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Discussion Forum

frost protected shallow footings

| Posted in General Discussion on February 28, 2004 05:31am

I currently live in a slab on grade ranch style house (with a twist). It is solar gain and earth sheltered on the North side. I would like to remove the earth sheltered aspect of the house. By doing this, it would expose the footings to the frost. In regards to issue 161 of Fine Homebuilding (Energy Efficient from the Ground Up), the frost protected shallow footings designed by Al Rossetto are very similar to my own situation. The only difference is that there is no compacted gravel (4 to 6 inches) under the footings, but is rather clay.

My question is how important is the 6 inches of gravel under the footings? Can I use additional insulation and drainage to remedy this situation? What are your thoughts on this? We live Southwestern Ontario where there is a 4 foot frost level.

Thanks for your input.

MikeQ7

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  1. Piffin | Feb 29, 2004 11:49pm | #1

    it would be cheaper to sell and move into a regular Ranch.

    The six inches would be a bare minimum to my thinking. I look for 8 to 18" depending on what is inder it in the first place.

    You need to understand the proinciples behind shalow frostprotection. The thing you are protecting against is frost heaving the foundation. When water freezes, it expands and moves whatever is in the way.

    So tyhe typical way fo protecting a foundation is to get it deeper than the frost goes.

    For shallow foundatijns, you need to do two things. one is to insulate it with foam panel boards so the heat is contained under the building.

    The other is to make ABSOLUTELY CERTAIN BEYOND A SHADOW OF A DOUBT that no water will be contained in the soils under the house foundation that can possibly freeze.

    it's really quite simple. If you are in a desert with no water, you won't see any ice cubes.

    So the reason for the gravel is to provide passage for water to drain AWAY from the foundation. It is a major component of the whole system.

    Keeping that in mind, when you remove the earthworks berem from the back side of the house, you have to make sure to cu5t far enough that any surface water is being led AWAY from the house and not alowed to pond near it. That means you will be doing a major amt of earth moving to get where you are going

    Next thing - This house was designed to function as an extension of the earth. no telling what other problems you may cause by changing the way things work. Start with doubling or tripleing your utilities bills as an assumption.

     

     

    Welcome to the
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    Knowledge FHB Campus at Breaktime.
     where ...
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    1. DavidThomas | Mar 01, 2004 09:10pm | #4

      To add on to (or summarize?) what Piffin said:

      You need three things to have frost heaves:

      1) Freezing temperatures

      2) Water

      3) Fine-grained soils.

      To NOT have frost heaves you must eliminate one of them entirely.

      I would hate to count on drainage to eliminate any possibility of any water.

      Stripping off the current cover will allow freezing temperatures to a depth they haven't been before (that is bad).

      The gravel mentioned in the article is to eliminate fine-grained soils near the foundation.  

      Leaving the clay in place, allowing deeper frost depths (by removing soil) and making the drainage worse makes frost heaves more likely because of all 3 factors.   There are houses up here that are 2" to 6" out of level.  It's okay if your hallway is like a bowling alley.  It is not okay for it to resemble a bob-sled run.

      Like Piffin said, sell this one and buy what you want in another house.David Thomas   Overlooking Cook Inlet in Kenai, Alaska

      1. Piffin | Mar 02, 2004 04:43am | #5

        Do you guys even attempt this sort of foundation in Alaska? With the permafrost, I know most of your foundations are a sort of floating slab anyway in essence, or is there a lot more I'm miossing. The whole concept of building on permafrost is a bit confusing for me. What are your basic rules up there for foundations? 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

        1. DavidThomas | Mar 02, 2004 10:29am | #6

          For a perimeter or slab-on-grade, get all the "brown dirt" out and replace with "NFS" - non-frost suspectible soils - sands and gravels. Always are some sands and gravels around in a glacial area.

          If fine-grained soils are too deep, then piles are an option but they have to be REALLY deep to resist frost jacking. I've seen 50-foot deep wells come 3 feet out of the ground in a decade (Fairbanks area).

          Beyond that transtion zone (roughly Fairbanks), the situation is the reverse of normal "snow country". The shallow, active zone thaws each summer but the frozen soil a few feet down is a permanent fixture. Get your piers in that and you're set.

          So in Barrow, the trick is NOT introduce heat into the ground. All buildings are up on piers so the winds can keep the ground underneath cold. Works fine until someone gets clever and skirts the building for more warmth or to keep the polar bears from gnawing on the ATCs and snowmachines underneath in the open crawlspace.

          Large buildings (hangers, hospital) are slab on grade but require active refridgeration to keep up with heat transmission from the building. There have been cases where someone shut down the cooling system and had a sloped slab a year or two later.David Thomas   Overlooking Cook Inlet in Kenai, Alaska

          1. Piffin | Mar 03, 2004 12:56am | #7

            Thanks, I'm sure it takes a lot of experience to judge what way to design something there. betcha have a lot of Chechacho contractors trying to fight nature and volunteering to be the butt of jokes up there. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          2. DavidThomas | Mar 03, 2004 02:31am | #9

            "betcha have a lot of Chechacho contractors"

            Yes, but I don't mind the newbies so much as the old-timers who expect to earn enough in a 4-month building season to cover all of hunting season, snowmachining, ice-fishing, and a few trips "Outside".  I much prefer someone who works most of the year and will go out of my way to keep them busy.David Thomas   Overlooking Cook Inlet in Kenai, Alaska

        2. reddog | Mar 03, 2004 01:19am | #8

          I, too, live on the Kenai Peninsula in Southcentral Alaska.  It's a misconception that all of the state has permafrost.  Although there are small pockets of it as far south as Anchorage, generally it's only found in the interior and northern parts of Alaska, and even in some of those areas, it's hit and miss.  Each building has to be engineered for its exact site. 

          As to the shallow, frost protected foundations...I have done a number of them in my geographic area and even retrofitted one on my own house that was originally built on piers.  After 13 years, no problems. 

          Like David says, you just have to get rid of the brown soil, which I call the organic layer that can vary from 6" to 2' around here.  From there, it's just a matter of getting the foam layer installed and covered.  Six inches of cover seems to be about the minimum amount of cover to adequately protect the foam. 

          Red Dog

          1. MikeQ | Mar 08, 2004 03:45am | #10

            red dog I have been researching the shallow frost protected footings in the documents I have downloaded It says "while a drainage layer is only recommended under wing insulation for heated buildings, a 6 inch drainage layer is required under unheated FPSFdesigns".I am planning to FPSF my existing footing but only have native soil under the footing.Iam planning on a 18" footing depth with 2" extruded poly vertically and24" horizontally as wings my air freezing index is 2000.As the under footing temperature does not drop below freezing nonsusceptible frost agrigate is not needed. For clarifiation on this check out http://www.cs.arizona.edu/people/jcropper/desguide.html  since you are the only contact I have had with experience in FPSF would you be kind enough to offer your suggestions thanks in advance.

            PS moving is not an option

          2. DanH | Mar 08, 2004 03:49am | #11

            Probably not suitable for Alaska, but I have seen a description of a shallow frost-safe footing that involved running foam insulation horizontally away from the foundation, several feet down and extending out several feet. The theory is that the insulation keeps the ground around the foundation from freezing.

          3. Piffin | Mar 08, 2004 04:15am | #12

            This might help you too, to understand what you are doing.

            I think I got it here from Gabe

            well, it wouldn't post as an attachment. Probably too big a file. e-mail me and I can send it direct 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          4. DavidThomas | Mar 08, 2004 10:26pm | #13

            "PS moving is not an option"

            Reminds of a friend who, of all the bizarre things, liked those GM-marketed, German-made Opels from the 70's.  And never made a good decision about them, spending way too much to restore them, get accessories, and keep them going. 

            He looked back and realized that the best car for him was the Olds Vista-cruiser station wagon his mom handed-down to him, complete with the vinyl wood grain on the sides. 

            He hated it and therefore made logical decisions about what to repair, and when to get rid of it.

            But, hey!  We've all been on projects where the crew is saying to each other, "Why didn't they just knock this thing down and build a real house in its place?"  Great for the billings - $80,000 trying to turn a house into something it is never going to be.

            David Thomas   Overlooking Cook Inlet in Kenai, Alaska

          5. Piffin | Mar 08, 2004 10:50pm | #14

            Eighty????????

            I've watched half a million go down wondering why they wouldn't let us start out with five gallons and a match. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          6. DavidThomas | Mar 08, 2004 10:53pm | #15

            Yeah, I'm small potatos.  At least until you find toxic waste on site.  Then I kick into high gear.David Thomas   Overlooking Cook Inlet in Kenai, Alaska

          7. reddog | Mar 09, 2004 01:47am | #16

            What I would recommend, and what I have done on some of the SFPF's that I have done is this.  Install a conventional footer drain around the perimeter of the house and direct its flow to a daylighted outlet or drywell, depending on your site.  Cover the entire drain with the horizontal foam insulation and protect the outlet of the drain from freezing.  No matter what, the bottom holes in the drain tile need to be above the bottom of your footing.   My own tweak on the design is to slope the insulation down and away from the footing.  This provides just one more layer of protection in keeping the surface water draining away from the building.  Also, keep the final grade sloped well away from the building...at least for 6 feet.

            Your footing being on native soil doesn't seem like a problem to me unless, like others have said, water and freezing temperatures get down there. 

            Red Dog

          8. MikeQ | Mar 09, 2004 03:19am | #17

            thanks again for your words of wisdom.

          9. MikeQ | Mar 10, 2004 04:35am | #18

            RED DOG COULD TELL ME THE REASON FOR MAKING ABSOLUTELY SURE THE BOTTOM HOLES IN THE DRAIN NEED TO BE ABOVE THE BOTTOM OF THE FOOTING.I SHOWED MY NEIGHBOUR," PARTNER IN CRIME AND FELLOW DIY'ER "YOUR eMAIL .WE WERE CONFUSED ON WHY THE DRAIN HOLES ARE ABOVE AND NOT BELOW THE FOOTING.

            THANKS QUINCY

          10. DanH | Mar 10, 2004 04:50am | #19

            Yeah, that confused me too.

          11. reddog | Mar 10, 2004 05:05am | #20

            I think my mind must have slipped out of gear..  What I meant to say was keep the holes well below the level of the finished floor.  Sometimes, the guys get carried away and excavate beside the footer to  make room for drain rock and inadvertently affect the bearing of the footing.  Sorry for the miscue.

            Red Dog

          12. MikeQ | Mar 11, 2004 03:14am | #21

            THANKS RED DOG AND ALL THE OTHER GUYS FOR  YOUR INPUT ON FPSF'S . IT IS ALWAYS NICE TO REAFFIRM WHAT'S WRITTEN ON PAPER TO ACTUAL EXPERIENCE THIS IS A GREAT SITE 

            QUINCEY 

  2. DanH | Mar 01, 2004 12:10am | #2

    Additionally, clay tends to be expansive with water, and removing the overburden would likely make it tend to expand and contract a lot more with rainfall variations.

  3. dIrishInMe | Mar 01, 2004 03:42pm | #3

    You said:  >> I would like to remove the earth sheltered aspect of the house. <<  Just curious - why do you wnat to remove the backfill?  Is it causing water infiltration problems, structural problems, or what?
     

    Matt

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