(back to blog index)


Sun, Feb 6
7:00 am

Cold Weather - Insulation vs Thermal Mass: We had record cold temperatures this week in Tucson as arctic air dipped way down across the country. Our nights were in the teens and days in the 30's. I was told we were colder than parts of Alaska! I can't ever remember it being that cold in the fourteen years I have lived here. Usually, storms bring cloudy weather, so my solar heating is not at its peak, but this cold brought clear skies, so I could really see how my sunroom works. I have no central heat.

My house is like a heating/cooling unit where I adjust it according to the outside temperature, to maintain the best internal temperature. I'll explain. As I studied natural building, I learned two of the main features in passive solar house design are insulation and thermal mass. Thermal mass heats up in the sun and insulation does not transmit heat. They are often confused. Think of a rock and a straw bale sitting next to each other in the sun. The rock would get warm as the sun shone on it. If the night had been cold, the rock might still be cool. The straw bale would not change temperature noticeably. The rock is thermal mass. When you burn your feet on a hot, sandy beach, you are feeling the effect of thermal mass. Anything made of rock, concrete, earth, sand, tile, and adobe is thermal mass. It gets warm in the sun and it gets warm from the light of the sun, not the transmitted heat. So, you can have glass in-between the sun and the thermal mass and it will still heat up. I've never heard of water referred to as thermal mass, but it must be, since it heats up in the sun also. I think rubber tires would be considered thermal mass, also. You get the idea. A thick exterior thermal mass wall will heat up in the sun and slowly transmit that heat into the house, unless the night cools it off before the heat reaches the interior of the house. I am no expert on adobe, thermal mass walls because that is not what I need in the hot desert climate.

I agreed with several articles I read, that the optimum house design is one with insulation in the exterior walls and ceiling, but thermal mass in the interior to maintain an even interior temperature and help hold onto your heating and cooling methods. If you are in a perpetually cold climate, then you might want thermal mass in your walls, so you can let the house heat up from the sunlight all the time and transmit that heat to the interior. I don't live in such a cold climate, which is why I chose insulation for my exterior walls. Most of the time, I am working to keep heat out of the house. You know how I insulated my walls - with straw bales - and if you read the book, then you know I used R-30 batts plus a radiant heat barrier in the ceiling. The R factor of bales varies with the bale, but it runs in the 30's to 40's. I have very large, solid bales, also. Insulation in the ceiling is very important. A radiant heat barrier is double-sided foil (stronger than kitchen foil) that is laid inside the roof to reflect the temperature of the roof back outside and the temperature of the house back inside.

Having said that, this is how my house works in cold temperatures: the south winter sun shines directly through the sunroom windows and heats up the earthen floor and the earthen plasters on the walls. I open the french doors from the sunroom into the house and let the heat pour into the house. When the sun goes down, I close the french doors and let the thermal mass of the interior maintain the temperature through the night. It does an amazing job and my house will only cool off a few degrees overnight. My house was about 70 in the days when I got home and cooled to about the upper-60's. I don't have thermometers around the house, but I should do that. I should put one in the sunroom and then in the main room to really be able to report on how it all works. Stay tuned . . . As a practical note, however, the sun blazing through windows for days on end, really does bleach the colors out of things. I cover my chairs and carpet in the sunroom with towels or sheets most of the winter to keep their colors. I know this is a problem if you have your south-facing windows shine directly into your living room.

This morning the outside temperature was in the upper 30's and my house is in the mid-60's with no heating used beyond the sun yesterday and last night. However, I do have baseboard heaters I can pull out in cloudy weather. Often, our coldest weather is cloudy and I really can feel how much this cuts down on the solar heat gain. But in this cold front, I was cozy. Now, I'll go outside, take the sheets off my plants and see what survived. Cactus is the most amazing stuff! I survives from about 20 degrees to 120 degrees on very little water. I'm just not sure if it was ready for the teens.

Keep it Simple - and be kind!