Insulation for a walk out bacement

Paddy · Aug 21, 2006

I'm contiplating the best placement of insulation for a walk out bacement.

Basic layout; It will be poured walls and finished as 2 guest rooms. The slope will make the bacement about 40% exposed. Back wall completely back filled, side walls with retaining walls 4' at the front- 60-70% exposed and front wall would be 100% exposed. The front will have sliding doors for a nice view to the lake.

My current home has a similer arangment but as a garage under my office. It has 8" poured walls and no insulation, no heat. In the heat of Summer it stays very cool, (I like that!). In the winter it can be chilly but compared to out side very fine. I keep my tractor in this space and she really likes the easy starts when it's 5 deg outside! I have had a water line freeze in extramly cold conditions. It feeds an out faucet on the wall that is completely above grade. It runs along the wall, down from the heated section above.

My gut feel is to capture the nice 50 deg earth temp any where below grade via, no insulation. And insulate any where above grade including a foot or two below. The insulation would be 2" blue foam covered with some sort of stuco.

What do you guys think? I'm thinking if I fully insulate I won't be able to take advantage of the year round 50 deg. I'm in Southern Indiana where we have hot Summers and cold Winters.

dmccarty · Aug 21, 2006

Insulate.

50 degrees in summer would be good. 50 degrees in the winter is better than 20 degrees but it would stil have to be heated to something comfortable. I thought about this with our slab but ended up putting in two inches of ridgid insulation under the slab per a Listiburek's construction details. It seems to work.

Later,
Dan

Highbeam · Aug 21, 2006

You are not trying to heat the home with the 50 degree dirt but the 50 degree dirt will absorb less heat from your home than the 5 degree outside air. The wall will experience less loss because the temp diff between inside and out is smaller. Insulation will help you since thermal transfer will be even slower. I would prefer the insulation on the outside so that the interior of the home will benefit from the thermal mass of the concrete. If you insulate the inside of the wall then you have the coleman cooler effect where only the air is at the warmer temperature.

Earth berming or underground homes benefit from a smaller differential temp between inside and out which keeps the inside from loosing and gaining energy from outside air.

BobRip · Aug 21, 2006

Insulate Concrete Form houses (I have one) have the insulation on both sides of the concrete. This make best use of the thermal mass. If you can only put it one place I would do it on the outside of the wall. Another inch on the inside would be nice too.

dirtydan · Aug 22, 2006

I would agree with Bob. I have seen basements done with ICF's and they are nice. Easy to put up and pour. I also used them for my foundation walls on my house though I only have a crawl space. My wife and I put them up ourselves. You can put sheetrock right over them if you want to have finished rooms.
Dan

Paddy · Aug 22, 2006

Bob/Dirty,

I have looked in to ICFs. My issue at this time is not how easy the ICFs go up but, "where is the best location for insulation?" I believe the ICF systems have the insulation on both sides for the convienence of the forming, not best placed insulation. HighBeam gave a fine example; Two frigs, one loaded with our best adult beverages, the other is empty. Open both doors for five min. The full frig will quickly return to a temp that is close to the starting temp. The empty frig will have raised temp. This is due to, no Thermal mass is inside the insulated envelope. The foam is best on the ouside of the poured wall.

The basement senario is an odd beast. You have two diff conditions, above grade and below grade and two heating/cooling conditions due to Winter/Summer. Heat is what moves, not cold. The Summer case, 95 outside, 74 inside and 50 below grade; The exposed portion of the wall, heat will move from outside inside. The lower portion heat will move in to the earth through the wall. Top description is not desired but the bottom is desired. Insulation at the above grade section will reduce the outside heat from entering. No insulation at the bottom will allow the earth to attract the heat from with in. Winter Case; 15 deg outside, 68 inside and 50 below grade. Both top and bottom sections, heat moves out side. The main diff is the rate. Because there is a much smaller temp difference, heat transfer is greatest on top at a ratio of 68/15 vs 68/50. Again, insulation would help on the top section but also would help to some degree on the lower section.

Since this is a lake house and the walk out rooms will be "guest over flow" during the busy Summer, I'm leaning to the natural cooling below grade. I will need to remove humidity either way.

Highbeam · Aug 22, 2006

An ICF wall is a step worse than a poured wall with outside insulation in regards to good thermal mass. You need to have the mass exposed to the conditioned space in order to make full use of it. A layer of insulation on the inside isolates the concrete mass within the ICF from the conditioned space. However, an ICF wall beats the heck out of stick framing and the ease of construction for ICFs might make them the preferred alternative.

Paddy · Aug 22, 2006

Highbeam,

Yes, thermal mass needs to be inside the heated/cooled space. Since I'm largely under ground in this portion of the project, stick framing isn't even in consideration. Just as you noted, the insulation does little good on the inside, I'm starting to think foam does little good on the outside below grade.

I'm sure ICFs are great for some folks and some situations. And I would say for would be more satisfied with ICFs than my curent timber frame if I was building it myself. But to maximize and to understand what is the best is my task. It may be cost prohibitive to have the best set up. Location plays a major role. An example might be as simple as picking your roof color. In a sunny cold climate, black would be best most of the time. In FL, I'll bet white! Same is true for thermal mass/insulation. High elevations in warm in day+cold at night, lots of mass and no insulation. We could take this a step further, you could live in a zone where all the above, roof color (solar good/bad), yearly temp swings and daily temp swings, make this task a moving target. So I feel you have two basic choices 1) Pick the solution that meets the need more days per year than the alternitive, 2) be able to make continious changes adapting to the current needs.

"Adapting to current needs". At today's low cost fuel and I do mean that, inovation is a balence. Having a roof change color daily/monthly would be great but expensive. A wall's thermal mass could be changed via water in jackets. A design I'm working on focuses on one of the best insulations known to man, a vacuum. A vacuum can achive R-values as high as R-250 per inch! If you were to use Blue foam it would take 50"! BUT, the problem with vacuum it is very tough to hold for any length of time and is very expensive. Hence my project deals with these issues.

BobRip · Aug 22, 2006

I think you know the answer for your situation, put the insulation on the outside. This will work fine.

For a house, (I know not your situation), the ideal situation would be a thin layer of insulation, a thin layer of thermal mass, and repeat many times. This is like a multipole electrical filter (if you speak electronics) and gives you the best R value/thermal mass affect.

Paddy · Aug 22, 2006

BobRip,

Interesting idea to explore. How does it work? I can imagine the heat is slowed by the R-insulation and if missed, absorbed by U-Thermal mass. And on an on.