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Fundamentals - Insulation
Use the right amount of insulation of the right type in the right
places.
How much is enough?
Insulation is measured in R value. Type 1 EPS for example gives you about R-4.17 per inch thickness (in cold climates) and Type 2 EPS is about R-4.5 (in cold climates). Assuming a proper wall design (see below) then the more insulation the better, but insulation costs money so you want the greatest thickness where it's needed most. When deciding where it's needed there are two considerations...
1) What is the temperature difference between the inside house temperature and that particular bit of the outside environment.
2) The fact that heat rises.
Taking the average over the year and assuming you are in the northern part of the USA (or equivalent cold place), the ground below frost level is the warmest, the ground near the surface is a bit colder, the air outside your walls is even colder, and the hot air is mainly trying to get out through your roof. Here are my recommendations for how much insulation is needed in the different places...
| Ideal Best Insulation | Rec Best Type2 EPS | My Type2 EPS | My EPS R | |
|---|---|---|---|---|
| Under footings | R-10 | 2.22" | 2" | R-9 |
| Under slab | R-20 | 4.44" | 6" | R-27 |
| Edge of footings | R-30 | 6.66" | 9" | R-40.5 |
| Walls | R-40 | 8.88" | 8" | R-36 |
| Roof | R-60 | 13.33" | 12" | R-54 |
"Rec Best Type2 EPS" refers to the recommended thickness of EPS (assuming EPS is the only insulation used). "My Type2 EPS" refers to the thickness of EPS (Expanded Polystyrene) that I am using. The thickness of EPS varies a bit in places on my design so I have tried to use an average value. In the case of the edge of footings, my EPS thickness increases as you go upwards towards the surface.
The EPS on my walls and on my roof is on the outside of the structure, which is where it should be for optimum thermal mass performance.
It is ok to vary a bit from the recommendations. In my case the walls and roof are 10% less insulated than I would like, but I made the conscious decision to not make the walls too thick because it was cutting into the floor area too much and reducing the visibility angle through the windows too much. My walls are 17" thick (sometimes thicker) and making them even thicker seamed not to be a good design choice.