How to Care About Air: thoughts on air tightness
Updated: Mar 18, 2022
The OG of Passive Houses, the 1973 Saskatchewan Conservation House (SCH) was a super high efficiency home that brought together many like-minded professionals who were looking to reduce their dependence on oil in a time of crisis. They largely succeeded and managed to build a house using rudimentary materials that are still widely available today. As Harrold Orr explained so eloquently;
The trick is to attack the big wedges of [the energy consumption] pie first and then do your best on the smaller wedges. When we did this to the proposed SCH, we estimated that we could heat the house with a candle, a 45 Imperial gallon candle, and in 1976 about 33.00 dollars worth of fuel.
The house was tested and came in at 0.8 air change per hour at 50 Pascals. How did they do it using merely 6mil poly, fiberglass and homemade windows? Care.
From the design stages to the construction stage, the Air Barrier System needs to be explicitly drawn, and ideally highlighted in a continuous red line*. For good reason, the OBC (and the NBC) doesn’t call it an “Air Barrier”, but rather an “Air Barrier System” (ABS). Because the air barrier isn’t a single product, it’s a continuous planar assembly that securely connects many products or building components into a single air tight system that includes roof or ceiling membranes to above grade walls to window and door frames to below grade walls to under-slab membranes to mechanical, electrical and plumbing penetrations. In PHA’s 10 Easy ways to Ruin your Blower Door Test, BldgTyp’s Ed May does a great job explaining common problems with ABS performance. But even if you know all that you still need to care.
Caring is about paying attention. Sweating the details. Let it be said that detailing an ABS for new construction is considerably easier than trying to cobble an ABS in an existing building. So hat’s off to all those who tackle the Deep Energy Retrofit of vintage buildings. Here’s a priority list of items to pay attention to:
It all starts in the planning stage: decide on how deep you’ll go and on which side of the existing wall your ABS will go. Talk to your mechanical designer and your energy modeler if applicable and establish a target air leakage rate. Your mechanical system will be sized using that leakage rate as opposed to a default rate. Whatever you decide the air leakage rate should be no higher than the rate used in the. Ideally, it’s below 1ACH50 for a house or less if it’s a larger building.** Pull the GC into the game and get them onside early. You may want to give them a bonus if they meet air tightness targets and or make sure they reign in their trades with holdbacks if their work assembling the ABS is shoddy. We tested a 20 story EnerPHit building in Hamilton last year with one blower door fan. Yes, one fan and I attribute that success to PCL in that they had their project managers got PH certifications and they brought the trades on early and converted them to the green side. Either way, the project manager must be on the job site when the first airtightness test takes place and needs to treat the testing process like their reputation depends on it.
Outtie trumps Innne: If there’s a choice, and exterior ABS is preferable to an interior one. The exterior ABS of the building typically won’t be penetrated as often by structural elements, electrical, HVAC and plumbing penetrations as an inside ABS. If it’s a heritage building where you’re forced to insulate inside, then consider a combination of flexible Self Adhered Membrane (SAM) and or liquid applied.*** The second benefit of an exterior ABS is it usually stays uncovered for longer which means when the air tightness test happens, leaks can be found easily and repaired. It was brought to my attention that 475 suggests an interior ABS and yes it works, but it’s more effort to detail. To 475’s point, permeability of the ABS comes into play here and if it’s an outie, should either have enough R-value or be vapour open to respectively prevent formation of condensation or allow trapped moisture to move out.
New meets Existing: Connections between the existing structure and the new additions are prone to leak air because they are often difficult to cleanly tie the dissimilar materials together. Best is to make sure that the transition membrane is flexible to allow for movement and if you’re phasing the build over time, think ahead and bury a continuous flap of sheet-good that allows the next phase to graft onto the existing structure easily and cleanly.
Transitions: Generally, the field of most rated ABS products doesn’t loom large as far as air leakage goes, but the joints where different products meet (roof to wall, window to wall, foundation to above grade wall etc) tend to be where human errors accrue. If the windows are being tested for air leakage (ASTM 783 or 1105) have them test the transition joint as well.****
Rock Paper Scissors: Rigid beats SAM/Liquid beats Unadhered flexible sheet good. First off, if it looks like it isn’t airtight, assume it isn’t. If the product’s installation instructions say don’t install in temperatures colder than 5oC, then don’t do it, ditto if adhesion won’t tolerate rain (or condensation). A rigid panel type ABS will take a bit more abuse and makes an easy substrate to transition to another product like a window buck or flexible sheet good. The panels still require meticulous sealing at all adjacent panels with either a compatible tape (roll it!) or liquid applied ABS. SAMs work well and are best applied over a clean solid substrate to cut off air flow that may be driven by convective flows or pressures so make it look nice and roll it! Fishmouths in lapped seams (drain the rain!) need to be pressed out and avoided, so attention to detailing is key. The image above shows a spray applied membrane being simply tested with dish soap and a vacuum; bubbles are bad. Unlike panels or SAM that are made in a factory, liquid applied membranes are manufactured in situ and more care is required. As always, RTFM applies for each product and make sure the trades have them and read them. Finally, an ABS needs to be robust, so whatever components or systems you use make sure it can handle the abuse of the construction process and still have enough left in the tank to last over a 100 years.
Testing day. There’s a saying in the biz, “Test early, test often.” Especially if it’s the PM’s first tested build. As soon as the ABS is substantially complete, put a fan on it and get some numbers. It’s always prudent to review the transitions previously mentioned. If the enclosure is heated and it’s cool enough outside, you can use infrared cameras to pinpoint air leakage as long as the ABS isn’t covered up in layers. Alternatively, you can use theatrical fog too. If the test fails, book the next test and start the remedial work on the ABS making sure the repairs are tight.
In the end, if you have trades people and labourers that care, you can make any building air tight. Even if you’re ABS is in a less than ideal location and even if you’re using basic materials. All the best ABS are a labour of love that should last a century or more.
* If you’re not familiar with The Journal of Light Construction, pick it up! It’s a wonderful technical resource aimed at builders. Full disclosure, it’s inexpensive and published by Huber, the folks that make Zip Sheathing.
** For PHIUS the most stringent requirement (p.22) is 0.060 CFM50/ft2 or 0.080 CFM75/ft2 of gross envelope area.
*** Liquid applied membranes require a nominal thickness and truth be told, there’s a lack of experience in installing the product. Often they claim to be installed by either paint brush/roller or spray applied or trowel applied. What I can say is that whichever you choose, make sure the applied thickness meets or exceeds the manufacture spec but also test it in the field by applying soap and water then cover the test area with a chamber under negative pressure. There should be no bubbles, no matter how cute and tiny they are.
**** Sadly, window tests merely confirm that the installed product performs as it did in the lab and has nothing to do with the installation of the window. It could if you spec it though. Too many professionals mistakenly think window tests check the integrity of the joint between the window frame and the rough opening.
Header image: "File:FensterIR Blower Door.jpg" by Sönke Krüll is marked with CC BY 3.0.
Other images: Greg Labbé
Greg Labbé has been working in the high performance building industry for over 20 years including 5 years with the BlueGreen Group conducting air tightness tests and enclosure diagnostics. Greg is currently a Building Science Research Lab Technician at Ryerson University. Greg created High Performance Design Meets Boots on the Ground while at the BlueGreen Group and passed the “BOG” torch to PBC in 2018. In other words he is the BOG Father.
Articles on the PBC website reflect the views of the author and not necessarily those of PBC.