Shallow Insulated Foundations

[dwolff]

Does anyone know how much insulation you need to properly insulate a shallow post and pier foundation?  When I say how much I mean what R-value and how far out do I need to go horizontally in each direction.  I bought a manual on this from the US government but they only covered slab and crawlspace construction.  The frost is 48" deep in my area.  Thank you for any replies.

[Kevin]

Well I did one for my cabin in Northern Vermont but cant tell you how I made out till this spring.
But I went out almost 10 inches al the way around.
Kevin

[John Raabe]

The general (perhaps conservative) rule of thumb is that the total insulation depth/length (down and out added together) should equal the frost depth.

See if this report has anything more to add:

http://www.toolbase.org/docs/SubsystemNav/Foundations/4495_RevisedFPSFGuide.pdf

[Amanda_931]

Wouldn't post and pier insulation be different?

You have to make sure that frost heave won't move your posts, but this won't have anything to do with insulating your house, will it?

[John Raabe]

Yes, for the house, the insulation is in the floor so no house heat is keeping the foundation warm (which is somewhat true of a slab floor).

Still, a shallow post and pier foundation can benefit from foundation insulation if the soil is expansive. Several people who have built the shallow Little house foundation have been in cold areas and have put insulation (sometimes only straw) around the piers and have not had any movement.

[dwolff]

John, if I understand you correctly,I should be able to get by with a 4'x4' piece of foam insulation over my footing if I dig 2' down.  The next question would be what R-value would I need.  Being conservative would R-10 be enough?  Would it be better to use the type of foam that it put under cement slabs?  Thanks.

[JRR]

To my mind, the principle would be the same for shallow foundations or footings under posts.   In either case you are trying to slow the vertical transfer of heat upward from below.

Using the link above.....I would use the insulation R values and dimensions recommended at the corners of shallow footings....as shown for unheated buildings.

[Amanda_931]

OK.

And this afternoon I finally opened the March Fine Homebuilding magazine and found the problem staring me in the Q & A section on pg 104.  Not unfortunately in the online site.

The guy--from Camden Maine--who wrote the answer is currently using some sort of patent pier form but says that anything to keep  the frost from grabbing the concrete (schedule 40 pipe, even plastic sheeting) works just fine.  

The picture shows 2" rigid foam board  around the pier  a few inches down.

While it looks in the picture like the diameter of the foam is about the height of the pier, there's not much said about the size.

Like my builder friend here, he likes crushed stone around the bottom, and (river?) gravel in the upper parts.

[John Raabe]

Yes, 2" foam (blueboard or pinkboard so as not to absorb moisture) would be about right. This is R-10.

Crushed rock for the footing since it won't roll about and settle. River rock is fine for the backfill.

[dwolff]

The article mentioned by Amanda in Fine Homebuilding shows a plastic tapered form to help prevent frost heave.  They use  gravel on the bottom and they use the R-10 foam 1 foot down as another safety measure against frost heave.  I'm sure this system works very well, the problem I have with it is it isn't a shallow foundation.  He digs down 5 feet.  For a person like me building in a rural area doing things by hand in clay mixed with lots of rock this system isn't as attractive.

[Amanda_931]

random thoughts on the subject.  Unedited brainstorming!

Guy in the article does not have any crushed rock UNDER his forms.  Doesn't mention that he tamped the #*&$+!! out of the bottom of the hole either.  I don't know which is more reasonable in this case.  I can see a case for both ways.

But since part of the purpose of the patent forms is not to give frost anything to hold on to, then schedule 40 pipe (or plastic sheeting around a sonotube?) might do the same, and maybe you could ignore some of the depth.  

What if there was never much if anything in the way of sunlight on your piers?  Or if you only dug all the way down with those on the south side?

I have a tractor with an auger attachment to help with digging in my rock+silt soil (the septic tank guy says that all that silt came from the Dust Bowl, carried by the winds--but then why do I get all the silt in the spring that apparently comes out of rock?)

Locally rented Ditch Witches (right expensive little babies) are apparently toothed so that they can dig up rocks and won't balk at a layer of shale.  But I'd hate to use one for a handful of holes.

But if it's not quite worth cranking up the tractor for a hole, then one of those 16 pound digging poles works fairly well.  Post hole diggers help if you are taking loose stuff OUT of the hole, but not otherwise.  One of the Stiles' books books mentions the slow digging holes for piers process, and then ruins the commiseration by saying that they got all their holes dug in about the time it has taken me to do ONE!  Busting through shale ledges is a lot of fun!

Do shallow foundations work best if there is a DRAIN TO DAYLIGHT from them?  Is this something one has to do with poles?

Doesn't sound like the best possible way to have to work piers and poles--quite a lot more annoying for digging than doing the foundation--even if it might not be quite as much work.

[John Raabe]

Remember that the whole thing about frost depth is that it is the deepest level that exposed open ground is expected to reach 32ºF. Foundations built closer to the surface than this might have footings that will freeze, but only if the soils are both saturated and expansive (clay or fine silt).

Is this a problem in your soil? Many times not:
• not if the soil is granular and water drains away
• not if the soil is wet clay but is insulated to hold the warm earth heat.
• not if the wet clay soil is drilled out and has crushed rock down far enough to rest on unfrozen ground
• not if the wet clay soil is heated by warmth from the building above

[Amanda_931]

Thanks John!

[Hal Nash(Guest)]

If the frost depth is 42", does that mean that one needs to insulate the ground around the pier in a 42" radius?  And a foot deep?  That basically means digging a 7' (84") wide hole one foot deep.

WOW!

That seems like quite a lot of excavation, especially if one is putting up 16 or 20 piers!

Why do that when you can augur out a 12" or 16" hole with a tractor and set concrete piers or treated wood piers?

OK.  Enough of that.

Would building codes allow one to insulate around the surface of a shallow pier using a plastic tarp covered by 6" or so of wood chips or some other natural insulating material?

That's what I did around my cabin...but then there are no building codes that say anything about doing this in my area.

[John Raabe]

Several people have built post and pier footings and insulated them just as Hal suggests. All have reported no problems.
(Here's a link to Hal's earlier post - http://www.countryplans.com/bbs/messages/2985.html)

Whether this is because the straw or sawdust works so well or because the soils were not water logged or because the frost depth was nowhere near as deep as expected I do not know.

A 42" frost depth is pretty severe! In those climates you can understand why most folks build a basement (they are most of the way there with the excavation!)

Here is a good link on frost protected footings http://oikos.com/esb/43/foundations.html

[hnash53]

Hal said before:  If the frost depth is 42", does that mean that one needs to insulate the ground around the pier in a 42" radius?  And a foot deep?  That basically means digging a 7' (84") wide hole one foot deep.

WOW!

That seems like quite a lot of excavation, especially if one is putting up 16 or 20 piers!

------------------------------------------------------------
I'd still like to know if what I said above it the case.

Would someone please refute or verify this?

Thanks.

Hal

[glenn kangiser]

Remember that the whole thing about frost depth is that it is the deepest level that exposed open ground is expected to reach 32ºF. Foundations built closer to the surface than this might have footings that will freeze, but only if the soils are both saturated and expansive (clay or fine silt).

Is this a problem in your soil? Many times not:
• not if the soil is granular and water drains away
• not if the soil is wet clay but is insulated to hold the warm earth heat.
• not if the wet clay soil is drilled out and has crushed rock down far enough to rest on unfrozen ground
• not if the wet clay soil is heated by warmth from the building above

It seems that would only be the case if you are in a problem soil area.  Even then something like the
Big Foot footings could allow soil movement without affecting the foundation.  Interior footings may be protected by the building above -proper drainage on a hilly site can minimize moisture getting into clay to cause expansion and freezing problems.  If you are in a real problem area it may pay to see what others have done or consult with a local professional such as an engineer or building official who is aware of local practices.  It is not really possible to nail down an answer like that in a forum like this as the members (and off duty professionals) here are only providing suggestions of what may work in some places.  There are so many variables that each site must be specifically evaluated by a local professional.  You can bet that they will cover their tails and try to design something that will work without a problem, so checking other time proven methods in the area under similar conditions may prove less costly - especially if you're in an area that doesn't require permits.  In a permit area you may need engineered footings anyway.  A good engineer will design footings that don't require an excess amount of materials.  Shop for the good engineer also.

Sorry if this sounds evasive but we can't get too much more specific here. :-/

[jraabe]

You can always do a simple test.

Dig some soil out of the bottom of the footing hole (or wherever you want to do this test). Put it in a jar and pack it down. Now mark the top or snap a digital photo and stick it in the freezer.

Pull it out next day and see what it did. Has it mounded up? Is there ice crystals poking up in the air?

This isn't a very scientific test but it will give you some idea of what your soil will do when it freezes. If it didn't do anything then that's information too isn't it?

Now, let's go see what's on the Tele  

[Amanda_931]

Of course there's a difference between insulating posts and insulating foundations.  But what John said was that insulation (out and down) should equal the frost depth.  So if frost depth was 8 feet, 4 down and 4 out, or 7 out and just one down.  Or vice versa.

both links refer to foundations--as opposed to piers, although the first one mentions adjustable foundations as one solution to frost heave. (as long as either the heaving, or trying to relevel the house doesn't break things )

Here are two links posted in another forum explaining the concept--the first is 55 pages of .pdf--time to make a cup of coffee or tea if you've got dial-up.  But this is the one that tells you about R-value of insulation and degree days, and minimum depth.  And differences between foundation of heated buildings and semi- or un- heated ones.

Hmmm, wonder if you can pretend that a post foundation is an unheated building?  One might run into problems about compressibility of the insulation, because those talk about running the insulation under the whole building.  The .pdf link does mention posts on one of the last pages.

http://www.cs.arizona.edu/people/jcropper/desguide.pdf

The second one's Oikos, with color diagrams, a lot briefer..

http://oikos.com/esb/43/foundations.html