Is a concrete basement or an insulated slab on grade the best foundation choice?
Traditionally, home construction often starts with a concrete basement 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 basement foundation may be the best option, but if space allows there is a strong case for avoiding basements 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 basements ‘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 your total building costs, which does not include finishing the inside basement 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'.
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 basement, no basement 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 basement 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 basement 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 basements.
So those are the “pros”, and I would be remiss if I didn’t share the “cons”.
- Concrete is hard, and consequently not as comfortable for standing. It’s a good idea to have cushioned mats where you stand frequently, like in the kitchen.
- If your kids wipeout, the stakes are a bit higher than on a shag carpet or pine floor, and if you drop a glass, you definitely won’t be drinking from it again.
- Without a basement you will need to account for a main floor laundry and utility room, so factor that into your plans. A utility room can be noisy, so try to situate it away the general living areas and be sure to soundproof the walls.
Be sure to check out The EcoHome Slab-on-grade Technical guide too
Foundations with a basement 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 basement, 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 basement is experiencing moisture issues, as most do eventually. A common mistake in basement 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 basement 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.
Top Tips: For a healthy, mold-free & comfortable basement:
- Test for Radon & install Mitigation if called for - because it's better to be safe than sorry
- Build using the best practices for mold free basements as oulined here
- Ventilate using a HRV or ERV, because let's face it - fresh air improves any Man Cave!
- How to choose between an ERV & HRV system for basement ventilation & Mold prevention - EcoHome Guide
- 'How to build basements that don't stink' - Dr. John Staube, University of Waterloo
Top Tips: For Building a Slab-on-Grade Foundation:
- Insulate effectively by Installing 8 inches of Roxul mineral wool below a concrete slab floor
- Follow the advice in THE ECOHOME A TO Z SLAB BUILDING GUIDE
Further reading for Slab on Grade or basement construction:
- Best practices for basement construction - Ecohome Guide
- How much insulation does a basement need - EcoHome Guide
- How to finish the exterior of Insulated basements and slabs - Ecohome Video Guide
- Why is a Slab-on-grade Foundation more Eco-Friendly than a Basement? - EcoHome Discussions