Dana, we would greatly welcome your thoughts about this analysis and, more importantly, about how well this assembly will perform. Thanks in advance.

Posted By sailawayrb on 04 Jun 2014 11:44 AM
Dana, we would greatly welcome your thoughts about this analysis and, more importantly, about how well this assembly will perform. Thanks in advance.

Yes, it would be greatly appreciated...

Here is an interesting article.

In an experiment designed to measure moisture entry into cavities, it was found that barometric cycling caused the wetting of cavities about 10 times faster than by vapour diffusion alone.

Maybe it's time to forget about vapor diffusion analysis?

Another conclusion: "...buildings in the Canadian climate should have all construction cavities breathing to the outside wherever possible."

It also talks about how important building pressure control is (since it can be used to make sure that air pumping in/out of the cavity is dry air).

Posted By jonr on 10 Jun 2014 12:30 PM
Here is an interesting article. It talks about how pressure changes (due to wind/barometric/temp) can pump 10x more moisture in/out of a wall/ceiling than vapor diffusion will. Maybe it's time to forget about vapor diffusion analysis?

A conclusion: "...buildings in the Canadian climate should have all construction cavities breathing to the outside wherever possible."

It also talks about how important building pressure control is (since it can be used to make sure that air pumping in/out of the cavity is dry air).

I don't think disregarding vapor analysis calculations is a proper approach.

As far as breathing to the outside and Canadian climates go, yes, that is the current consensus with any cold climate area (Zone 5 or higher).

A slightly positive pressure is good for a home. Mechanical ventilation via an ERV or HRV will provide the proper pressure and ventilation.

"A slightly positive pressure is good for a home."

A slightly positive pressure can be actively BAD for a home in a cold climate, inducing far more moisture condensation/adsorption into susceptible materials along the exfiltration paths than would occur from mere stack effect or wind.

My choice would be to run negative pressure during the heating season and positive at all other times. The former for wall/ceiling moisture and the latter for that plus pollution and humidity control (assuming you have some device for bringing in outside air and then filtering and dehumidifying it). Rebalancing a HRV twice a year sounds doable.

In most full-time energy efficient (i.e., well-insulated and well-sealed) residential buildings the goal is to reduce cooling/heating loads and uncontrolled infiltration is typically the biggest driver of these loads in these buildings. In most residential buildings, ERV/HRVs are generally setup in a neutral, equal pressure scheme to provide your ventilation air. Consequently, in most residential buildings using ERV/HRVs for ventilation, uncontrolled infiltration is relatively small and vapor diffusion tends to be the dominant building assembly moisture mechanism in play. As such, doing a Glaser dew point analysis to ensure your residential building assembly performs as desired is entirely appropriate.

However, in part-time energy efficient commercial buildings, ERV/HRVs are sometimes intentionally run out of balance to achieve a preferred pressurization scheme. As I mentioned in the other thread, in commercial buildings you could have the exhaust air rate exceed the fresh air rate by about 10% to create a slight negative pressure in the winter. In the summer, you could have the fresh air rate exceed the exhaust air rate by 10% to create a slight positive pressure. These preferential pressurizations are intended to control which way air will flow through the commercial building envelope at different times of the year. In the winter it is often better to encourage dry winter air to pass in through the building envelope as it will not condense on its way in. In the summer, it is often better to encourage dry room air to pass out through the building envelope as it will not condense on its way out. Consequently, in many commercial buildings using ERV/HRVs for ventilation, controlled building assembly infiltration/ventilation may be relatively large and may be the dominant building assembly moisture mechanism in play. As such, a Glaser dew point analysis would likely be invalid/inaccurate because vapor diffusion is the only building assembly moisture transport mechanism considered by this method. Building assembly air infiltration/ventilation, building assembly material moisture capillary transport, direct rain intrusion/wetting, and solar heating moisture transport mechanisms are NOT considered. You would really need to do a much more accurate/complicated building assembly moisture analysis using a transient software model that is capable of considering all of these moisture transport mechanisms as well as the humidity, pressure, and temperature initial conditions.

Yes, this preferred pressurization scheme can also be used to good advantage in full-time energy efficient residential buildings too, but you will need to carefully consider and properly address combustion air requirements (e.g., appliances needing combustion air) and makeup air requirements (e.g., appliances such as stove exhaust fans needing makeup air). Of course, you will also need to carefully design a building assembly with this preferred pressurization scheme in mind too. In short, this is an integrated, multi-discipline design/build problem and one really needs to know what they are doing to successfully play this game. The home owner also needs to be fully aware of what is going on to ensure that this integrated building assembly/HVAC system will continue to operate as designed or the consequences may be disastrous. Frankly, we wouldn’t trust very many HVAC pros or home owners to get this right.