That is a common question, and a hot topic that often brings dissagreement, but we like that though! So much so that we have a page highlighting the pros and cons, check it out -
Slab on Grade or Foundation and Basement; Which is Best?
But to address your questions specifically, I've included your questions in the response to make it easier for both of us -
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?
We have typically found above-grade walls to be a more affordable building option, but no, it is not a huge difference. A basement will result in a much higher excavation cost to begin with, but also, concrete is a more expensive structural material compared to wood, so an above grade wall assembly can usually achieve a higher performance level for a lower cost since you can use wood as the structure. Yes, you will have a greater amount of above grade wall to build, but as mentioned, that can cost less than the same amount of below grade wall.
On the other hand, there are ICF advocates (insulated concrete forms) that would argue in favour of basements, including bottom to top ICF walls; those builders would argue there isn’t a great difference in cost.
Personally, I wouldn’t base the decision as much on cost as a few other issues, namely: durability, quality of life, and ecological impact. Cement manufacturing emits a significant amount of greenhouse gases, and sand that is suitable for concrete production is becoming an increasingly scarce global resource, where wood is renewable. that's why we as an organization like to promote the more sustainable options, and limiting the use of concrete is a large part of that, ergo we like slabs.
As for durability – basements walls cannot dry to the exterior, so more caution must be taken in designing and building walls below grade. Also, basements flood, that’s just a part of life. We can take steps to mitigate that with drainage and sump pumps with battery backup systems in case of power failure during storms, but it is impossible to predict levels of precipitation long into the future except to say that it doesn’t look good. If your house is above grade and not in a flood plain, you will never have to worry about it flooding.
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?
More than being a valid concern is the part about wrapping your head around it, so that's all relative. I have no problem with embedding the infrastructure in the concrete but I would have a hard time wrapping my head around the idea of digging a hole in the ground to live in it when I could be living above grade. It's more a matter of what you're used to, if you go to California for example, you'd have a hard time finding a basement because they build all their houses on slabs and don't give it a second thought. And... if a drain DID have a leak and a the occasional drop made it to the ground below, no you probably would not know, nor would you probably care.
Where concern is valid is - you cannot change plumbing in the future so you need to be sure you are happy with the house layout to begin with. In order for a drain to leak you’d have to screw it up in the first place. So hire a licensed plumber and don't worry. As for the water feeds and electrical lines, they should be put into conduits (a sleeve), so if there ever was an issue you can pull them out and replace them. HRV ducting should not be run in the slab, that should be run through walls and ceilings only.
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?
A hybrid can be done no problem, but it would add cost and building complication, so if it were me I wouldn't do it unless there was a very good reason due to the building terrain a house was to sit on. A hybrid would still leave you with embedding stuff in concrete as well as the durability concerns of a basement, so that sounds to me like it will ensure you are concerned out about it whichever one you choose :)
We are currently building a new demo house, on a slab, here is a video of the install if you want to have a peek. It is entirely above grade, and as I write, the house on top is being built, which will be insulated with cellulose (recycled news print).
Here are some other pages on slabs for your interest. Don’t hesitate to write back with more concerns, we love slabs and are happy to keep the discussion going.
I understand your desire to limit steel fasteners into the slab and thereby reduce their thermal bridge. But what about use of a metal flashing above the vertical slab edge roxul tying into the wall's water barrier for drainage? Won't this come into contact with the cement and be a thermal bridge around the entire perimeter of the building? Or have you come up with a great solution to this as well? Really like this detail with the zip ties!
Thanks Mike, no flashing goes through enough to act as a thermal bridge. What we did with the zip ties was really a Hail Mary in a way to attach the cement board, and while it works, it's a little softer than I'd hoped it would be. That can be solved though with a thicker coat of more flexible spray on parging.
It would have also been better had we installed double the amount we tie straps, or if we had a more creative and easier solution from the get go. Finishing the edge is for sure is a trouble spot with thickly insulated slabs, and is one of the many reasons I've taken a shine to the pre-fab slab kits. The insulation is fastened to the concrete during the pour, and all you have to do is parge the outside. Best regards, Mike http://www.ecohome.net/guid...
If you were to do it again, would you recommend 2" of EPS on the outside for finishing and 6" of Rockwool inside? And how exactly woud you finish the EPS foam slab exterior? Just a large angled flashing? Thanks a bunch for the awesome content, very inspiring. I might be testing your technique in cold Whitehorse, Yukon when building next spring. Cheers!
Hi Raphael,
No, we would not do it like that again, it worked but it was pretty labour intensive. We just built a new demo house and it is on a Legalett slab on grade with air heated floor, and the company takes care of that detail with a factory solution, They have an insulalated door threshold called ThermaSill that solves the problem entirely.