How to build a super-insulated slab (spoiler: it’s easier than you think!)
This image is not as complicated as it might seem and could save you a LOT of $$$’s on heating!
Originally published in February 28, 2020.
No longer just a “poor man’s” foundation, super-insulated slabs are the foundation of choice for energy efficient housing projects. A prime example are PassivHaus projects which are generally built on a variety of super-insulated slab designs. The image above is an example of a super-insulated slab design that are typical of the ones I use in my builds and plans. I built this exact detail last summer and it was simple to build. The version shown encapsulates a floating slab with Rockwool® board insulation that keeps the floor fully isolated and warm/cool depending on the season. I specifically designed this slab using the sleeper floor option since my wife spends extended time in the kitchen and her legs, understandably, resent concrete! While slabs are an ideal foundation for high performance, the concrete is unforgivingly hard. So, to meet our preferences, we built a sleeper floor system that gives the house even greater thermal integrity and our legs something easier to cook on.
The right slab design can manage terrain surprisingly well.
Another great benefit of an insulated slab - like this one with deep footings - is that similar to crawl spaces there are actually stem walls that can help manage modest slopes and wetter locations. Consider the detail again and you’ll see it’s basically just an unventilated crawl space that’s been back-filled with enough gravel to set some horizontal board insulation and the slab level with the perimeter walls. This keeps the slab above grade and dry.
Super-insulated slabs are more expensive - but they’re worth it.
While I promote this type of superior slab design there is some small additional cost for the additional gravel requirements and insulation. Some penny-pinching builders won’t like that. I also encourage people to install self-compacting fill rather than the cheapest coarse gravel option and I actually think purchasing better materials can often be the same cost - or better. For example, I don’t like to rent a mechanical foot to compact cheap gravel fill; so I typically spring for a more expensive self-compacting pea gravel. While this is certainly more Gucci than crushed fill, it’s the better choice because once installed, it’s done. I’m going to say pea gravel adds maybe $300-$400 to the cost of a slab. How much does it cost to rent a mechanical foot? Or to pay for the labor to do the work? That’s also worth considering.
Slabs don’t need toxic foams!
I don’t use EPS or EXP rigid foam board insulation in my slabs because both have been shown to leach toxic chemicals into the environment. I know many builders who disagree and insist on the up front savings of cheaper insulation options. Regardless, studies demonstrate the inherent toxicity of these products. For those interested in reading about the leaching capacity of brominated flame retardants like hexabromocyclododecane from expanded polystyrene (EPS) and extruded polystyrene (EXP) rigid board insulations I’ve provided a link to a 2019 article titled Laboratory studies on leaching of HBCDD from building insulation foams. And there are more, please feel free to email if interested.
No foam here! Installing Rockwool® slab insulation is an easy DIY job!
What should you use instead of foam then?
EPS & XPS foams do not really belong in/on anyone’s project - especially if it’s aiming to label itself with a “green” tag. It’s hard to think of a less green building material than EPS or XPS. Cleaner alternative foams exist, such as polyisocyanurate (polyiso) exist but should not be used under slab or in any below-grade application.
So what should you use? Rockwool®, which is a mineral wool made from post-industrial slag and volcanic material. It is a reasonable alternative EPS & XPS foam in slabs and for insulating houses in general. While Rockwool® is not entirely without drawbacks it is at least free of the toxic flame retardants used in foams. Builders often tell me, Hey, you can’t use Rockwool® under a slab! I’ve even had plans examiners say the same thing. Well, they’re incorrect, because you can! Rockwool® Comfortboard 80 is both IRC and IBC compliant. But they don’t need me telling them otherwise; the specs speak for themselves:
Here are some tips!
Okay, you’re convinced! That’s great news. This is going to be easy because Rockwool® is fantastic for making precise cuts, layering horizontally below slabs and fastening directly to concrete walls. I just recommend budgeting for the following things:
An inexpensive insulation knife like this one from Hultafors. The insulation cuts like a loaf of bread and many people actually use old bread knives!
Plasti-Grip PMF fasteners for installing the vertical Rockwool®. All you have to do is pre-drill a hole, set some insulation and then tap a fastener lightly into your concrete with a hammer.
Custom metal insulation protection to prevent backfill and weed-eaters from destroying the insulation.
One more thing. If you plan to use the slab with a finish floor you simply omit the sleeper system as shown in the above detail. Pour, polish and seal - oh, and insulate!
Thanks go out to the subscriber who posed this question last month. I will post replies to other questions received as the year progresses. Feel free to offer you own insights and experiences below!
