Does Your House Breath Air or Drink Water?


Andy from Elysian Energy came out to do our final blower door test.  We knew that we had a 1 ACH 50 from John’s test at the Meetup event with 30 people walking around and playing with the windows and doors.  After taking some significant measures to air-sealing the windows and joints, Jim and Andy came for a second test which had a result that I thought was too good to be true but we felt comfortable enough to proceed with drywall and everything else.  I think there was a discrepancy in how the total air volume was calculated…

Last Friday, Andy did our final testing the ACH at a number of different pressure levels and our final number was 0.52 ACH 50!  The number is not 0.33 but we think it is a very respectable one for our first attempt at Passivhaus.

Final Blower Door Test

Air-tightness really is a difficult thing to achieve if you do not plan ahead.  I think to get the air infiltration number even lower, air sealing must be done in conjunction with construction, i.e. tape as you build.  I know of a builder that tapes up each nail hole right after the nail is hammered in so as to not miss any.  It is obsessive but I am not sure if that effort is really excessive.  Air-tightness is a quality of construction issue.  We’ll never get to a 0 ACH 50 unless you build a house with a balloon but we can certainly get darn close…

The Importance of Air-Tight Construction

I know this is a huge topic but in an effort to share information and help me articulate the issue I must dip into the importance of air-tight construction.  We had previously talked about this but we only talked about what we need to achieve but never explained why.  Here are some of the “why”.

1. Energy conservation:  Did you know that you can have all the insulation in the world and the build can still perform poorly? A leaky building envelope means “uncontrolled ventilation”.  This means conditioned air can leak out and hot/cold air can leak in; thus decreasing the effectiveness of the insulation.

This is an illustration of air-infiltration from http://oee.nrcan.gc.ca

As you can see, air can leak in and out at a variety of places.  This is “uncontrolled ventilation” or what some typical builders talk about when they say, “You don’t want a tight house, your house needs to breath.”  Our retort is, “Your house does not need to breath, you do!”

2. Indoor Air Quality: Air can carry pollutants, moisture, odor and other undesirables into the building through these leaks.  If you cannot tell where the air is leaking in from, you have no idea what pollutants are being carried into the indoor environment.  As I like to say, “you are breathing like a frog.”  I love Kermit the Frog but we are mammals, so let’s have a tight skin (air-tight building envelope) and breath through a dedicated respiratory system (an Energy Recovery Ventilator or Heat Recovery Ventilator) that brings in fresh air through a controlled path and filters it before we consume it.

Do you want to breath the air that passes through this window?

3. Vapor Penetration

Air carries a large amount of vapor or moisture.  Each air leak has potential to cause water intrusion which in turn causes mold and structural damage if the moisture does not dry out.  In well insulated buildings, this problem can get worse because there is so much material that can trap moisture.  Therefore, air-tightness becomes even more important as building envelopes get tighter to keep water out in the first place.

This image from  Building Science Corporation paper RR-0412: Insulations, Sheathings and Vapor Retarders http://www.buildingscience.com/documents/reports/rr-0412-insulations-sheathings-and-vapor-retarders gives us a dramatic visual of how moisture laden air behaves with air-tight building envelopes and leaky building envelopes.  Over just one heating season in a cold climate, 30 quarts of water enters into a 1in sq hole in a building envelope whereas in an air-tight building envelope, only 1/3 quart of water enters into the envelope through diffusion.

Water is the biggest enemy of buildings, keep it out.  By building an air-tight envelope, water intrusion becomes water diffusion.

So, we’ve concluded that a house does not need to breath and does not need to drink water either.

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3 responses to this post.

  1. Great post!
    I used to really protest when folks (esp. builders) insisted that a house needs to breathe, for all the reasons you mentioned here. But lately I’ve found it’s a better to just run with the house/human analogy, and agree that a house (like a human) needs to breathe, but that it must breathe though a controlled and filtered source — we humans breathe through nose & mouth, not through our skin…. houses should too. In your house the ‘breathing’ will be via the HRV, not the walls.
    So, you hit the nail on the head with the frog comment, I think.

    Reply

    • Hi Jack,

      Thanks for reading. I agree builders have that tendency because it’s so simple to think ” a house needs to breath” so that we can breath. I think in a couple more years it is going to be very difficult for them to say that. As jurisdictions adopt 2009 and 2012 IRC, the air-infiltration requirements will go down to 3 ACH50, which is not your “breathing house”. They can continue to argue all they want but by code, in order to stay in business, they will need to build much tighter homes.

      Reply

  2. Posted by Peter on 2012/07/19 at 3:33 am

    I am self building at the moment (not to PH Standards). This article helps ( or hinders) my thinking of how to achieve the regs. on ventilation. Have agreed with the window man not to have trickle vents I now have to achieve ventilation at this late stage. I have been looking at positive input as the most economical solution. I ask myself where does it go? Through holes there in every building is the reply. I did consider mechanical trickle vent at the lowest point to create a path of least resistance as the plaster board example above. Now after reading your article I wonder if positive input will prevent ingress of moisture. Any thoughts?

    Reply

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