I had a quiet moment to step back and sieve through some of the information we had researched and accumulated over the past months.
Amongst them, a podcast posted on the greenbuildingadvisor.com page with an excerpt from a buildingscience.com seminar called “Building Science Fundamentals” taught by Joe Lstiburek and John Straube.
This kind of resource is priceless and gives me the chance to check once more on the technical details of our design and may be stumble across opportunities for further improvement.
The podcast outlines of the four principal control functions of a wall, listed below in order of importance:
- Rain control layer
- Air control layer
- Vapor control layer
- Thermal control layer
The water is a little muddied by the fact that some control functions can be combined in one layer. Take our project for example. Our masonry building shell (mass storage wall) is the rain control layer.
The spray polyurethane foam (SPF) is our air control layer, essentially providing us with an airtight building shell. The SPF also performs as a vapor control layer, which, in our case, allows the wall assembly to dry out to the exterior and interior, depending on the season. Last but not least, it is also our thermal control layer.
I appreciate products that can do more than just one simple thing, meaning that I get something for my money.
The podcast explains that if one flips the wall section by 45 degrees, one ends up with a floor section. Flip the floor section by 180 degrees and you get a roof section.
The control layers of the floor must connect to the corresponding control layers in the wall, which must in turn connect to the corresponding control layers in the roof.
What really stuck with me was a design review recommendation that suggests taking a pen and tracing each control layer on the plans, from the floor through to the walls and roof. Whenever my pen has to leave the paper, while tracing a control layer, I have come across a design flaw. And it didn’t take me long to find one.
The thermal, vapor and air control layers are switched from the interior of the building to the exterior at the top of the foundation wall.
That switching of the insulation from the inside to the outside would represent a design flaw, or does it?
After thinking about it for a while, I noticed that the root cause for this design flaw lies a few feet farther down and unfortunately is something that I cannot address. I am talking about the damp proof between the footing and foundation wall … or lack thereof.
Because my old limestone footings and foundation wall has no damp proof layer, I have to find ways to let the foundation wall dry out.
That can only occur effectively to the inside of the building, because the outside of the foundation wall is covered by soil. We will leave the foundation exposed in the basement and install a mechanical ventilation system with an Energy Recovery Ventilator (ERV). That system would supply sufficient air movement and exchange to assist in the drying process.
I find myself in the position where I need to balance the need of adequate moisture management against an apparent design flaw. The moisture management aspect overwrote the principle of contiguous control layers in the wall.
My common sense tells me that this is a good weighing of the priorities and I hope we won’t regret it.