Airtightness Explained: The Ugly Truth

Why do our buildings have to be airtight and is it healthy to live in an airtight building?
This video sheds light on the critical yet often overlooked aspect of airtightness, as it affects not just our comfort and energy bills, but also our health and environmental footprint. Discover the relationship between building envelopes and the forces of nature, and learn about the importance of proper implementation in achieving true airtightness.
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My name is Anton Dobrevski and I'm an architect and accredited Passive House Trainer who is passionate about designing High-Performance and Healthy buildings. On this channel, I'll be talking about Building Science and the Passive House Standard, more specifically about Insulation, Airtightness, Vapour Control, Heat recovery ventilation systems, Thermal bridge-free designs, Fresh Air, Healthy Buildings, Comfort, Innovative Building Products, and I will show you around some of the projects I am working on. You'll also find videos revealing the thinking, the process, and the ideas behind the making of Passive House buildings. If you are an Architect, Engineer or Builder hit that subscribe button!
⌚️Timestamps:
00:00 Intro
00:32 4 Control Layers
02:44 What is Airtightness?
06:05 Air vs Vapour
08:21 Design Principles
12:00 Don't Make This Mistake!

[@CMCraftsman]

I think you did a great job describing the process of achieving an airtight building and you had the control layers in order of importance but I believe the conversation about ventilation should have been at the beginning of the video to also keep the order of importance.

[@ScottyDMcom]

Some time ago I watched a video where the builder (Jake Bruton) and architect (Steve Baczek) framed, sheathed with Zip-R, taped, and sealed a new house, before they cut openings for the windows and all but one door. They wanted to understand how well Zip and basic sealing worked, before complicating things by adding windows and other penetrations. The result was an ACH-50 of 0.3, even before they added AeroBarrier. The video is here: kzbin.info/www/bejne/f57dmaaKgdqAntE Anton, you know, but others may not realize: It's common for professional builders to sheath over windows, then cut them after fact. Some might even tape the joints (except they don't waste tape over the windows because they're going to cut them out anyway). For this test Jake and Steve left the sheathing over the window openings, and even taped them, to run their test.

[@tealkerberus748]

Lungs also are airtight but well ventilated. They work a lot better if they don't have holes all over them leaking uncontrollably.

[@tealkerberus748]

I live in a climate where it's hot (tops of 43 C) in summer, and cool the rest of the year, rarely dropping below freezing but even now in the middle of Autumn it's in single digits every night. What I'm being taught in class is that we shouldn't use vapour-impermeable barriers anywhere in our walls, because whether that barrier is on the inside or the outside of the insulation envelope, at some time in the year it will lead to warm moist air infiltrating through the insulation and hitting the cold vapour barrier, creating condensation. So the recommendation is to have vapour-permeable materials all the way through, and focus on stopping air movement so the only way vapour moves through the wall is by permeating through each layer, not carried wholesale by an air leak. The question I'm grappling with, is how well will this actually work? If the amount of water vapour inside the wall is only what's been able to get there straight through the plasterboard, and then it passes through the insulation to the outside - say a cement sheet cladding - will it pass through that too, or will it get cold enough to condense and drip down inside the wall anyway?

[@AntonDobrevski]

Every building must have an airtight layer. By having the airtight layer, the only way for moisture to move through the wall is via vapour diffusion. In general, it is recommended to use intelligent airtightness systems which are vapour open in one direction ( e.g. allowing the structure to dry out to the inside) and vapour closed to the other side (e.g. not allowing the internal vapour get into the wall). An example is the pro clima intello plus system.

[@edvb9]

Hello Anton, it is a quite interesting video. You mention that the airtight layers should be on the more hot and humid side of the building. Could you share a examples for Tropical climates (Hot - humid) which (oppositely) have only two seasons, a wet season and a dry season? How this airtight layer can be effectively placed in such environment? One may think that the (constant) circulation of air could benefit for a better cooler indoor confort. I find tricky for the hot - humid climates.

[@AntonDobrevski]

For tropical climates, the airtight layer (which should also act as a vapor barrier) should be on the outside. And the construction should be able to dry out to the outside so moisture doesn’t build up inside the construction. Air circulation should be prevented inside the construction in any case, as it might result in mold and condensation and moreover, we can’t control the indoor humidity if warm, humid air is coming into the building uncontrolled. The air (and humidity) should be controlled via the ventilation system.

[@frankcrawford2998]

Unless you are assuming a combined air and vapour control layer, it is the vapour control layer that goes on the warm side of the wall assembly, and only 1 vapour control layer can be installed. The air control layer can go anywhere in the wall and you can have more then one air control layer. Otherwise excellent explanation and editing.

[@AntonDobrevski]

That's right! I indeed assume the air and vapour control layers are combined, as in most cases the vapuor control layer also fulfills the function of the airtight layer. Glad you enjoyed the video, thank you :)

[@Dancing_Alone_wRentals]

Hello,. I have an existing 1960s home. I want to put 1" foil faced Rmax sheets over the subsiding. Then cover it with Tyvek house wrap. Over that 3/4" battens to act as a rain screen. I'm not planning to tape the foil sheets. My concern is that they may create a vapor barrier. (I have these materials already, Rmax & Tyvek are made by the same company. I made a video on this question. tHanks for any ideas!

[@AntonDobrevski]

Hi! The Rmax sheets with the foil (e.g. Rmax Thermasheath) will indeed create a vapor barrier. Therefore, my recommendation is to tape the foil sheets. I would also advise you to reach out to the supplier and ask them if it's needed to combine the Rmax sheets (with the foil) with the Tyvek house wrap, as it could be an overkill.

[@Dancing_Alone_wRentals]

Hello @@AntonDobrevski tHank you for the reply. I had called DOW who makes both products and the one person I spoke to there said to not tape, and to wrap with Tyvek. They make the Tyvek product so that is to be expected. I imagine they were showing bias, but I can't find where people use these foil faced sheets. I bought them years ago, and intended to use them inside under the drywall. This was a different home, different project. These days, people speak about using insulation on the outside of the wall framing. tHanks again for your reply. PS. I'm tempted to try Tyvek over un tapped foil sheets. This application would be for the second story of this home...the first story is brick veneer so more than likely it would still be drafty. I've also come across information that says foam insulation doesn't work at very cold temperatures. In the end, I am looking for a way to improve, but not damage. (I have a video that shows some of the project)

[@ScottyDMcom]

@@Dancing_Alone_wRentals: How will you attach the Tyvek to the foam? Most builders attach house wrap directly to the sheathing with staples or cap nails, but you cannot nail or staple into foam. Plus, if you watch builder videos here on YT, you'll run across a few where the builder attached the house wrap, then when they come back the next day they discover sections have blown loose. Finally, that beautiful water/air/moisture-proof foil face will be filled with holes. For these reasons I feel the advice to _not_ tape your foam, but instead wrap it with Tyvek, is beyond stupid. Although expensive, I've found aluminum foil tape sticks like gangbusters to foil-faced polyiso. Most modern polyisocyanurate uses butane as a blowing agent, which gives the foam the unfortunate property of losing R-value as the temperature drops. It the early days they used one of the freons as the blowing agent: R-value was about 7.5 per inch and it remained high at low temperature. Then that got banned. They switched to a different freon, until that too was banned. A third blowing agent is available, but expensive, so most makers use butane, which is only R-6.5 per inch and isn't worth beans when it gets cold. Bummer, because polyiso is otherwise wonderful stuff: air, water, and vapor-tight, plus it's self extinguishing (there must not be a lot of butane in there). Extruded polystyrene is most of those things, except (unless chemically altered) it burns like napalm: melts, drips, and sticks to things as it burns. The blowing agent for any foamed polystyrene is air, and the R-value actually goes up a little as it gets colder. R for extruded is 5 per inch. Extruded polystyrene can be used in direct ground contact, such as under concrete or on the exterior of a basement wall. Expanded polystyrene (bead board) is similar, except R-value is lower (3.5 to 4 per inch) and it leaks, but only slowly. Saw specs on carbon-altered expanded polystyrene, which was vapor open and water closed, and R-value was 5 per inch. Another downside is that most solvent-based products dissolve polystyrene, but latex paint, glue, and caulk are fine. A few months ago Home Depot had 2-inch extruded polystyrene and 2-inch foil-faced polyiso for almost the same price (within about 40 cents for a 4 x 8 sheet).