Slab on Grade vs Foundation & Crawlspace or Basem*nt? (2024)

Isan insulated slab on grade the best foundation choice? Or is a basem*nt better?

Traditionally, home construction often starts with a concrete basem*nt foundation - But is that the best choice?

The first factors to consider when designing the foundation system for a home are lot size and soil conditions. When space is limited (with total footprint and height restrictions) a basem*nt foundation may be the best option, but if space allows there is a strong case for avoiding basem*nts altogether and choosing to build a slab-on-grade instead.

Concrete foundations - pros & cons:

For a modest sized home, a concrete foundation will cost you easily between $20,000 and $30,000. Factor in a subfloor and finished flooring, you’ll be lucky to stay under $40,000. Building basem*nts ‘just because’ invites unnecessary costs, potential humidity problems, and greater environmental consequences.

The production of one ton of Portland cement releases one ton of greenhouse gases, and the average home foundation wall uses between 75 and 100 tons. It will also account for between 10 to 15 percent of the total building costs, which does not include finishing the inside basem*nt walls, or building a floor on top of it.

And if you remember the 'One-Tonne Challenge' that asked Canadians to reduce thier carbon emissions by one tonne in a year, well this one move could offer you a century of success in that. It can also be much cheaper, more ecologically sensible, and offer a better quality of life to build 'up' instead of 'down'.

Check out our video building guide: passive solarslab-on-grade construction

Slab-on-Grade Construction is best if:

A slab floor is a fantastic way to start a passive solar home, as your entire floor surface consists of several inches of thermal mass to absorb heat. A slab is a great place to bury electrical work, plumbing, central vacuum tubes, phone and internet lines, speaker wires, and most importantly, tubing for radiant floor heat.

A slab-on-grade means no basem*nt, no basem*nt walls, just one slab of concrete on which you build your house. They aren’t suitable for all building sites, which we will get into later, but for now let’s assume you can build on one.

There is a lot of prep work to do first, you need well-packed soil, proper drainage, insulation, vapour barrier, and a lot of mechanical infrastructure before you pour. None of that should discourage you, that will all be necessary under your basem*nt floor anyway.

You can put flooring material on top of a slab, but simple finishes include acid staining, adding colour, cutting tile patterns, or the simplest and cheapest is just polish it and call it done.

All the electrical, plumbing and other mechanical infrastructure you will have to do anyway throughout your house, so don’t think of it as added cost. In fact with a slab that can all be done cheaper. Laying out plumbing and wiring on a flat surface is a lot easier than drilling a thousand holes and fishing wires through studs and joists.

At a cost of about $8 a sq. ft. to pour a concrete floor plus a couple of thousand to polish it, a finished slab over basem*nt construction can potentially save you tens of thousands of dollars during construction and leave you with a final product that is extremely durable, energy efficient and does not pollute your indoor air as so many finshed flooring products do, not to mention moldy basem*nts.

Be sure to check out The EcoHomeSlab-on-grade Technical guidetoo

Foundations with a basem*nt make sense if:

As we mentioned, not all sites are suitable for a slab. Municipal restrictions for footprint and height may leave you with no option but to build down in order to have sufficient space.

If your building site is on a slope and will require terracing, a foundation can often be the only solution.

In such cases, be sure it is well-insulated and well protected outside from moisture infiltration and it’s always best to insulate the outside of your foundation rather than the inside.

By doing so, you greatly reduce your risk of interior condensation and you keep the concrete walls as thermal mass to help balance temperatures.

Be sure to use a foundation wrap or membrane extended over the footing, and proper drainage.

Regardless of the base you build on, insulate the surface extending out 4 feet from your home to keep either your foundation or slab from freezing. This is done by grading away from your foundation, but stopping a foot or so below your final desired height. Lay down 3 inches of extruded foam, then the final 8 inches or so of soil.

If you have to build a basem*nt, it makes sense to use the space. But living underground is not the most ideal situation at the best of times and especially if your basem*nt is experiencing moisture issues, as most do eventually. A common mistake in basem*nt finishing is doing it too soon, before concrete walls are dry. Allow 2 years for concrete to shed excess moisture.

All walls need to breath in one direction and since the exterior foundation walls are buried in wet ground, they must breathe to the inside. A vapour barrier prevents that from happening and you will inevitably trap moisture inside your walls.

Depending on the situation there are other options that can be more durable and affordable that will better protect your home air quality.

Insulating under concrete floors is essential

Many building codes have only recently insisted on the inclusion of insulation below concrete basem*nt floors, and as always, it is important to realize that the requirements of building codes are the bare minimum only. Increasing sub-slab insulation to R15 or R20 (or more) will make floors much more comfortable and reduce heating costs. The added cost for additional floor insulation can be paid back in as little as a couple of years though energy savings.

TopTips: For a healthy, mold-free &comfortable basem*nt:

if called for - because it's better to be safe than sorry
Build using the best practices for mold free basem*nts as oulined here
Ventilate using a HRV or ERV, because let's face it - fresh air improves any Man Cave!
- EcoHome Guide
'How to build basem*nts that don't stink' - Dr. John Staube, University of Waterloo

TopTips: For Building a Slab-on-Grade Foundation:

Insulate effectivelybyInstalling 8 inches of Roxul mineral wool below a concrete slab floor
Follow the advice inTHE ECOHOME A TO Z SLAB BUILDING GUIDE