you have adequate ventilation to make sure that you are safe and comfortable and that you don't get mold on your books. You didn't mention open or closed cell but I highly recommend that you consider a vapor open assembly, such as dense pack cellulose or rockwool.

    I also recommend you install a simple through-wall HRV like the VENTS-US Twin Fresh comfo (

  3. ranson | | #3

    All resistive electric heat is essentially equally efficient, if you're using it to heat the air to the same temperature. (If you keep the room cool, but sit close to a radiant heater to keep warm, it will use less energy, but it doesn't sound like you're doing this.) If you want increased efficiency, you'll have to use a heat pump of some sort, like a mini split. A very small (6000 BTU) mini split would probably still be overkill for your building. It will also keep your building cool during the summer, and can be mounted up high on a wall.

    I would reconsider your insulation. Assuming your insulation is between joists and studs, you're well below code for the ceiling. Don't believe spray foam salesmen who claim that foam lets you get away with less insulation. With only 240 square feet, your energy bill is unlikely to break the bank, but you could do better.


  4. GBA Editor
    Martin Holladay | | #4

    Unlike Ethan T, I don't think you need to worry about toxic off-gassing from the spray foam insulation. That type of problem is extremely rare.

    Any of the electric-resistance heaters you have listed will work just fine, although your energy bills will be lower if you install roof insulation with a higher R-value. Depending on how you read the building code, your roof insulation is either just barely at code minimum levels, or below code minimum levels.

    If you want to save energy, install a ductless minisplit. It will use between 33% and 50% as much energy for heating as any of the electric-resistance heaters you suggested. You can use some of that saved energy to operate the minisplit during the summer, thereby lowering the indoor relative humidity for your books in July and August.

  5. ranson | | #5

    New York has adopted IECC 2015, so I think the ceiling insulation requirement nowadays is R-49.

  6. Robert Opaluch | | #6

    What's the R-value of the insulated floor? Would you consider electric cable heating embedded in the floor? That would keep heater away from books and may reduce heat stratification (cooler at floor, warmer at ceiling as heat rises from wall-mounted heaters). What is your finish floor material?

  7. user-7066239 | | #7

    Everyone, thanks for pointing out that this plan does not meet code for R-values in the ceiling. I am following up with my insulation contractor about this. I'm not sure why he suggested an R-value so low, unless I somehow misheard him when we spoke on the phone. The framing is 2x6, so even if we use closed-cell foam, at R-6.5 per inch, that's only a total of R-39, right? How would we reach the requirement of R-49 and still keep the sloped ceilings? I assume the shingles and roofing add a little, but do they add enough?

    We had considered a minisplit, as we're putting those into our house right now, but the cost for a dedicated unit for the library was just prohibitive. We plan to install a conventional thru-the-wall AC unit that includes a dehumidifier setting.

    As for ventilation, we have a ridge vent in the roof, and my insulation contractor mentioned adding soffits. Ethan T, your suggestion to add an HRV is interesting, and I do worry about getting enough air exchange if we use spray foam. I'd love to hear everyone's thoughts on how to ensure we have enough ventilation.

    Robert, Electric cable heating in the floor -- I hadn't heard of that one! I'm not sure of the R-value of the insulated floor -- it's been pre-fabricated, and back when we ordered it, I didn't even know about R-values -- I was just like, oh it's insulated, perfect. I need to go back and ask. We haven't decided how to the finish the floor -- perhaps laminate or cork.

    I do have a few qualms about spray foam, but given the sloping ceiling, I don't think there's any other option that gets us close to the required R-value. I spent some time last night reading about mineral insulation (rock wool), which might work for the walls, but I don't think my contractor has experience with it (he does cellulose and spray foam), and we'd still need spray foam for the ceiling, and to figure out the vapor situation for the rock wool.

    Again, thanks all. I'm learning a lot from the discussion.

  8. GBA Editor
    Martin Holladay | | #8

    If you read the following article, you will understand why your spray foam contractor is suggesting that you install less than the minimum code requirement for R-value: It’s OK to Skimp On Insulation, Icynene Says.

    If you read another article -- this one: How to Build an Insulated Cathedral Ceiling -- you'll learn about a number of ways to achieve an adequate R-value in an insulated sloped roof assembly with 2x6 rafters. Most methods require you to add some materials to create a thicker space for adequate insulation.

  9. STEPHEN SHEEHY | | #9

    Elias: have you thought about using some reclaimed insulation above the roof sheathing? And if you are concerned about the cost of a minisplit, why use expensive foam between the rafters, since you still suffer a significant thermal bridging effect through the framing? I think reclaimed foam above The roof and fluffy insulation between the rafters would be cheaper and you could meet code. On sidewalls, think about Zip-R searching and baths between the studs.

  10. user-7066239 | | #10

    Stephen, My shed is pre-fabricated by an Amish builder a state away from me, so the roof sheathing, shingles, joists, and rafters are already constructed and assembled. If I follow you correctly, that means that it's too late to add insulation above the roof.

  11. onslow | | #11


    My wife and I have quite literally tons of books, some of which resided in bookcases on exterior walls at our former home in Illinois. We likely experienced seasons and humidity that are pretty close to New York state,
    One problem you will discover over the winter is just how effectively densely packed bookcases interfere with heat distribution and wall temperatures. As you are already skating close to the edge thermally, I would not recommend setting back the temperature over the evening or weekends.

    I did not realize how much effect the books had until we moved and discovered signs of mold/mildew behind the bookcases. Admittedly, our walls were less insulated than your current levels, but the pattern of mold was pretty clearly matched to the shelves, with the encyclopedias having the biggest effect. Our new shelves are on interior walls only.

    Books are quite a bit denser than wood which is typically rated at R1.5 per inch. so you can imagine that 10" deep volumes racked across a 36" shelf will interfere with heat getting to the wall. The net effect is moving your dew point inward on the wall profile, which in our case became the surface of the wall behind the bookcase. You can perceive a milder effect if , in the winter, you have had to pull a sofa away from a wall to chase cat hair.

    Heat can be tricky in some situations, as temperature for a room is partly based on air temperature perception as well as the temperature of the objects in the room. It is quite possible to have a sufficiently pleasant air temperature and distinctly cold objects in a room that is heated with moving warm air. Set back thermostats can make sitting down for breakfast a bit bracing depending on how far you let things "chill" over night. The air will feel warm, but your bottom might not.

    Regardless of heat sources, the rate of loss through the walls,ceiling and floor is modified by the objects we put in a room. Sofas, bookcases, hutches and carpeting all modify whatever heat is distributed either by directly insulating or disrupting flow of warmth to exterior points.

    A mini split will provide a rapid response for air temperatures and maybe with the moving air even help get enough heat behind a sofa. It still won't have much effect on a floor insulated with a thick rug or a wall blocked by books. In very cold weather, the walls and floor may well be cooler than desired for those areas.

    Radiant can appear to amplify this effect simply because the air temperature is effected indirectly. Air is poorly heated by cove heaters (which I have a house full of) and it is only over time and with very good insulation that all objects in a room stabilize at a livable temperature. The air temperature is largely driven by heat given off by the objects in the room. A problem I have covered elsewhere on the forum.

    Based on what you have described for the structure, the floor heat seems like the poorest and potentially costly option. A lot of your heat will exit downward without better insulation and the much slower transfer of heat to the rest of the space will likely amplify the bookcase effect. The enviro type heater will more successfully transfer heat to some of the air and some heat to the objects by radiant, but anticipate running them without setting back for nights or weekends.

    For the protection of the books, long term, I would consider some form of shelving that keeps them off the wall by a few inches and not have a backing panel. Certain Mission style cases with the multiple slats instead of solid casework might be a viable option.

  12. Expert Member
    MALCOLM TAYLOR | | #12


    As usual Roger has provided a very useful answer.

    My summer project at home last year (a builder's equivalent of a holiday I guess) was a shed for my wife. It is about half the size of yours at 100 sq ft, and holds several thousand books. We keep it at about 68F in the day and unheated at night, using a portable oil-filled electric heater. Because there is no real source of interior moisture we haven't found any mold problems. The shed maintains itself at almost 10% lower humidity than the house.

    Being in the PNW I only used 2" of EPS on all six faces, and we keep the books an inch from the back of their shelves for some air movement. I don't think the books are affected at all by the temperature swings, it's the changes in humidity you need to monitor.

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