The Straw Bale Heat-Hog House: Energy Use Before Plaster Completion

Written by Andrew Morrison

House made of moneyI just received this email from a friend who is almost finished with his straw bale house. Unfortunately, his house is not performing the way he would have hoped. As the email below says, there is still interior plastering, drywall taping, and caulking to complete. In addition, the foundation stem wall is not insulated which likely has a big impact on the performance. If any of you have personal experience with a situation similar to this, please comment here. Thanks.

Hi Andrew,

We have a straw bale heat-hog house. Granted, our house hasn’t been plastered on the inside yet, still has seams between the sheets of blue board, and hasn’t had any caulking put on any joints where cold air could be infiltrating…BUT, it is plastered on the exterior, and it does have all the insulation in place. The insulation is R38 in the vaulted wing, and in the central entry wing, and R50 blown into the bedroom wing. With our radiant floor up and running, our 18kw electro boiler keeps the house at a constant and comfortable 60 degrees. I checked out the electric meter after returning from vacation, and, HOLY COW!, the boiler has racked up 3000 kwh in a little over 2 weeks time!

OKAY, the boiler ran non-stop for about 72 hours, bring the slab up to temperature, so I was hoping that most of those kws were devoured at the onset. As an experiment I noted the meter last night at sunset and again this morning at sun rise. No one opening or closing doors, just the cold night vs. the insulated house. During the night the meter racked up another 175 kwh. Ouch. Now, I never expected that just because we were building w/ straw bales, our house would be one of those ‘heat with a candle / cool with an ice cube’ jobs. We’ve too many windows, too much perimeter wall area.

I guess my question is can I hope that the heating load will diminish once 1) we finish up the interior plastering and caulking? 2) insulate the perimeter of the slab with some rigid insulation. (right now, the full 2′ height of the slab edge is exposed; backfilling alone would probably be of some help). 3) put up some window treatments such as blinds or curtains to pull during the night? 4) scrap the electro boiler and go with some sort of heat pump? (4 is not really an option for the immediate future).

I was hoping that 2 and 3 wouldn’t be necessary, but am thinking now that they will be. All of our radiant circuits are laid out so that they heat at the perimeter first and wind their way into the center of the slab. I think they lose a lot of the energy in the hot water at that cooler perimeter of the slab, thus requiring a higher amount of heat input from the electro boiler. (For example, when the house is holding steady at 60 degrees, putting my hand on the floor in the center of the room, the floor feels neutral to the touch. At the interior of the straw bale wall, not even near a door, the floor feels cool to the touch. )

Finally, most of this is me thinking aloud. Don’t feel you need to perform an energy audit or anything in depth. I’m really just curious if you’ve seen a a performance jump in your straw bale houses once the final plaster and caulking are complete.



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28 Responses

  1. How big is the house? I haven’t built a strawbale house yet but I have been doing research with my parents. Something that keeps coming up is the size issue. It seems to me that everyone thinks that just because your house is strawbale, you can build a 12,000 square foot monster and heating and cooling will be as cheap as a little cottage. Is this the case with the house in the email?

  2. Hi Adrien,

    My house is only 11,000 square feet.

    Just kidding. It is about 2300 square feet. Certainly not a cottage but not a monster either. The caveat is that the house is built around a court yard, so instead of being a square or rectangular house, it’s really two rectangular pieces connected by a long, thin entry that bridges the two wings. This means instead of a 45 ‘ x 45 ‘ box, with a little under 200 feet of perimeter, we actually have a little over 350 feet of perimeter.


  3. Hi Scott,

    I’m a building inspector in Wyoming. What climate are you in? If you are even moderately cold like us then your slab will suck the heat out of the house if its not insulated. Create a thermal break and insulate the exterior of the slabe all the way to the top of the slab if you can, and far enough down to keep below the frost depth in your area. belive it or not, this is one of the biggest heat sinks for homes.


  4. Scott,
    A few questions popped up while reading.
    1. Where is the house, exactly? North, South, East or West? Mountain, hillside, valley?
    2.Exposure to the South (ie sunshine) and how the house’s long axis (or axes if you’ve got 2 boxes) is oriented to this direction? This being established, how much sunshine does it get daily. Here in Germany this time of year you can go for weeks without sunshine, but plenty of rain, fog or snow(all the while with below freezing temperatures). In my home state of SC, you can be facing the sun directly, both summer and winter, but only catch a glimpse through the shade cast by the 100′ plus trees.
    3. You state too many windows. Is there more glass exposure on the north or south. What percentage is the glass exposure to wall size?
    4.How airtight is the house? Have you tried to see if there are any air leaks?
    I plan to build my timberframe/strawbale house in 4-5 years and have being studying the process for over a decade. I appreciate that yours hasn’t fully met your expectations and would really be interested in your progress/solution(s). Good luck and please keep this site posted!

  5. My straw bale house is 11 years old, 1750 sq ft heated with radiant heat warmed by a gas water heater. We insulated the slab and the used styrofoam block footings. My gas bill is annualized at $83 a month at 7000 ft altitude where the temperature is often in the single digits.
    My house has lots of south facing windows and is much warmer after hanging insulated drapes. Available at fabric stores, the insulated fabric is a bit pricey ( about $16 per 60 inch wide yard), but greatly reduces heat loss at night. This may be an expensive investment, but easier than watching the meter run wild!
    Your idea about the heat pump may be your best long-term solution. Not a choice here on volcanic rock.
    Best of luck in resolving this issue.

  6. Thanks to those who have taken the time to respond. I appreciate your thoughts.

    The house is oriented so that when the sun is shining, there’s no need for additional heat. So far that part has worked very well.

    In our county either perimeter insulation or a thermal break between the footing and the interior slab is required for a slab on grade. I talked them out of it in lieu of providing insulation beneath 100% of the slab. I’m learning that that was a mistake. Last week we put 2″ rigid insulation around the whole perimeter of the slab.
    Energy use dropped by about 25% right away. We still need to backfill.

    Additionally, there are air leaks in the home as it hasn’t been plastered or caulked on the interior yet. The use of insulating drapes in spots sounds like a good one to me too.

    Thanks again,


  7. Scott,
    Thanks for the update. This is good to know. Perhaps I should follow your lead and build my straw bale house in SC the same way. Our hot months far out pace our cold months and that would definitely help in passive cooling!

  8. Chip,

    Well…I would provide a thermal break between the footing stem wall and the slab, or at least insulate the perimeter of the slab as I recently did. As I learned, it doesn’t pay to heat the edges of the slab any more than necessary.

    We have a online diary of our home construction process, if you are interested in checking it out.


  9. I’ve enjoyed your blog for the last nine months. The most recent entry about
    the small straw cabin that heats up quickly prompted me to email the following

    I’m also interested in wooden boats and a recent publication reviewed small
    cast iron stoves that struck me as a good idea for energy efficient straw bale
    homes. These stoves are designed for heating small drafty areas, i.e. boat

    The company website is here:
    This may be an option for folks that want the heat from a wood stove, without
    the large BTU output of modern wood stoves.

  10. I wonder if having the huge windows and large cold slab require for most passive type solar homes is worth the loss of heat at night or in cold weather.
    Would it not be better to have small windows for light and fresh air while using some type of active solar collector to heat water for use in raidiant floor systems that do not require slabs? There are hard wood floor systems for heat.
    Maybe some combination of geothermal and active solar heat or heat pumps would be more effective in a house that has less glass and have a conventional basement or crawl space well insulated from the ground.
    What about fire place? Do they loose more heat out the hole in the roof than a fire and produce? Better to have a close well insulated roof?
    Thank you for any comments.

  11. Funnily enough Steve, my answer to all of these questions is very similar to the one I just left as a response to your other post question. To me, it is all about design. If the majority of the year is hot, then design the house to handle the heat. If the majority of the year is cool, then design accordingly. The questions you raised are all good ones and ones that should most definitely be addressed in the design phase. I do not believe there is a one size fits all answer to the questions; only a best practice response. If the windows are needed for the heat intake during the winter but cause huge heat gain in the summer, they probably were not design right. The same goes for anything in a house that is intended as an asset but ends up upsetting the balance of the home in general.

  12. We’re in the planning stages of a bale house in Bonanza OR. Our design has 800 s.f. on the slab, plus about 200 s.f. loft/ 2nd story. There will be 3″ rigid insul outside the foundation stemwall; don’t know about a thermal break yet. Our heat loss “UA” value I calculate to be 157 in daytime, 126 at night. (This would lose 8478 BTU/hr during a 20deg daytime, and 8316 BTU/hr during a 0deg night.) This is based on having _good_ thermal insulation in all the windows at night — 2″ rigid insul. Of course this is a commitment to putting them up in evening and taking down in AM; we believe that we owe the planet (and our budget!) some effort. We still haven’t decided where to store them in the daytime – maybe fold them in spaces adjoining the windows.

    I’ve done solar thermal modeling for a year, using Bonanza temperatures and southern Oregon estimates for sun, and found that our whole backup heat demand should be about 3.2 million BTU, which equals 950 KWH, for a cost around $60. This ignores the wood stove we will install. (Incidentally, I agree that a fireplace loses more heat than it adds; but a wood stove installed with a combustion air intake duct is well worth the trouble & cost.)

  13. We have added a straw bale addition to our existing frame house. We are enjoying our addition but we have a problem keeping it warm during extreme cold temperatures (-30C or -20F or below. We have not been able to get the temperature above 15C (60F)during these cold spells. We have a cement floor with radiant floor heat. The floor is very hot during this time but the room stays cold. We thought we just did not have the window sealed well enough and have correct that and have even put plastic on the outside to help further in that area but nothing seems to help. We have never had any problems heating the older part of the house. Can you give me some ideas or suggestion on how to improve the heat? O during this cold weather we have also tried using additional electric space heaters to no avail. Thanks in advance for any help in this area.

  14. Can radiant heat flooring work too efficiently in a straw bale home? (I read in additon to being expensive it can overheat a well insulated space). I am building a workshop that 22ft run south, and 42ft run east/ west. We are on a budget, and I am looking for a heat source that is the most economical long term for the shop.
    Any suggestions?

  15. Chrissy,
    Radiant can be expensive to use if you try to change the temperature all the time. The key to its efficiency is its ability to stabilize a room’s temperature. The best method to use is a static temperature method. To accomplish this, set your thermostat to the temperature you want and then leave it alone. Once the room stabilizes, it will be very efficient; however, if you try to bump the temperature up a few degrees and then drop it down at night, for example, you will end up paying out the wazoo for the heat. I personally think radiant heat with bale walls is an awesome combination and I would recommend it highly. If you need to add cooling to the equation, then radiant is a bit harder to use unless you try using cool water in the system (this is experimental right now, but has some great potential).

  16. Hi Andrew,

    When the sun is out, our house warms up and doesn’t seem to need the radiant heat input. I’ve been wondering if the slab slowly cooling throughout the day could possibly increase the additional heat load at night, once the sun sets and the temperature drops back down near freezing? I’ve also been wondering if there was a way to increase efficiency of the system by circulating solar heated hot water during the day to keep the slab temp. up so that the boiler doesn’t have to work as hard during the evening and night?


  17. Scott,
    Did you have the system sized by a professional or did you guys lay out the system yourself? That is a huge potential for inefficiency. Radiant systems are simple once in operation but are very technical to get right in the design phase.
    The slab’s cooling is what makes the mass so wonderful. That heat is released into the house and keeps the temperature of the room high without the use of the boiler. The slab should never cool down much below that of the rest of the room temperature because the system should kick back on as it hits the registered temperature. Be sure to set the temperature and leave it alone. Don’t try to adjust the heat in the house with the thermostat. It should stay at the same temperature all the time. Any adjustments need to be made over the course of a week and no less. If you have a fireplace, that is a great way to get a quick heat boost, radiant does not work that way at all and will cost you a fortune if you try and run it like a heat pump or “regular” heat source.

  18. Scott,
    See if you can find a copy of teh last editon of Mother Earth News. There was an interesting article that covered what I believe to be exactly what you’re talking about.

  19. Thanks Andrew. It was designed and sized by professionals, installed by us. Some of the problem must be that I’m either turning off the thermostats during the day (since pasterers are always coming in and out and leaving the doors wide open), or I have been changing the temperature up and down a lot, depending on whether we’re working inside or outside that week. We probably won’t know until next winter where things really stand – though I don’t know insulating the slab did improve things notably.

  20. Thanks Chip. Our library had a copy of that issue of Mother Earth News. That is along the lines what what I had been thinking. Good to know it’s been implemented successfully before.

    One correction – in the above post # 19, the last sentence should read “… – the I DO know insulating the slab did improve things notably.”

  21. When I was younger, I lived with in an appartment with my mother over an unheated garage.
    We had uninsulated wooden floors, single glazing and one coal (later Gas) fire, that heated the three rooms.
    So to keep warm, small is good. Over the mantlepiece we had a tilted mirror to reflect the heat from the fire. On the floor we had a persian carpet, but any woollen carpet will do, it was big enough to sit around the table but it never covered the edges of the room where the sofa and the cupboards were.
    Our curtains were made of thick cotton velvet,the kind you see in theaters, and many a night we had frosting on the windowpanes but not inside of any of the rooms.
    To make the evenings comfortable, my mother had taken out all inside doors, and hung them with more velvet curtains to keep out any drafts.
    In the middle-east where nights and winters can be cold, people pile up rugs one on top of the other to keep small children (toddlers) warm. But you could also use home-made or industrial felt, (feltslippers are the warmest)or old woollen blankets. For keeping the heat (or the cold) out, cover your windows from the outside, nothing can beat wooden hatches. The metal ones become too hot themselves. Use windows that open at the bottom, in my experience they ventilate better, you can smell the fresh air comming in, and when the outside air becomes too humid, or sudden drafts occur, you are the first to know, so you can close them just before the weather changes.
    love your blog Andrew, Conny

  22. Hi Andrew,

    I thought provide a quick update to this thread. While I’m still not happy with the houses winter performance, things have gotten a lot better. Insulation added to the perimeter of the slab helped, as did installing thermal draperies on all the french doors. And the wood stove certainly makes things cozy on the coldest January nights.

    In my opinion, there were several errors which contributed to the problem. First, by not designing a thermal break between the footing stem wall and the concrete slab, the slab could loose heat to the exterior. Even installing 2″ rigid foam (In retrospect, I’d install 3″ as insulation is cheap, all things considered) at the perimeter can only help so much. Second, some of the designed runs of radiant tubing were too long for the diameter of tubing. For 5/8″ tubing, runs should be kept under 250 linear feet and some of ours are close to 500. Considering that the tubing heats along the perimeter first, and that the perimeter is under-insulated, I believe that a lot of heat gets sucked into the huge concrete footings before it ever makes it further along into the slab. Second, we could have bought tighter, more well insulated doors. That would have cost more money, but I can see now that eventually that investment would have paid for itself in lower utility bills. Third, utilizing an electric boiler is killing us. A geothermal loop with a heat pump would have – as I’ve come to understand it – increased the efficiency at the heating end by a factor of 4. That’s four times the heat for every KW of electricity put into the system.

    It’s fortunate that our house performs so well during the summer months. Six months out of the year we have negative power use – this takes the sting out of December, January and February. Ultimately, we’re looking at adding a heat pump, or, if we can swing it, a geothermal loop with heat pump.

    Thanks for the forum.


  23. Scott,
    Glad to hear that life has become “better” but your lack of satisfaction is going to help you and all readers of this site, as long as you keep up dated with your progress. I know that reading it has helped me to fine tune the design for my timber frame and its basement.

  24. I’m assuming you live in a cold climate that does not get enough direct heat gain (sun)to provide passive heating. The number of windows installed, on the north side in particular, is critical. Too many windows on the north side = more heat loss. Equally important, is the ceiling insulation (should be at least R-40). Are the windows triple pane, Low-E? You don’t mention having installed rigid insulation under the slab, this is essential, no matter where one lives — if you are in a cold climate (Midwest or New England, for example) 3″ of rigid under the slab would be about right, again, to avoid heat loss. Since you are not feeling any heat when touching the slab, I suspect a combination of things are happening: heat loss due no under slab insulation and possibly a poorly designed distribution system (pex tubing layout and zone designations). The water temp with Radiant Floor Heat doesn’t have to be very hot in order to generate proper heat. Is there a circulation pump moving water throughout the pex tubing? If so, is it working properly?

  25. Thanks Andrew. It was designed and sized by professionals, installed by us. Some of the problem must be that I’m either turning off the thermostats during the day (since pasterers are always coming in and out and leaving the doors wide open), or I have been changing the temperature up and down a lot, depending on whether we’re working inside or outside that week. We probably won’t know until next winter where things really stand – though I don’t know insulating the slab did improve things notably.

  26. Andrew, You mentioned in the Denver design seminar something about a very small water heater being sufficient for in slab hydronic heating. Can you suggest some reliable sources for further information about this? Also, is underslab insulation considered essential if one is going to use hydronic heating? Does this negate the cooling potential of the slab in summer?
    Finally, is this the correct place to ask a question or is there a way to start a new thread?

  27. Hi Enga. There is no way for readers to start a new discussion on the blog, so this is a fine place to ask your question. 🙂

    I would talk to a “Green” plumber who specializes in radiant floor systems for the boiler or water heater. So much depends on the details of the design: location of the house, square footage, window and door square footage, ceiling heights, etc. They can give you a much better and more thorough answer that is building specific.

    I think under slab insulation is a good idea as it limits what you are heating or cooling to the material above the insulation (more or less). If you don’t use it, your thermal mass is certainly greater and thus your storage capability; however, that may make for a less efficient system in the end. I think it’s better to control what is and isn’t part of the system so it can be more accurately designed and maintained.

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