Slab-On-Grade Construction - Technical Guide
Deciding to build your home with a slab-on-grade foundation instead of a conventional basement or raised crawlspace foundation can greatly reduce your environmental impact during construction, and has the potential to offer significant financial savings as well.
By replacing concrete with lower-impact and better performing building materials like high performance insulation, you can eliminate many tons of greenhouse gas emissions from your building materials as well as during the future operation of the home.
Slab-on-grade construction is a somewhat uncommon method of home building that replaces the conventional foundation wall and basement or crawl space with a concrete slab that rests directly on grade. The technique lends itself to most types of terrain, with the exception of heavily sloped sites, and increasing the performance of slab on grade floors is simply done by adding insulation - yep, we have a guide for how to insulate slab on grade floors when building, here.
Why build on a slab instead of a basement?
To save money and carbon emissions, and have a healthier and more durable house. For a detailed guide on choosing between a Slab on Grade or a Basement for a home foundation see here, but we'll give conclusions below.
Typically, the construction of a single family home in Canada & North America starts with a poured concrete footing followed by an 8-inch thick foundation wall, generally 8 to 10 feet in height.
In a cold climate zone a basement foundation counts on being buried at depths of between four and five feet to protect the footing and basement floor from freezing temperatures. Since this creates additional floor area underground, it is a natural desire to claim that space, and so born is the basement rec room or man-cave - and it's important to understand how to choose between a crawlspace or a slab foundation too.
Whether you start construction with a slab-on-grade or a foundation, in the absence of bedrock both will be resting on dirt. So one is not more 'stable' than the other, or more suceptible to frost heave than the other, if it is properly insulated around the perimeter.
In the case of a 'walk-out basement' you effectively have a slab, only with a very expensive and poorly insulated wall. Envision a slab-on-grade as a four-sided walk-out basement with much cheaper and better performing walls, though if you do need to build a basement to optimize sqft on your building plot, then learn best practice for insulating when building a new basement here, or if you have a basement and need to learn how to finish a basement & insulate correctly to avoid mold, see here.
In cold climates, a slab-on-grade can easily avoid frost heave with the simple addition of skirt insulation, which protects the perimeter of the footing so that it will never be subjected to the freeze/thaw cycle, and instead remain close to the relatively constant 8 -10 degrees Celsius temperatures of the earth. You can find a step by step detailed guide to building a frost protected shallow slab on grade foundation using foam forms here.
Skirt insulation around slabs and foundations prevents concrete from freezing © CMHC
Skirt insulation is a very simple and cost-effective solution that could (and should) be applied to basement foundations as well, as it would help reduce heat loss by keeping basement walls at a much higher temperature.
In addition to significant cost savings during construction, a slab-on-grade will reduce the risk of problems caused by humidity and water infiltration, which are typical challenges faced by basements.
What's wrong with basements?
The costs - ecological and financial:
Concrete is very expensive to purchase and extremely energy intensive during manufacturing. The process of building a basement requires 3 costly visits from a team of concrete trucks; once to pour a footing, again to pour walls, and a third time to pour a basement floor. For every ton of concrete produced, one ton of greenhouse gases is released into the atmosphere.
Once a basement foundation is completed, a subfloor must be built on top. This is another cost that will be incurred in order to create a surface on which living space is built, compared to a slab which is ready for construction.
* A home built on a slab-on-grade has a lower risk of flood damage, which is favourably looked upon by insurance companies and can be reflected in your premiums. Basements can also be susceptible to Radon gas infiltration, so to learn how to prevent Radon gas levels being high in homes, see here - or to specifically find out how to remove Radon gas form basements and crawl spaces, see here.
Health, durability and comfort:
Despite how common finished basements are, there is a general lack of understanding of the additional challenges posed by building underground. This can mean that in a lot of cases, the proper building techniques are not being followed, according to the principles of building science.
Case in point: walls need to be able to dry in at least one direction. In a cold climate, the walls above ground should be designed to dry predominantly to the outside. But, since soil is for all intents and purposes 'water', basement walls need to dry to the inside.
Common building practice does not account for this, and a disturbingly high number of homes have moisture damage and mold, which is in part blamed for an increase in respiratory illnesses.
This happens for a number of reasons, the first of which is that we finish basements too soon. Concrete is largely made up of water, and with soil on the exterior a foundation requires a minimum of 2 years to fully dry to the inside.
Secondly, as the National Building Code now requires basements to be insulated and new home buyers usually want the space finished, the cheapest way for developers to do this is to treat them as they do above ground walls. So basements are generally insulated on the inside, long before unwanted moisture has left, and in the same manner that we build above ground where walls can dry to the exterior.
By sealing moisture sensitive materials (wood and fibreglass insulation) between a wet concrete wall and a polyethylene vapour barrier, we are inviting mold. A slab-on-grade foundation avoids this all together.
Why we like slab-on-grade construction
Quality of life:
In comparison to a basement, building above ground has the advantage of providing more natural light. It also helps maintain clean interior air quality as it reduces the possibility of mold.
What's more, slab-on-grade construction can make your living space more comfortable. Thermal mass within the conditioned living space has the ability to absorb and store heat, greatly helping to regulate interior temperatures. Homes with significant thermal mass inside the building envelope are also easier to keep cool in summer.
Bringing a building project to the point where it is ready for main floor framing can be done much cheaper with a slab-on-grade than a basement. With a slab, the same milestone is reached without having to build an 8-foot concrete wall, nor do you have to build a wooden subfloor on top.
Comfort and efficiency:
In the absence of 5 feet of dirt, a slab-on-grade in cold climate zones requires additional measures to avoid frost heave, so it includes levels of insulation that otherwise seem to be omitted from basement construction. That insulation can be paid for with the thousands of dollars that would have gone into purchasing concrete for a foundation wall.
Slab floors also easily accommodate radiant floor heating, which offers a very balanced and comfortable environment, transforming the concrete mass into one big radiator.
One of the great advantages of radiant floor heat is that the further you get from anything radiating heat (imagine a woodstove), the cooler it gets. So heat is concentrated at ground level where we are rather than collecting in the highest points of our house, where we aren't. This facilitates an overall lower temperature, without sacrificing comfort. Warm feet are happy feet!
Reducing your environmental impact:
The slab-on-grade building method reduces your impact on the environment in two different ways: by greatly reducing the amount of CO2 produced in the manufacturing and transportation of materials, and by providing - dollar for dollar - a wall with much better insulation.
By building a slab-on-grade home, you are choosing to substitute foundation walls with above-ground walls. In other words, you are replacing concrete walls with a much more affordable and energy efficient wall assembly.
This is why Slab-on-Grade foundations are frequently chosen for Passive House certification in North America, Passive Solar Homes & LEED certified homes.
Slab on Grade Foundation © Yanni Milon for Ecohome
Sacrifices, challenges and solutions:
In order to be able to make an informed decision about this type of foundation, there are several precautionary steps that must be taken and challenges to be addressed.
Your state or municipality may require plans that have been approved by an engineer, and some may not be familiar with slab-on-grade homes. Be sure to check with your municipality or state before beginning construction, and even before getting too far into your building plans.
While we are firmly in favour of slab-on-grade construction as a concept, we recommend carefully considering your options before moving ahead with any plans. There are many legitimate reasons to begin construction with a basement foundation:
- A slab-on-grade will require more above ground space, so in order to have the same size of home you will need to build either out or up. You may run into height restrictions where you choose to build, meaning you may have to build out instead of up. This is not always possible, certainly if your site is an urban infill lot, leaving you two options: a smaller house or a basement.
- Despite their disadvantages, basements are often very practical since they provide a significant amount of storage space. Without a basement, everything that would have been down there must fit into the rest of the house or in the garage or shed.
- Basements usually house mechanical rooms. Keep this in mind during the design phase should you choose a slab, as mechanical systems will now need to be housed on the main floor. And don't be stingy with that space - think about everything that may potentially need to go there: a furnace, boiler, water heater, air exchanger, water softener, septic pump, sulphur tank, central vac, etc.
Given that you are committing a certain area of the main floor to mechanical systems, this is as good a time as any to plan some storage, and maximize the efficiency of that room. Along with some space for storage, you could consider including laundry facilities, or even a pantry in that space.
With the amount of action going on in a mechanical room, it will be a bit noisy. In order to mitigate that, those walls should include sound reducing measures.
Plumbing systems are normally accessible from the basement or a crawl space, but not so with a slab. The nature of a slab-on-grade means plumbing systems will be permanently fixed in the concrete and not easily modified.
Installing a second toilet on an existing drain pipe is virtually impossible in this situation, so plan ahead. Worth investigating is the concept of 'flexible housing design' where future changes are anticipated so that the necessary infrastructure can be put in place at the time of initial construction.
Floating slab / Monolithic slab:
The term floating slab refers to a two-stage slab construction, where footings are individually
poured, and the centre floor of the slab is poured after footings have cured. The forms of a monolithic slab are designed so that both footing and slab floor are poured at the same time.
We have found no great advantage to support either method - the main reason for a pouring a monolithic slab would be to reduce the visits by the concrete trucks to one only.