footing insulation 1

[georam]

In general, the depth of foundation or footings should be below the frost depth penetration. However, if this is not practical, there are methods to raise the foundation above the frost depth. But it has to be compensated by placing insulation layer (styrofoam). Any guideline in terms of thickness, type of styrofoam , and location of this insulation ?<br>
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Thanks<br>

 

[jfmann]

Constructing a foundation above the frost depth does not sound like a good idea, even with insulation........the only way I can see insulation being useful is if there is some heat source nearby.......such as the heat from inside a building........the insulation would then be used to retain heat by slowing down heat loss to the ground for example.....however I could be missing something......if so let me know!!<br>
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There are some useful publications about Styrofoam........if I dig them up I will pass info along.......for starters try Dow Corning.......perhaps web site.......

 

[TAC]

Insulation can reduce frost penetration, but it would have to be placed some distance above the foundation bottom and extend some distance beyond the edges of the foundation for optimum effect. Placing insulation directly against the foundation itself will have no positive effect. I can't imagine that it would be cheaper to insulate the footings rather than extend them below frost depth.

 

[harry]

There is an ASCE article which deals with this issue. It is "Design of insulated Foundations" by Eli Robinsky and Keith E. Bespflug. published in the Journal Of Soil Mechanics and Foundation Division September 1973.

 

[majdal]

In STAAD-III


Regarding the column design, How can I design columns with wedth of 20 cm, knowing that when i try to do this it says that &quot;column depth less than 10 inch, design ignored&quot;
Though if i did the design manually, i will get a result, so why cant staad do it???????
How can i go around this [sig][/sig]

 

[kdhenry]

NAHB did some research in this area. I have a publication titeled something like &quot;Shallow Depth Insulated Footings&quot; or something to that effect. Just moved and I cant find it.

You might also refer to the &quot;Building Insulation Design Handbook&quot; from the Oak Ridge National Labroraty (Publication DE88-013350)

You might also search the net for the foam insulation societies.

Karl D. Henry, Architect [sig][/sig]

 

[ishvaaag]

Regarding STAAD, No idea. Small columns of RC 6 inch wide were made to support stairs in the forties. Codes have become more stringent and some programs follow. In any case you can analyze it in RISA 3D or other package, or use whatever approximation or substitution you see good.

 

[dik]

We often use insulation to reduce frost penetration, for both new construction and renovation work. The first I encountered the use of insulation was about 40 years ago; it was being used for highway work in Michigan (was a kid at the time and my dad stopped so I could ask one of the people why it was being used). The material is typically EPS (expanded polystyrene) and it permits the geothermal heat within the earth to keep the frost from beneath it. It must extend away from the structure a sufficient distance that the 'frost path' is still greater than the normal depth of frost penetration. The equivalency is approximately 1&quot; per foot of soil. Dow will provide assistance in determining the thickness required and in their earlier catalogues they had a set of charts and diagrams. Another thing we do is ensure that all moisture is intercepted by means of a really good perimeter drain.

The insulation is installed either just below the topsoil or above the footing. The former so that the grass isn't 'burned' due to the reduced, impermeable surface.

 

[Lakelander]

I'm from northern Minnesota and this type of construction, especially for residential, is becoming very common for structural slabs on grade. Frost depth here is 5-6 feet deep and it usually becomes much cheaper than excavating for full depth footings if a full depth basement is not desired.
As dik mentioned, the earth provides heat, a constant temp of 50 deg F - always. For an area such as here, this can mean a differential temp as much as 100 deg F compared to the outside temp. It is therefore important to provide a means of stopping frost penetration into the foundation area. Keep in mind, frost mainly travels downward - not horizontally.

Generally, I use 2-3 inch thick(2-3 layers 1 inch thick, lapped) type 4 extruded polyethylene (R9-13.5, blue board or pink board depending on brand) along the vertical sides of the thickened slab and 4 feet out from the slab around the perimeter. This is conservative but utilizes the full sheet of insulation board-usually in sizes of 4ftx8ft.
Some items to remember:
Make sure the structure is heated and insulated BEFORE freezing temps have ocurred.
Provide drainage as dik also mentioned. This is important since heat is conducted thru moisture much faster and can subsequently freeze.
If the vertical insulation above grade remains exposed, it must be covered with a protective coating against UV damage.

I have also used this concept in designing &quot;frost-free&quot; footings for columns ,piers, and walls by pouring directly on the insulation under the footing.

A good guide to use is in the 2000 IBC for residential construction. (Starting at R403.3). There is also a free energy program called MNcheck.

http://www.commerce.state.mn.us/pages/Energy/Builders/ECodeMain.htm#Software

 

[dik]

Lakelander:
polyethylene or polystyrene?

Have used insulation for horizontally as well as vertical frost movement (some foundations with slightly over a foot of cover. and have never left it exposed to the elements (uv deterioration). Also for protecting pipes and culverts against frost jacking have used the DOW Hi-40 and Hi-60 load resisting insulation.

 

[Ibeam]

Lakelander,
Most homeowners do some landscaping around thier house- shrubs, bushes etc. If the insulation covers 4 ft. of perimeter away from the slab, then..no planting in that area?

 

[Lakelander]

Ibeam,
The soil conditions for growing will be different than with no insulation. Because the insulation is rather shallow, the soil may dry out faster. Also, I'm not sure what problems one may have with roots damaging the insulation board (which of course you don't want), depending on the plant. My two cents....Choose plants that tolerate drier conditions and have shallow growing roots and I don't think there would be a problem.

I have, by design, used less of a distance out from the foundation for the perimeter insulation when the characteristics of the soil (well draining, compacted gravel or sand) and the topography of the building site(ex. near other structures) have allowed it. As I said before, 4 ft. is conservative. I have used as little as 2 ft. For protection of the insulation board where digging may occur, place cement board or treated plywood on top of the insulation.

dik,
My mistake (I should proof-read better). It is polystyrene, not polyethylene. Thank you.

I also want to remind everyone when building a slab on grade, it is important to slope the existing soil away from the bottom of the perimter of the slab so that a &quot;bathtub&quot; effect is not created by the excavation for the slab/foundation.

 

[CarlB]

Some additional information on insulating foundations. In Anchorage, Alaska the code requires footings of heated structures to be buried at least 42&quot; below exterior grade (to bottom of footing)-unless alternate methods are approved. Typical frost penetration is 6'-8' in snow-free areas, and 10' in cold winters. Two inches of insulation is placed against the exterior of the foundation wall above the footing. This effectively directs building heat below footings to prevent frost below them. Alternate insulation schemes and footing depths are allowed. I've seen thickened edge slabs protected with 2' vertical insulation, then 2' horizontal insulation (both 2&quot. See following link for Anchorage UBC code amendments regarding foundations, Section 23.15.1806.

http://www.muni.org/services/departments/public/bsdhandouts/97ubcpublic.htm