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Foundation close to grade on helical piles – closed or vented mini crawl space?

owenqs | Posted in General Questions on

Hello
I have been doing some reading on the various articles on pier foundations and crawl spaces and have found myself somewhat confused. Please help!  Buildingsciencefightclub on Instagram posted a topic recently on sealing crawl spaces that got me thinking.
Context:
– small 300sqft newbuild cabin, not fully occupied in winter so thermal comfort less of an issue than longevity and effective building techniques
– zone 4c or so, lots of rain and humidity 
– due to zoning rules, building must sit as low as possible 
– site is foot access only. No concrete pumping for example.
– site is basically flat and a perimeter drain would be straightforward to install to direct water away from building 
– No services need to be run in the ‘mini’ crawl space, so access should never be an issue I hope

The building will sit on helical screw piles which provide the bearing capacity in poor soil and the uplift we need due to high winds. A proposed steel frame will sit on the piles with 8″ joists in between the steel beams. 
The floor will be covered with 7/8″ T&G OSB with glued seams and water vapour diffusion resistance value of 50μ.  

We need to sit the floor structure 4-6″ off the ground (undersides of steel beams). My plan was to insulate under the joists with 1-2″ of rigid foam seams taped, then rockwool between the joists. Spray foam the awkward areas where the screw pile heads meet the steel beams. Around the perimeter leave it completely open to facilitate full ventilation with just stainless mesh buried into the ground to stop rodents making a home. 

Now further reading suggests maybe I should install a thick layer of poly directly on the ground (with no insulation underneath or is a couple inches beneficial?). Screw the piles through the poly then reseal the penetrations. Then at the perimeter dress this poly up to the walls and completely insulated the perimeter outside the steels with 2″ thick rigid foam. We are proposing exterior rigid foam anyway so we could just carry this on down to below soil level (how far?). So essentially condition this space, which would require a lot less awkward insulation work actually.

Our heating is a small wood stove and two baseboard heaters, but there will be a small dedicated HRV too to keep internal humidity down in winter.  
Your expert thoughts greatly appreciated. 

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Replies

  1. charlie_sullivan | | #1

    I think your first idea is fine. My only worry is that you might, someday, need to get under there to fix something, and it would be good to put a little thought into what you would do if that need arose. Maybe you could jack the whole cabin up enough to comfortably crawl under, do the work and then lower it again. So maybe it would be good to design it so that that is feasible.

  2. owenqs | | #2

    Hi Charlie - with no services underneath and the small footprint we hope to not need under there! But thanks!
    FYI I've found this article which leads me to believe either approach should be okay though sounds like my latter is preferable
    https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

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