What is the best method of building a slab on grade?
Can you recommend the best method of building an insulated slab on grade suitable for the laurentian mountains in the event that bedrock prevents digging out a foundation? On your site, you discuss Frost Protected Shallow Foundation - or FPSF., 8 inches Rockwool rigid board under the exterior of the slab, Tpye III EPS foam under footings and a skirt of R8 insulation extending out 4 feet. ARe these suitable for such conditions. Have these materials been tested to see if they retain their insulating properties over time?? If not, the concrete slab will heave, crack, etc. Also, do you have a list of foundation contractors who have experience with building these slabs?
Having built several LEED certified High performance homes on slabs with different methods, our preferred method on how to build a slab on grade would be with an engineered prefab raft slab kit. The reason we like that system is the consistency of design and performance. It is not that individual designers and builders can’t pull off a slab successfully, but we have seen many mistakes along the way, and often sub-optimal performance. This page here covers the pros and cons of a basement vs slab on grade as a good starting place–
Slab on Grade vs Foundation & Crawlspace or Basement?
In there you can see our slab on grade installation video of our previous demo house when we did it by hand. It works fine, but we do now prefer the raft slab system, which we tried in our latest demo house, our LEED Platinum prefab eco home kit house on a solar air heated slab.
The videos on those pages will give you a good idea of the difference in the systems. The major advantage we found with having a slab provided by a specialized design firm is the attention to detail, the tech support, and the innovative solutions to common problems such as the insulated door sills for slab floors, where custom designed slabs usually resort to removing insulation to allow concrete to overflow door sills for a solid stepping surface.
To address your concerns about material type - no, the slab built on Roxul does not and will not experience frost heave, It has a perimeter skirt to protect it. The Rockwool insulation will hold up fine over time as it is a mineral wool (stone dust). At the time we encouraged Roxul (Rockwool) to pursue the sub-slab market, we like it since it is a recyled material, it performs really well, and they have no fire retardants. But they chose not to pursue that market, so you are best to stick to EPS foam we woul say, it is tried and true.
As for finding contractors, where are you located? We have a directory of green building professionals about to be launched soon but it is not currently ready, but if you choose the prefab slab kit route, Legalett supplies across North America and they may have a list of local installers.