Last weekend was a reckoning of sorts.
We've been focusing on making the house as air tight as possible ever since the framing stage. The structural insulated panel (SIPs) construction goes a long way, but ensuring a tight envelope requires attention to small details, like filling or sealing off all cracks and gaps around windows and doors and between panels. The foam and caulk guns have been our constant companions of late.
Blower Door Test
Despite returning home with plenty of dried foam in my hair and stuck to my arms, I still wasn't sure that we had caught every potential intrusion point for cold air. The way to find out is with a blower door test, and Dave borrowed the equipment to do this from his office mates at O'Brien & Company.
Dave set up a computer-controlled blower with a cloth skirt that allows it to fit tightly in a doorway. The fan depressurized the air in the house and very precisely measured how much air was getting in. As it turns out, very little.
The first test gave us a scare. It read about 3 air changes per hour (3 ACH at 50 pascals). This was only slightly better than code and wouldn't be nearly tight enough for us to achieve our zero-energy goal. This didn't seem right after all the air-sealing work we put in. Thankfully, once Dave taped off the external vents, the actual results were much better: just .50 ACH, which is better even than the extremely rigorous Passive House standard of .60 ACH!
Isn't It Unhealthy to Tightly Seal a Home?
It's true. In the interest of conserving energy, building codes have been evolving over the years to require homes to be more airtight. However, building technology and regulations for ventilation haven't always kept up. In some cases, poorly ventilated and poorly designed homes have become "sick," meaning mold and mildew begins to grow in the walls, endangering the health of the inhabitants.
This often happens when the moisture from cooking, showering, and the occupants' breathing raises the humidity in the poorly ventilated house. This moisture is held harmlessly in the warm inside air. Unfortunately, damage starts to occur when it condenses at those places where cold outside air infiltrates through the building envelope (walls) or thermal bridges (areas where cold passes from the outside through solid, conductive surfaces, like wood framing lumber or metal fasteners). The warm, humid air hits these surfaces and water droplets begin to condense, sometimes inside the walls, creating the perfect environment for mildew.
SIPs panels have the benefit of not being prone to this in-wall condensation because they are made of solid EPS foam, sandwiched between two OSB (oriented strand board) skins, like an ice cream sandwich. The dew point, that theoretical point within the wall where the temperature is low enough to condense the water out of the interior humid air, is actually somewhere inside the solid, closed-cell foam, where no air can reach.
Despite the benefits of SIPs construction, no house can rely completely on the wall system to avoid condensation. In today's tighter houses, especially one like ours that is sealed almost as tight as a submarine (I exaggerate, but you get the point), it's critical to have a mechanical ventilation system. HVAC designers can choose to just use a system of fans and ducts to pull in unconditioned air from the outside, ideally through a filter, and exhaust warm, humid interior air to the outside.
A more energy-efficient method is to install a heat recovery ventilator (HRV), which is what we did. We chose a LifeBreath model, slightly oversized for our house. This large piece of equipment, which dangles from chains from the ceiling of our mechanical room, has a heat exchanger inside. This allows the unit to not only circulate fresh air throughout the house but to preheat the cold air coming in with the warm air being exhausted.
Any mechanical ventilation system in a tight house is designed to operate continuously. It's a different concept than owners of older homes are accustomed to. In an old, leaky house that has enough gaps and cracks to allow the interior air to be replaced with outside air every hour or so, one doesn't have to worry about ventilating air unless to control for point sources of humidity, as with a range hood in the kitchen or a bath fan over the shower.
In addition to the continuously operated HRV system, our house employs strategies to take care of point-source humidity as well. A button can be pressed in either bathroom before taking a shower, which will ramp up the air flow in the HRV system temporarily. Also, we will have a balanced range hood vent system, employing two ultra-quiet inline S&P brand fans, provided by our Artemisia Lab partner Canby Sales. One fan exhausts humid air from the range hood to the outdoors, and another brings fresh air from the outside into the kitchen. This prevents negative air pressure to build up, as would be the case if we only ran the single, powerful range hood exhaust fan.
We're looking forward to breathing fresh, clean air, while avoiding the cold drafts of leakier construction methods.