### Image Credits:

1. Lyndon Than

1.
Mar 6, 2017 10:31 PM ET

Thanks in advance for the data
by Charlie Sullivan

I for one will be very interested to see that data on whether you get any moisture detected by the sensors. Thanks for setting that up and sharing the thinking about what might or might not be problems, and thanks in advance for sharing the data as to what actually happens.

2.
Mar 7, 2017 9:50 AM ET

Dew Point Calculations
by Gordon Franke

Very interesting, thank you for posting. Your approach is similar to an example that was posted back in November at http://www.greenbuildingadvisor.com/articles/dept/green-building-blog/pr... but without the foam. I posted what is currently the last comment on that article asking about just such an approach.

Your average annual temperature in Toronto is around 47 F. Ignoring your heat storage strategy, the assumption would be that your slab will hover around 47. Using the same calculations as in my above-referenced comment, the dew point of your indoor air should vary between something like 45 in the winter and 50 in the summer. If the slab temperature is 47 then you probably will get some condensation.

In my area the average temperature is 56, which I think means that I can insulate between the slab and floor with less concern over condensation. However I'm just doing back-of-the-envelope calculations and hope an expert might chime in to correct me if I'm missing something.

Look forward to hearing what you discover as time goes on.

3.
Mar 8, 2017 12:20 PM ET

Too complicated?
by stephen sheehy

I don't follow the logic of not putting foam under the concrete slab and simply separating the slab from the foundation wall with more foam. What is the benefit from being able to take up a few floorboards and observe any moisture? Then what? How do you fix the problem?

Building this floor required an additional 18" or so of foundation height (to get the same headroom), plus the lattice, expensive insulation and the floor system. And then, the result is a floor made from framing lumber that will shrink and the resulting spaces will fill with the usual basement crud. How much additional cost is this over putting reclaimed foam under the slab and eliminating the condensation risk altogether?

As Gordon points out,some condensation is likely. Why risk that, especially when it could be avoided by putting the insulation under the floor?

4.
Mar 8, 2017 12:33 PM ET

Stephen
by Malcolm Taylor

I agree. It would have been helpful if the logic behind the various building assemblies had been laid out a bit more, as opposed to the steps taken to deal with the problems choosing them may pose in the future.

5.
Mar 8, 2017 10:31 PM ET

Basements never flood or leak... and subslab Roxul
by Anthony Mach

I've seen many homes on site visits with wood subfloors made with 2x3 or 2x4, either flat or standing. It seems to have been popular in the 1970's and later renovations. The clients are typically planning major renos and usually they have moisture problems with those subfloors. One house had a major plumbing leak from the well pump and it flooded a thousand square feet or so of the subfloor. The solution was to remove that soaked wood subfloor and not reinstall it. Most basement renos are done with dricore panels or a dimpled Hdpe membrane. If the clients wants a bit of insulation, then 1.5" of XPS with 2x4 sleepers on the flat, or no sleepers if it's floating laminate flooring. As they are older homes, there's not much headroom to be wasted.
Also, it's possible to install Roxul under the slab, as they did at Ecohome's Edelweiss demo house.

6.
Mar 30, 2017 3:39 PM ET

Edited Mar 30, 2017 3:47 PM ET.

Logic behind the assembly
by Lyndon Than

Bit slow to respond to some of these comments - my apologies.
The moisture we might expect is from flooding - not so much condensation. Thats really the reason for the removable floors. The house that sat on this location in the past had a number of floods. These were a result of what seems to be climate change. It seemed the basement was fairly dry until recent years, in which many many houses in Toronto flooded in their basements.
In my neighbours' house, the basement slab came up one year. He had no insulation under the slab at the the time - but I think there was no relief for the water - and the whole slab just cracked and lifted up in the centre. Floods happen for a variety of reasons, including sewer backups, ingress from the grade, and rising water tables. We've implemented a few flood defense features in this house, but keep in mind the following: Water is about 40% the density of concrete. If your sub-slab insulation is more than 2.5 times the thickness of your concrete slab above it, I would say there is a risk the slab will lift and buckle if groundwater levels rise to something like the slab level, due to flotation of the sub slab foam.
This gives some reason to consider insulation on top of the slab. But the major reason we took to having insulation above the slab was to keep the concrete on the outside of the building, avoiding the thermal bridges (or complex thermal details) that tend to result when there are crossovers.
There is also something I'm not really sure I understand properly. The idea of condensation on a cold concrete wall - If this happens, it means the moisture in the air contacting that wall is being removed - provided the condensate has a place to go. Well, if no new air is provided to that space, then the condensation stops after the air dries. In other words, I don't really understand the concern if there is no bulk movement of air to provide a constant supply of moisture. That means this arrangement should work fine as long as we provide an air barrier at the finish floor level or thereabouts. There is a moisture barrier under the slab. We've even placed a layer of rigid foam under the slab - i had purchased a little too much of it. With so much insulation above the slab, the insulation below should do very little to raise the slab temperature. However, the paths for moisture to enter the floor cavities would be virtually nil - other than in a flood situation.
Same with our walls. We have roxul insulation in contact with cold concrete walls - but so what? There are air barriers to prevent the bulk movement of air.

When there is a flood, however, (and I've been through a few, in finished basement space), we can remove the floor boards and ensure the concrete at the base of the floor cavity is drying out. We can also physically remove this water using pumps, etc. I imagine removing only sections of insulation where the concrete surface is lowest. Despite having a sump pit that the concrete surface slopes toward, there are still a couple of slightly low spots.

LT

7.
Mar 31, 2017 9:45 AM ET

Edited Apr 27, 2017 4:13 PM ET.

by Jon R

> If your sub-slab insulation is more than 2.5 times the thickness of your concrete slab...

In such a case, I'd review BEopt results regarding the cost effectiveness of so much foam (as compared to other options achieving the same net result). Or consider not building a basement where flooding is known to occur.

Constantly humid cavities can create a mold problem - even if there isn't enough airflow to create continuing condensation. And unless you are going to depressurize it, the mold odor will escape to the interior.

8.
Mar 31, 2017 10:49 AM ET

BeOpt is about cost effectiveness, PassiveHouse is not (@ Jon R)
by Dana Dorsett

The focus of PassiveHouse is about hitting particular energy performance targets, not about the cost effectiveness of hitting those targets. If it were about cost effectiveness you'd never see the levels of slab insulation found in most PassiveHouse designs.

If flotation of the slab during high water events was a primary driver of the decision to go with slab-top rock wool, splitting the difference putting a lesser amount of foam under the slab, with the remainder of the R as rock wool above would mitigate the summertime moisture accumulation potential.

9.
Apr 27, 2017 2:30 PM ET

Why not eliminate the slab altogether?
by Ethan T ; Climate Zone 5A ; ~6000HDD

If you are going to go this route, why not consider, as the builder in this JLC article (http://www.jlconline.com/how-to/foundations/a-basement-floor-without-con...) does, eliminating the slab altogether and just putting your insulation, subfloor, and flooring above your crushed stone and vapor barrier?

10.
Apr 27, 2017 9:49 PM ET

Why not Eliminate the basement slab altogether
by Lyndon Than

Eliminating the slab is an interesting idea. In our case, the footing were large enough and the design of them is such they could have been designed as grade beams so the slab is not needed. Most of the time, the slab is a structural member holding the basement walls against earth pressure.