Tuesday, April 6, 2010

Thursday, February 18, 2010

Super insulated Frost Protected Shallow Foundations, a proposed building technique by Andrew Johnson

Disclaimer: I am not an engineer. Consult and engineer and get proper approvals before beginning any non-traditional construction technique.

In cold climates, a traditional foundation requires footings below the frost line so the foundation does not move and crack with frost heave as the ground freezes. A basement makes this easy, but for buildings above grade, this issue must be addressed.

Traditional foundations require what is called a stem wall or frost wall, typically 42" deep in Fairbanks, to rest on footings below grade under the frost line, and support the building above. Here's an example from the City of Fairbanks:

This is relatively expensive in both labor and materials, requiring as many as three concrete pours, additional types of labor, and additional types of heavy equipment.

The basic outline of construction is:
1. grading
2. excavation of trenches for the wall
3. forming and then pouring the footings
4. building the wall out of concrete blocks, formed concrete, or treated wood
5. applying insulation to the perimeter
6. backfilling
7. preparing a subslab substrate of compacted sand or gravel
8. pouring the slab

The Frost Protected Shallow Foundation is an alternative foundation that eliminates the frost wall. It is being adopted in the USA, and has long been used in Norwegian countries.

The basic outline of construction is much simpler:
1. grading
2. spreading and compacting Non-Frost Susceptible (NFS) gravel
3. digging, often by hand, a slightly deeper perimeter for added strength, since this part of the slab is the footing.
4. forming and pouring the monolithic slab
5. applying insulation to the perimeter

To avoid frost heave problems, the foundation is protected from frost in three ways:
1. Final grading is sloped away from the building to keep runoff water away from the foundation, so there is no water in the soil to freeze.
2. Non-frost susceptible (NFS) soils, usually granular gravel without fines, is used under the perimeter down to the frost line, so if it does freeze, it doesn't move.
3. Perimeter horizontal insulation beyond the footprint of the building is placed to keep the ground above freezing. In Fairbanks, this can require as much as 4 inches thick extending 60 inches away from the building at the corners. Additionally, heat from the slab perimeter escapes and heats the ground to keep it thawed.

The astute among you may see a pattern in that list: any one item would prevent frost heave.

This is true, frost heave only occurs if all three conditions exist:
1. Water saturated soils
2. Frost susceptible soils
3. Frozen soil temperatures.

Why attempt to prevent all three? Primarily because it’s a good safety factor. It's hard to predict what might happen over the life of a building. Clogged gutters, erosion, etc, can cause soils to become water saturated.

In today’s environment, striving to be as energy efficient as possible, we must seek additional ways to increase energy efficiency. At the same time, perhaps we can improve construction techniques to save time and money. We can do this with a slightly modified FPSF design:

The Super-Insulated Frost Protected Shallow Foundation design modifies the FPSF to reduce the heat-loss to the ground by fully encapsulating the slab (and thus the entire building envelope) in insulation.

This is done by adding insulation under the perimeter footings. Higher density XPS foam with a rating of 60 PSI is available. This is 8640 PSF, greater than most local soil bearing specs. Perimeter insulation is reduced to a single layer 24” out, primarily to aid drainage.

Additionally, extra care is taken to ensure the soil under the building is non frost susceptible and will remain dry. This is accomplished by ensuring a full 48” of excavation, lined with geotextile filtering fabric, with a drain system installed and filled with NFS soil, compacted in appropriate lifts, and graded carefully to ensure drainage.

You will note that there is a lot of foam under the slab, which could become costly. In addition to extra insulation in the hottest part of a building heated with in-floor heating, this also reduces labor. By building the thickness at the center of the slab, the slab can be poured on an easy to prepare, flat compacted gravel pad.

Due to the cost of foam, there are modifications that can be made to reduce cost. Concrete is cheaper than foam, so the slab can be thickened. Additionally, A very shallow trench can be carefully dug to accommodate one thickness of foam.

Previous design, real world application:
Experience has shown that digging an 8” trench to accommodate a 4” slab with a 12” parameter, and equal foam underneath, is difficult. Compaction in the trench causes the trench walls to erode, and fitting foam to the base of the trench and the trench walls is time consuming. There will be a slight void, which the concrete will have to span, so extra rebar at this area may be considered. This is how my garage was built, and two winters have shown no settling, but I think the flat pad or 2” trench is the way to go.

Sunday, December 6, 2009


Yesterday friends Karen and John came over, and we headed out to the power line where the spindly Alaska trees grow a little better with more sun. Here's what we found!

Then today we put up 4 strings of LED christmas lights outside. They are the big C9 size, like I used to have on my house as a kid. I like the big ones, they make the house look more like a gingerbread house.

LED Christmas lights in detail:
LEDs seem to be the new thing. In this C-9 size they only use 20 watts for 200 lights, compared to about 1400 watts for the same number of incandescent bulbs!

The amber and red colors are not quite as bright as the old incandescent ones, but they are much more vivid. Green is about as bright and the blue bulbs are brighter since the old blue bulbs cut out most of the yellowish light produced by incandescent bulbs. For some reason the yellow bulbs are quite dim.

I have a couple 25-bulb sets of white incandescent C-9 bulbs in the back yard for yard lighting, and they are much brighter than the LED bulbs, but I could have 3400 LED bulbs and use the same amount of power. Still, the LED bulbs seem to be very visible but they don't really illuminate anything around them.As an interesting side note , LED bulbs happen to shine brighter and last longer the colder it is.

LED bulbs seem to be 2-4 times more expensive than incandescent bulbs, if you can even find them any more. The bulbs should last at least 5-50 times longer, but I wouldn't expect sets to last forever. Wires can crack, mass low cost manufacturing results in pretty low quality so sometime LED sets just die. All that aside,even if they outlast regular lights somewhat, they save installation time and tons off your electric bill.

Saturday, November 14, 2009

Another shed

Marcy and I did another craigslist weekend shed project to help pay the vet bill for Roland. This was a water shed. The floor is designed to hold 15,000 pounds, and the roof is designed to withstand falling snow from the house. It will get siding in the spring.

Saturday, October 31, 2009

Happy Halloween!

Here we are, Fry, Leela, and Nibbler.

Friday, October 23, 2009

Shed Building

Paul and I (with some help from Ty Keltner) built a shed for his new house. Fairly simple, shed roof, no roof trim to save money:

I enjoyed it, so I put a craigslist ad offering sheds built, any style. I got a response and we ended up building this:

I didn't get any photos earlier, but here's some of the building process.

The temperature was dropping to about 20 degrees, so we were slowed down some by finicky nail guns. Also, daylight was short. Without those obsticals, this would have been a weekend project. As it turned out, we needed to spend a couple hours Monday and Tuesday evening to finish up.

The owner was very pleased and Paul and I made some extra money while getting exercise. We might try to do more of these next summer.

Thursday, September 3, 2009

Wednesday, August 5, 2009

Tuesday, August 4, 2009


There are numerous fires burning around fairbanks, including one alarming one that just sprung up some 30 miles from our house. It was the driest July in history, I believe I heard. Tonight it has been fairly clear down at our house, but the smoke is rolling in and has made the sky a wild yellow color. This photo is with the white balance set at daylight, which would yield true colors on a sunny day, and slightly blueish colors on a cloudy day.