Why am I devoting an entire post to the movement of air you ask? And why now, at the early design stage?
As it turns out, moisture buildup in walls is one of the biggest problems in building. Moisture can get into walls from both the outside (rain and humidity) and from the inside (bathing, cooking, house plants and breathing). Both sources of moisture often get into walls by hitching a ride with air. Once moist air is in the wall it can condense into water and degrade your insulation value and cause mold and mildew problems, not to mention structural issues if serious and left untreated.
Since we’ve decided to build a super-insulated house, the more insulation you use, the more this can become a major concern. I’ll explain this more in a future post on wall sections. For now, trust me, its a problem.
There are several ways to control moisture in the wall, by limiting its ability to get in, and then giving it a chance to dry out when it does. I’m just going to consider the first part in this post (limiting moist air infiltration into the wall), and its related topic ventilation. Because if you build a tight house to keep moisture out of your walls, you need to provide a way to get fresh air into the house.
Infiltration and tight houses
As the literature goes, old houses blow. Literally. Old houses were so loosely built (that doesn’t mean sloppy, just not sealed well) that air blows right through the house, through cracks in the wall and floor, electrical outlets, window and door frames and basement and attic openings. It may not seem like a lot, but its a bit like the dripping faucet problem. It doesn’t seem like a lot, but it really adds up over time. As outside air blows through your house, all that nice warm moist inside air that you spend so much money on to create, simply leaks out.
This isn’t all bad. You need fresh air to replace old air filled with odors, excessive moisture and other yummy stuff from cooking, cleaning, washing and breathing. The key is build as tight a house as you can (limit infiltration), and use mechanical ventilation to provide fresh air and to vent odors. Mechanical ventilation can also be used to preheat the cold incoming outside air with the warm outgoing inside air, which is what makes it more energy efficient than just letting the wind blow through your house. We’re cover this a little later.
So its that simple you say? Build a tight house and ventilate mechanically.
Well this is where it starts to get complicated. What constitutes a tight house? How much fresh air is the right amount?
First, defining what makes a tight house means understanding a bit about how to measure air infiltration in a house.
Measuring Infiltration with a Blower Door Test
Air movement is often measured in cubic feet per minute (CFM) or air changes per hour (ACH). A value of 2 ACH means all the air in your home is replaced twice in 1 hour. CFM and ACH are mathematically related, so you can calculate one if you know the other. I find ACH an easier number to understand and use in my calculations, so I’ll use that.
A blower door test measures the air leakage of the house by depressurizing the house to see how much air seeps in to replace the air that gets sucked out. Air pressure in measured in Pascals, or P for short. It is sometimes tacked onto the CFM or ACH, e.g. 5 ACH50P (or simply 5 ACH50), means 5 air change per hour measured at 50 Pascals.
In my research, a tight house is defined anywhere from less than 5 ACH50, to less than 0.6 ACH50, and everywhere in between. The PassiveHouse standard for a tight house is 0.6 ACH50, which is the tightest standard I’ve seen. Based on my research, a 1 ACH50 target seems very doable for our house, considering my OCD tendencies.
But if your blower door test results show 5 ACH50, that doesn’t mean the house air is changing over 5 times per hour under normal usage, that’s only then you have a big fan trying to suck out all the air in the house. So we can’t use that number in calculating how much infiltration is adding to your ventilation needs.
We need what is called a normalized value to approximate what 5 ACH would be under normal conditions. Research on the web says to divide ACH50 by X to get ACHnat or ENIR (estimated natural infiltration rate). X depends on climate and location. I’ve seen ranges from 17 (Minnesota) to 30 (Florida). Of course this is an approximation. When the wind blows outside, more air is forced into and sucked out of the house than when the wind isn’t blowing. But it helps in understanding how much of your ventilation is being provided by infiltration. I’m using an X factor of 20 figuring we’re closer to Minnesota weather than Florida.
So if I shoot for a target of 1 or less ACH50 in the blower door test, that means I can estimate a value of 0.03 ACH due to infiltration. For you energy nerds, that means I would loose roughly 5.5 BTUs/hr/1° due to infiltration, or over 11,000 BTU’s on a day when its -15° outside. Compare that to a leaky house (1 ACH) which might loose over 383,000 BTUs due to infiltration on the same day.
Now on to the next question.
How much fresh air do you need?
The code recommends that you need 3 cubic feet of air per minute (CFM) for every 100 square feet of conditioned space, plus 7.5 CFM per occupant. Our conditioned air square footage is 1200. You derive number of occupants from the number of bedrooms plus one (2 people as assumed for the master bedroom and 1 for each additional bedroom). We have a bedroom and an office, but by code standards, that office is another bedroom (which also effects the size of our septic system).
But you get the idea, we would need approximately 60 CFM. CFM can also be measured as air changed per hour (ACH), which is an easier number to think about. 60 CFM = .35 ACH. Which means that a little over a third of the air in the house changes over every hour.
I should note that lots of other folks recommend no less than 2/3 ACH, which is another third more than the code recommends. I haven’t figured this out yet. So when I do I’ll let you know.
If you remember from above, I’m shooting for less than 1 ACH50 or just 0.03 ACH due to infiltration. That doesn’t meet the minimum required value of .35 ACH. So I need to provide an additional .32 ACH in mechanical ventilation. Note that many older houses have a value of 1 or more air changes per hour just in infiltration. This means they don’t really need any mechanical ventilation because all their ventilation needs are being addressed by their leaky house.
Most folks get mechanical ventilation through a forced air system which can provide heating and cooling. In our case, we don’t need cooling. And I haven’t determined what type of heating or how much we need yet. So if we just look at pure ventilation (.35 ACH) and ignore heating and cooling (assuming we just suck in the -15° outside air and blow out the 72° inside air), we would loose roughly 134,000 BTU on a very very cold day, just to get some fresh air in the house.
If we used an air exchange system as mentioned in the early part of this post, we would be able to recover some of the heat that was being blown out of the house. If we assume an 80% efficient heat recovery ventilator (HRV) then we only loose 27,000 BTUs. That is a lot more efficient than the 383,000 BTU’s lost by just letting the wind blow through your house.
All this assumes my calculations are correct. Every time I go through them I find some horrible mistake. I’ll revise this post as I get more accurate numbers. But I’m getting closer. Hope you enjoyed, I certainly did.