Hello people I've been needing some advice on the best type of foundation to use for a sip construction? I am building 3 townhouses and was planning on having a concrete slab foundation but I am reading that may not be the best way to go.

Posted By Self builder on 15 Aug 2017 07:31 AM
Hello people I've been needing some advice on the best type of foundation to use for a sip construction? I am building 3 townhouses and was planning on having a concrete slab foundation but I am reading that may not be the best way to go.

Self builder,
95% of the foundations I have seen have been Slab on grade when using SIP Construction and actually in my area for any type construction,
unless there is a compelling reason not to? I would always recommend slab on grade foundations

I'm worried about the panels rotting when in contact with the concrete.

Yes as you should be , but there are rot proof panels available

I guess it would depend on the location and common practice in that area. I am more inclined (and in this neck) to lean towards a unvented crawl space using ICF's for the stem wall. By hanging the floor joists inside the crawl area and doing the same with any 2nd floor joists; you would have a continuous vertical plane of insulation from footing to top plate; avoiding tough to seal bands.. While some consider a crawl so much wasted space; it does afford a convenient area for the routing of plumbing/electrical etc. With careful planning I could see a situation where there would be no penetrations of the SIP walls. Air sealing such would also not be much more difficult than a slab.

A crawl also allows for mods in the future which is very difficult in a SOG. A three row ICF stem wall seems extremely doable for a DIY (or a competent builder), while I see a SOG with proper locations of plumbing etc. more of a challenge than I would attempt.

An advantage of a concrete slab (on grade or suspended) is that you can economically have a high performance hydronic radiant heated floor and save $$$ on floor finishing by using colored/stamped concrete or acid dyed polished concrete. Of course some people love a solid heated concrete floor and some people love a creaky flexing wood floor.

From an installers perspective the slab allows the use of scissor lifts to work on the high walls, set the roof panels with no crane and set ridge beam.......................................................................

Self Builder - You should just determine which type of foundation works best for your project, and then adhere to the SIP manufacturer's installation details, as well as local building codes, of course, and you'll be fine.....OSB SIPs are placed on a 2 x treated plate so that the skins do not come in contact with the concrete. Following is a link where you can see the subject detail.....Good luck with your project (BTW, it is always interesting to see where the GBT participants are building their projects, if you care to share) http://premiersips.com/details/103%20(8-16-07).pdf

Water management is critical on both the site and building shell. The homesite should follow FHA-HUD lot grading standards and soil adjacent to the home should be a minimum of 8 inches below finish floor and slope away from the house. Any concrete adjacent to the home should slope away from the home at 2% or more (2% is max for ADA at door landings). There are numerous details for proper wrapping and flashing of the shell as well as proper protection around penetrations. However if the site is not done right it's very difficult (expensive) to fix it at the building. Less than $500 of dirt may cost $10k in concrete, drains and waterproofing (that still don't perform as well the added elevation of a few truckloads of dirt).

Why not use PWF SIPS for the foundation? Where are you? There are 2 suppliers of PWF SIPS that I'm aware of, one in Canada (http://www.greenrpanel.com/prefab-home/no-crete-foundations/)
(they ship to the US) and one in Minnesota (http://extremepanel.com/sipsproducts_sipsfloorsfoundations.shtml). You could do a PWF SIP "frost wall" around a slab on grade type construction but the grade outside needs to be 8" below the PWF top and properly sloped. This constraint means, usually, one or more steps. The PWF SIPS can be on gravel "footings", in trenches. This construction is commonly used for attached garages in the northern states. The SIP serves as perimeter insulation for the slab and with under-slab insulation the floor heat loss is well controlled. Similar, thicker, PWF SIPS can be used for a crawl space or basement. FWIW I'm in the late planning stages of a PWF SIP on granular fill with a full basement and attached garage that will be fully ADA accessible thru the garage, no steps, the house floor "I" joist system is on top mount hanger inside the PWF SIP walls and the garage slab slopes 2 %.