Passive Solar Index Home Case Study

When I drive by houses doing exactly what you are planning, sometimes I cringe at what I see happening. There are many different types of board insulation and it’s very easy to add the wrong type, or probably a surprise to some, but the wrong amount.

Some rigid board insulations are a vapour barrier, some are not. Some are air barriers, some are not. XPS foam insulation at about 1 inch thick will act as a vapour barrier, EPS foam insulation will start to act as one at about 2 inches, and polyisocyanurate (the one with the foil on both sides) is a vapour barrier at any thickness (foil is waterproof). You can read all about rigid insulation panels here.

Installing insulation that acts as a vapour barrier on the outside will not serve you well, except if you add enough of it, to the point where about 2/3rds of the total insulation is on the outside. At that point what you are doing is building a R.E.M.O.T.E wall, read more about that here. What that means, is if you have 3.5 inches of fiberglass in the stud cavities, you can add about 7 inches of EPS foam on the outside with no problem, because you will then be keeping the interior of the wall warm enough that the ‘dew point’ is no longer an issue. We would suggest EPS because of the plastic based foams this is the least problematic environmentally. Here's a link to a product that is simple solution to achieve this, it is an EPS foam panel with a metal rail embedded in the center for exterior insulation that is used to attach it to the wall and that the strapping is then attached to for installing siding. You would just screw it to the wall, then screw strapping to the same panel for installing future siding, which sure makes the job quicker. 

If you prefer a wood solution over plastic (though EPS is really mostly air), what you could also do is use a Larsen truss wall system as a double stud wall, (see more here) which are ideal for improving wall insulation when renovating homes & for building new homes with high performance insulation systems such as Passive House or LEED. You can then either fill the new cavity with dense packed cellulose insulation, or even consider the latest kid on the block, natural based hemp insulation products.

What is really important is that you don’t affect the wall’s ability to dry. For that reason if you wanted a simple "one size fits all" exterior insulation skin option but don't want or aren't able to add 7" to the exterior of your home, we would recommend Rockwool rigid board insulation (known as Roxul prior to re-branding). It’s sort of a failsafe insulation in the sense that air goes through it, moisture goes through it and water drains through it. You can add as much rock wool as you want, or as little as you have to if there are other considerations like property line setbacks or such, because it won't alter the basic function of the existing wall.

After that, install a vapour permeable drainage plane, and vertical strapping to allow moisture to vent and water to drain, which won’t happen if you install strapping horizontally. We have a page on how to install siding here where that is all explained in detail.

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.