Insulated Mass

Insulated Mass

On Earth Tubes Andrea said…
I love following your journey to your own home and how your family has actually truly built it yourseves. I do have a question. You had mentioned you will be earth sheltering your home and you have mentioned the work of Haite. But you have insulation under and all around your house that would isolate the house from the earth. Why? Dont you want the warmth of the earth in the winter and the coolth in the summer?

The diagram above shows how we setup our wall. The 50 tons of thermal mass (grey) of our Tiny Cottage is on the inside of the insulating envelope (pink). Outside of the pink foam is a parge of PVA fiber cement textured to look like stone that protects the foam board from the sun, weather, insects and chickens. On the inside is a parge that looks like tinted plaster but is made of a fine sands concrete to give it greater durability.

When thinking about contact with the soil, remember that the earth is not a good insulator, rather it is a good flywheel to temper the seasons. There are several reasons why we insulated with pink foam outside the thermal mass:

1) Here in Vermont our soil temperature is fairly low, about 45° to 50° F even down fairly deep. The typical quote is that the deep temperature is in the 50’s or 60’s but it isn’t like that here, especially in the winter when it matters most. Thus our winter soil temperature is too low for comfort for Holly especially. Thus if we thermally connected the house to the soil we would be constantly pumping heat into the earth to try to warm the entire mountain up to the 60’s. Rather a futile endeavor.

2) We’re on ledge. The ledge acts as a wonderful conductor letting the heat of the earth out and thus cooling the ground. In some places the same piece of ledge that we’re sitting on is also exposed in the middle of the winter to high winds and -45°F temperatures. The soil/ledge is pretty cold so I don’t want to bleed heat out to it during the winter – thus the Tiny Cottage sits on a pad of insulation over crushed stone. In the good news we have deep, early snows which keep our frost depth pretty shallow, only about 3″ in most places. Still, I wouldn’t want to bleed heat into that cold frozen soil.

3) We live in an area where plenty of water runs down the mountain in the spring and fall. In fact there is a run that goes right by and sometimes under the cottage through pipes I setup to channel it. The flowing water conducts heat away further making the soil not all that great a storage medium. Since the entire mountain flows with water at times it would be hard to avoid that. What we did do is put a swale and berm above the cottage to direct much of the flow away. We also put pipes and crushed stone under the cottage. The closed cell insulation under the cottage helps keep the floor warm.

4) It will be several years before we get to berm the back wall of the cottage. I want to build a small extension to the back of the Tiny Cottage which will end up inside the berm. This will be our new root cellar. Once that is done we can look at doing the berm. So until then we need insulation on those walls even though they’ll be interior walls in the long run.

5) Not all the walls off the tiny cottage will be bermed. All the areas where the windows are on the south, east and west walls will be exposed. So the stone, brick and concrete there need to be insulated or we’ll leak heat.

While the earth does work great for warming the incoming air in the winter, it isn’t much of an insulator or a thermal storage in our situation. In a dry place like Colorado where John Haite is things would be very different because his soil is so dry. Doing an umbrella helps keep off the down falling rain moisture. Unfortunately the umbrella won’t help with the water that flows along the ledge boundary below us.

Our solution for the Tiny Cottage is we enclosed a large thermal mass, of about 100,000 lbs, inside of an insulating envelope. The sun coming through are large windows warms the mass most of the way to a comfortable temperature. Our little wood stove brings the house up a bit more making Holly a happy camper. The wood stove is surrounded by thermal mass so most of its radiant heat goes into the house’s mass for storage and is then slowly released. When we do berm the northwest, north and northeast sides of the house the earth will temper our climate a bit more from the cold outside winter climate. The deep snows we get also help and the berm already lifts the wind keeping it off of the cottage.

Earth sheltered techniques are valuable ways to build, not just for heating and cooling but also for minimizing maintenance. However, they must be adapted to each climate and situation. What will work in Arizona might not work right off the page in Louisiana, Vermont, Alaska or by the sea shore. Each place has it’s own special little challenges to keep up our creativity. Be cautious of plopping down a design from another area into a novel situation.

Outdoors: 56°F/38°F Mostly Sunny
Farm House: 65°F/61°F Chicken Run Up
Tiny Cottage: 66°F/61°F

About Walter Jeffries

Tinker, Tailor...
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3 Responses to Insulated Mass

  1. karl says:

    if you don’t mind, what is your parge recipe that you used on the exterior of the foam insulation?

  2. Karl, it is a simple portland cement and sand mix with the addition of PVA fibers and a little lime. See this post.

  3. Barb says:

    Great blog! I love your tiny cottage.

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