What are the factors I should consider for choosing between a slab on grade and basement?

Shaun B
Updated: Aug. 3, 2020

We are in the planning stages of designing a new home to be built to passive house standards, although not interested in certifying it.  We are trying to decide between having a basement vs.  a slab.  The house will be around 3000 ft2 and we do have room to do either option.  Soil conditions are not a factor, we are located in Uxbridge, ON.  We are on well and septic.  My goal is to heat/cool with an ERV and mini split system, not radiant in floor.  I have several questions about the merits of each option, and would appreciate advice.

1. Cost - I know a slab is cheaper but having a basement that we can finish allows for a smaller footprint with less walls and roof material.  Is there a big difference in cost? 

2. Access for mechanical - I can't get my head around having to embed drains, plumbing, electrical, etc in concrete as opposed to putting it under floor.  Not to mention running ducting for ERV.   Is this a valid concern?   What if there is a leak in the drains, how would you ever know?

3. What about a hybrid - we don't need a full basement, what are the implications of having half of the footprint as a slab, and half basement?




Responses (3)

Mike Reynolds 5 years ago

A footing and frost wall requires a lot more excavation and a lot more concrete, and it's done for the most part to accomplish the same thing that can be done with a perimeter insulation skirt. Instead of digging down to build a footing and have your load bear below the frost line, you can just lay down a sheet of insulation and move the frost line.

And sometimes a skirt isn't even required, that depends on the climate and how well the slab is insulated. That is one advantage to the Legalett system, each slab is designed to meet the needs of it's specific climate (here is a video simulation).

Legalett is also a particularly good option when you are attempting to meet Passive House standards as they have seamless insulation from below the slab up the entire perimiter as well. They also have a door detail we really like. In order to create a solid surface that can we walked on, the insulation of a slab is often removed outside of doors to allow concrete to be poured there instead. This results in a lot of heat loss under all doorways. Below you can see the Thermasill where a door will be, it is a wood fiber cement board that is about R1.7 per inch. 

When building on perfect soil conditions Legalett will sometimes be a more expensive option than hand-made slabs with wooden forms since it is engineered as a raft slab, but if you have a high water table, soil with lower compressive strength or expansive soil (like we had in where the Wakefield demo house is), it can often come out cheaper (read more about building on poor quality soil here).