Growing food in urban cores with the Urban Farm Unit

There are conflicting thoughts on this kicking around the web, but we will share what we know as best as we can. However, please be sure to check with your local building inspector regardless to ensure you meet any regional qualifications to build.  

4 inches of EPS rigid insulation can meet code requirements as a vapour barrier only if it is overlapped, to make sure moisture does not pass through the butted joints if you are only installing single sheets.

Some building certification programs such as Passive House will have higher standards than building code and will require that you add a slab vapor barrier, and to achieve LEED for Homes Certification a Radon barrier radon evacuation stack is mandatory.

To continue about radon – exposure to radon gas in homes is the second greatest cause of lung cancer. There is no way to test radon levels in a home before it is built and no way to install a barrier after a slab is poured, so I’d certainly install a radon barrier it if I were you since you are building in a high-risk area.

Of the relatively few builders that seem to be are aware of radon gas health issues, many will just use a standard 6-mil poly vapor barriers as a radon barrier, which is better than nothing, but I’d want a thicker one for my home.

There is a barrier called Radon Block, which is first of all a much thicker and puncture-resistant membrane, and manufacturers claim it is 100 times less permeable compared to standard poly of the same thickness.

If a Radon Block barrier is more than you can afford in your budget, I would at least consider a thicker poly membrane of 10 or 15 mil. With only 6-mil it is almost a foregone conclusion that you will puncture it by walking on it, dropping tools, piercing it with rebar and reinforcement mesh, mesh chairs etc, so to ensure protection against radon I would choose the added protection of a thicker membrane myself. And for installation, if you can, lay the barrier down BETWEEN layers of foam, that way you walk on it once and then cover it. The only issue with that is that you need a strategy to remove or drain any water if it rains before you pour concrete. 

In our LEED V4 Platinum Edelweiss Demonstration house we installed a 10-mil underslab vapor barrier from WR Meadows, we found it to be extremely puncture resistant and our final radon test showed levels under 20 becquerels, far lower than the 200 becquerel level that Health Canada considers unsafe for longterm exposure. 

As a final thought as well, remember that radon isn't only going to enter homes fromt the soil, but radon gas in water is another potential source that should be considered to ensure safe indoor air quality in a home.

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. 

• Best practices for insulating basements
• Choosing between a slab and a basement