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I am figuring out the foundation for a small wood cabin for my back garden, due to height being a premium I was wondering if I could put the wood directly on an existing garage foundation. Providing I add a DPC under the log cabin?

The foundation would consist of (in reverse order)

  • Existing concrete slab
  • Damp proof course sheet larger than the cabin
  • The cabin base wood and insulation
  • The foil backing of the insulation as well as aluminium tape for a vapour barrier
  • OSB deck

The sides will be later covered with a house wrap which would go over the top of the DPC.

My concern is that there could be condensation but I am not sure

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  • Where in the world are you? What are your local codes? Maybe maybe not . How thick is the slab and footings? How tall and what type of walls,,, there are many different types of cabins. I have been involved in a few from the heating and electrical mostly but if I can ask these questions not answered you need to do your homework for your area!
    – Ed Beal
    Jul 18, 2020 at 0:57
  • I am in the UK. I didn't refer to the foundation as I am not asking about if the foundation is deep enough to support the weight of the building. I am only asking from the perspective of making sure moisture or water can't rise up from the existing foundation. Jul 18, 2020 at 11:39

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A damp proof course (DPC) is required “Under Floor Space” where there is evidence that the “water table rises to within 6” of the building perimeter or if surface water does not readily drain away from the building”. (See ICC 1807)

Likewise, wood joists within 18” of the ground or beams and girders within 12” at the perimeter or under a building in a crawl space shall be “naturally durable or preservative treated wood”. (See ICC 2304.11.2.1) Btw, this will help anchor the building to the foundation in seismic and high wind areas.

Likewise, under floor space shall have ventilation through the exterior walls, except where it’s a basement or cellar. (See ICC 1203.3)

Likewise, under floor insulation is required of R-25. (See ICC Table N1104.1(1)) Vapor barriers are installed on the heated side where the building is heated predominantly throughout the year.

So, to answer your question, yes you can install your cabin as you describe with the above requirements, including crawl space vents.

Your situation is a little different because you really don’t have adequate space for crawl space vents. In this case, I’d recommend you eliminate the floor insulation and increase another element in the building envelope. (See ICC Section N1103 Alternative Systems, i.e.: thicker wall insulation, thicker ceiling insulation, better insulated windows, etc.)

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  • Thank you for your answer, very clear Jul 19, 2020 at 15:56
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As far as moisture coming up from the the foundation you need a sill barrier to prevent moisture from wicking to the wood. I usually use a foam barrier in areas with high moisture have used neoprene it is a bit more expensive but keeps the wood separation the best. My area uses DPC to stop concrete to concrete moisture transmission but it should work as well as any other foam sill barrier for your log cabin walls.

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