Tiny Cottage Temperature Stability

Saturday, June 30th we finished up the last of the major fencing projects. The major planting was done except the occasional succession planting. Home school reports were completed and ready to send to the department of education. I’m making good progress on the wholesale/retail pastured pork sales. All on schedule. That meant that July 1st, after months of only small labors while we planted and got ready for summer, we were finally able to turn our attention back to the tiny cottage we are building up on the hill by the upper pond. This will become our new house this fall as I have no intention of living in this old house another winter.

It isn’t that the old house is too small for our growing family – far from it. The new cottage is a quarter the size of the winter portion of the old farm house. I realize most think bigger is better and the mere idea of five people living in 252 sq-ft is enough to drive some over the edge into insanity. But the reality is we spend much of our time outdoors year round and in the winter we already close our old farm house down to a core of about 1,000 sq-ft for about five months. Even that small winter portion is hard to heat and the whole requires a lot of maintenance. The new tiny cottage will be self maintaining and heating. It will be easier to live in – everything in the cottage has meaning and function.

Even now, although unheated other than the sun, sealed tight with no opening windows yet, single pane glass and with no ventilation the tiny cottage is keeping itself at an even and comfortable temperature varying by only a couple of degrees night to day. The secret is the enormous amount of thermal mass in the small space. The concrete and stone are soaking up the daily dose of passive solar gain without letting the cottage overheat and then keeping it warm at night by releasing a some of that heat back to the room.

According to everything I had ever read about passive solar heating I’m doing it wrong. The writers stress that it is important not to have too much glazing area, especially in small areas because it will cause overheating. The recommendations I’ve read over the years talk about recommended glazing of 8% or 9% generally to a maximum of about 20% with dire predictions of overheating if you go beyond that. We have 162 sq-ft of windows and only 252 sq-ft of floor space. That’s a window to floor area ratio of 64%. What they don’t take into account is having a very large amount thermal mass. Most construction is stick built and it’s hard to get 100,000 lbs of stone and concrete in 252 sq-ft house when you’re building with wood. Since the tiny cottage does already have almost 100,000 lbs of concrete and stone the high window to floor ratio works. In the end we’ll have a little bit more mass, be living in the house and cooking to warm it as well.

Years ago I build a greenhouse, with little thermal mass, off the south side of our farm house years ago and yes, it did overheat during the day in the winter often getting over 100°F – the trick was we blew that heat into our house to heat it. Then at night in the winter it would drop down to the low 40’s – we didn’t heat it and closed it off from the house at night. That greenhouse had some thermal mass, a few hundred gallons of water and a few thousand pounds of soil in the planters. This worked very well – that temporary structure served us well for a great many years until the ice storm of 1998 ripped it down. While it was there I used it to experiment and develop many of the ideas that we’re implementing now. With the tiny cottage the house itself is the solar collector since we have such massive windows wrapping around the east, south and west sides of the house. Keeping everything passive means there is less to fail, less to maintain and more time to invent new things. I don’t like doing maintenance.

Interestingly the mass within the tiny cottage feels pleasantly cool during the day and slightly warm at night. This is because the thermal mass is a little below the day time air temperature, shown below as daily high/low, and then a little above the air temperature at night. In the cottage the daily variation in temperature is around three or four degrees Fahrenheit. The outdoor temperature varies much more greatly over almost a 30°F range and the daily swing in the post and beam wood framed farm house is about 10°F.

For a while I was measuring the temperature inside the living room walls to compare the thermal mass to the air temperature. Fortunately my wife is very understanding about me drilling holes in the walls – she knows they’ll get covered by the final stucco parge. In a nutshell, as is to be expected, the day temperature change of the wall lags a little on the rise and then the night temperature change lags a little on the drop with the thermal mass staying about a degree off the extremes of the air temperature. At this point the sun isn’t actually hitting the interior walls that much during the mid-day because of the angles of the overhang. I did that to avoid over heating the house in the summer although in retrospect it wasn’t as necessary as I anticipated. During the the winter the sun is low on the horizon so it reaches all the way to the north wall of the house directly heating the mass.

If I had the money I would implant thermal probes, light probes and such all over the house and hook them up to a mux and computer. Then I would be able to actually log the real time temperatures so I could analyze and graph them. It would be very interesting but not likely given the high cost. I get a good enough approximation using my two min/max indoor/door thermometers. The good news is that the data I have collected fully supports the theoretical models I made so collecting a lot more data wouldn’t give me more real answers. It’s a case of diminishing returns. Perhaps I can thank the lack of cash for avoiding wasting time on such fun projects – Still, it would be fascinating…

It is very interesting to look back over the log of temperatures since the tiny cottage was closed in. We used only a little electric heat to warm the cottage during the very coldest period of the winter when it was extremely windy and -20°F. Other than that the cottage has kept itself above freezing and even reasonably comfortable through the winter, spring and now summer. An important thing to understand is that the thermal mass started out cold in January because we did not close the cottage in until the very last days of 2006.

After having gone through the summer to warm the mass the house will be that much warmer and stabler this coming winter. My goal is a house that will keep itself warm in the winters and cool in the summer. I’m watching the daily temperature readings with fascination to see how all my theory and calculations play out in the real world – It appears to be working as I anticipated. It is always nice to be proven right – especially when the project is cast in concrete…

Outdoors: 82°F/53°F Sunny, 2″ Rain in three days
Farm House: 73°F/63°F
Tiny Cottage: 71°F/68°F

About Walter Jeffries

Tinker, Tailor...
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2 Responses to Tiny Cottage Temperature Stability

  1. karl says:

    thermistors aren’t that expensive–ten bucks max. the input devise for a computer is a bit pricey but could answer tons of questions, control loads of devises and be great fun. just think your house could be online (re: daycreek style) besides, what a great home school project.

  2. cat says:

    So much work!! way to go! :) can’t wait to see the finished product..i’m sure you can’t either..heheh

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