Dispelling the old myth "Houses need to breath!"
How many times have you heard "You can't build tight homes, houses need to breath!"?
At Gomex Engineering, we like to split this issue into two parts. Occupant comfort and building durability.
Regarding occupant comfort, we agree houses do need to breath but the fresh air coming in has to be mechanically controlled, and properly filtered before being sent throughout the building, in order to maintain the highest possible indoor air quality for the building occupant's health, safety and comfort. Drafty homes that "breath" with out any mechanical control will cost you in heating/cooling bills for the life of the home.
Regarding building durability, older drafty homes, also allowed wall assemblies to get wet, but more importantly, dry, in the summer months as well. This is why these older homes have stood the test of time. As long as wood studs have adequate days to dry and compensate for its days getting wet there is no long term damage done and durability is maintained.
Prior to the 70's, drafts were responsible for up to 40% of a building's heating bill. In the 80's, with the introduction of the R2000 Home Standard, tight homes were being built without adequate knowledge regarding building science. Sure, heating bill's lowered, but so did indoor air quality. These tight homes, without adequate ventilation, created lower oxygen levels. They also generated excess humidity, which created the perfect environment for black mold, fungi, mildew, bacteria, dust mites, which led to an increase in symptoms and allergic reactions. In severe instances, this was leading to wall assemblies rotting out and builders were left footing the bill on remediation costs. Rightfully so, this was when tight homes tarnished it's reputation, and when 'Buildings need to breath!' was born with builders who experienced these sorts of call backs. Nowadays, building science has progressed significantly and we now have known wall assemblies that are designed and built to last. Hygrothermal analysis software has vastly improved to predict very accurately if a certain wall assembly is going to be prone to rot or moisture issues.
So how do we build tight, improve indoor air quality and ensure maximum durability for the life our buildings?
BUILD IT TIGHT.
When designing a tight air barrier, we want to try and eliminate as many building envelope penetrations as possibly, through smart design choices, appropriate equipment selection etc., however, since we can't eliminate all building envelope penetrations, we also want to provide specific air sealing strategy details around these penetrations and adequately prepare for them.
First, let's discuss options for avoiding penetrating the building envelope. Instead of a range hood direct vented outdoors, consider using a recirculating hood. If you aren't constantly deep frying or cooking heavy with oils a recirculating hood is more than adequate and eliminates a building envelope penetration. A ventless condensing clothes dryer also eliminates the need to adding an exhaust vent through your building envelope.
After working through your design and eliminating all possible penetrations, we turn to the penetrations you can't avoid and highlight specifically how these penetrations need to be addressed. It helps at the start of construction if there is a designated person who is responsible for all the air sealing throughout during construction. Standard construction in the past has used spray foam to try and air seal these penetrations. In Passive Houses, we know now that spray foam is not considered an adequate air sealing strategy and more work is required to properly seal a penetration. There are several more effective tapes and liquid applied products that are actually designed for air sealing.
Taking the time in the design stage to make sure these details are sorted out and easy to understand will save time in the construction phase and also be more likely to be followed by the trades. With respect to the importance of airtightness, it is not uncommon to see a belt and suspenders approach to detailing air tightness at specific junctions. Certain parts of a building don't afford builders the ability to go back and seal something very easily (or at all) should a leak be discovered during a blower door test. It's these areas where its critical to be vigilant during construction and give extra attention to the design details and construction sequencing. A standard red line test can help identify junctions where air sealing or maintaining continuity of the air barrier could be an issue.
VENTILATE RIGHT.
Now that you have built a tight home, it is equally important to mechanically ventilate and bring in fresh filtered air. Passive House Certified H/ERV's have a 75% minimum required efficiency but there are available units that can go as high as 91% heat recovery.
Fresh filtered air plays a huge role in improving indoor air quality, removing indoor/outdoor contaminants and leaving building occupants healthier and safer. These units are designed to be ran 24/7/365. While they are using power this entire time, they are also continuously removing contaminants and bringing in fresh filtered air that building occupants are breathing in. The electricity consumption is minimal as these units are quite efficient and is negligible when compared to the health benefits to building occupants they provide.
VAPOR OPEN ASSEMBLIES.
Building durability is the second piece of the puzzle. Air moving through a wall assembly brings moisture along with it. Building tight prevents excessive moisture from entering your wall assembly, but it can also trap all the humidity generated indoors from showers, kitchen and laundry room. This is why it is critical to design wall assemblies to be able to dry out since no wall assembly is ever 100% perfect. Using vapor permeable insulation, and smart vapor membranes are some of the key elements in designing a wall assembly that eliminates the call backs.
In the end, you can have your cake and eat it too. Tight homes lower energy bills, mechanical ventilation improves indoor air quality, and vapor open assemblies ensures your investment lasts for as long as possible. Here at Gomex, we incorporate all three aspects into our building design.
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