More Building Science
I was recently part of a webinar where we discussed the basics of the four control layers: water, air, vapor, and thermal. These basic principles of building science were introduced to mainstream construction industry by Joseph Lstiburek a couple decades ago. One of the most influential articles on the topic can be found at Building Science Corporation’s website: BSI-001: The Perfect Wall.
In the webinar I was part of, the participants got into a discussion on whether the industry needs to add a control layer to the list. The organizer brought up the idea of adding acoustics as one of the control layers. Acoustics are important, addressing mostly occupant comfort. A second idea was also discussed: fire. This control layer would move away from occupant comfort and address property and personal risk. Let’s look at both these ideas, then you can make your opinion known in the comments.
The Acoustic Control Layer
Acoustics in building science is the study of sound within or around a structure. It could refer to an exterior noise affecting the building and its occupants, such as a busy highway or a home close to a noisy factory. It could also be an interior noise, sound that transmits between interior spaces, like the teenager who decides to take up a percussion instrument and practices in their bedroom.
The importance of this “control layer” is subjective. A retired couple living in a rural location may not be worried about acoustics, whereas a young family living close to a busy airport might have a different opinion.
Some of the best ways to achieve acoustic comfort in construction are simply designing and constructing well-built houses. Because sound is a vibration or pulse in air…
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12 Comments
I think you're spot on with these being important variables, though, as always, there’s some nuance.
Fire control absolutely deserves a spot on the standard list, although the performance of an assembly is a combination of the traditional rated control layer as well as the ignition resistance and fuel load on the exterior of that layer. E.g. an open gap wood rainscreen siding will provide a lot more stress an underlying control layer than if that same assembly had something that either did not provide fuel (fiber cement) or was more ignition resistant (minimally or unvented lap siding).
Exterior will be the primary concern for most situations, but interior also comes into play with garages, Energy Storage Systems, and non SFH occupancies. Even for areas not currently prone to wildfire, climate change and shifting home insurance risk tolerance could make fire resistance essential to maintain coverage at a reasonable price.
Acoustics, on the other hand, can be critical but is problematic to frame as a “control layer.” Like you noted, airtightness helps a lot, but beyond that, sound mitigation isn’t just about layering, it's about the interaction of mass, damping, and decoupling across the whole assembly. The “triple leaf effect” is a perfect example of how counterintuitive it can be https://www.soundproofingcompany.com/soundproofing_101/triple-leaf-effect
Pests and radon are absolutely real issues too, but like acoustics they also stretch the concept of a control layer with the needs for deflection for termites and mechanical removal for radon. At a certain point, we might need a broader framework, something like “control elements” to capture elements that go beyond a "layer".
bcade,
It's an interesting exercise expanding the layers, but I agree, the four are foundational, and others - even though important - are perhaps best categorized in another way.
Hi Randy,
We have been discussing protective layers vs control layers for the carpenters courses we run at studiohpdc.org
Some protective layers being insect control, rodent control,
Fire and acoustic could be described as protective layers that may need to be managed
I would add that designing in HEPA filtration into ERV systems could be another protective barrier as it protects against the microparticles in wildfire smoke.
A high performance building is designed and built to meet the challenges of the site
So a site-responsive design would incorporate the control and protective layers needed to match what nature presents
I am with you that we need to develop new language around this
Nick,
Protective Layers sounds like a useful way to distinguish them.
I'm with Malcolm "categorized another way".
Fire needs to be treated first and foremost as an entire building/landscape system. A wall "control layer" isn't all that helpful in conceptualizing, or even implementing, the solution.
Something else we should be striving for is to simplify assemblies. The more bits and pieces and layers and part there are the more opportunities there are to screw up in terms of installation, and they typically become more costly as well. Instead of continually adding stuff to solve a new problem, we need to step back occasionally and ask, "Is there something we can remove to solve the problem?" We can't always simplify, but when we can, let's try.
I have batted around the 'Wind Control Layer' for a few years now as the Europeans often speak of this. This is good to consider if one is using an air permeable insulation that may be exposed to the exterior or an interior insulation that may suffer from convective airflow. Each could degrade the efficacy of the insulation and adding a 'Wind Control Layer' would address this.
Another is the 'Structural Control Layer' This is often important to consider when tracing out the primary 4 control layers. Order of install operations often have to thread the needle between an air barrier and structure, for instance. Thinking about structure in tandem with the other control layers can be critical.
Lastly, 'Water Shedding Layer' is one that I use occasionally. There is the 'Primary Bulk Water Control Layer' which is the ultimate line of defense for water ingress, which is typically the WRB or roof underlayment. I believe this should be the one that gets traced onto the plans. But the siding and roofing to the lions share of water protection and it may be useful to use the term 'water shedding layer' as a heuristic. The term 'Primary' can be confusing as it implies the 'first' layer that water sees. Maybe using the term 'Water Control Layer of Significance' is more descriptive, but ultimately its a mouthful.
Josh,
On walls, our code makes the distinction between the "First and Second Planes of Protection". The first referring to what you call the Water Shedding Layer, and the second the WRB and flashing at the back of a rain-screen gap.
Between the regional differences in terms, and building scientists thinking up new ways to describe building assemblies, pretty soon we may see a new profession of building translators pop up.
I'm a fan of listing out as many elements as possible when one is undertaking any sort of design work and seeing if all elements are being accounted for (sometimes there are compromises). You can then treat it like a tic list when reviewing the design or the final product. With this mentality, I would certainly go ahead and list out everything beyond the fundamental 4 control layers, including fire, sound, pests, UV/radiation degradation, wind, gravity, seismic, soil stability, light (good and bad), unwanted guests, spirits, etc. (internet filter to keep out the ridiculousness of this world?)
This serves a different purpose than talking about the 4 fundamental control layers, which as the name implies, focus on actual "layers" and on continuity (though certain elements listed above certainly benefit from continuity). But controlling/protecting/managing (pick your verb) many elements, including the fundamental 4, really goes beyond simple layers, as others have mentioned in regards to fire management. There are always multiple elements at play.
Even the fundamental 4 are somewhat ill served by discussing them as layers, since many times they are not one material or one plane. Conversely, one material can serve multiple purposes. We all know proper thermal control transcends the 'type' of insulation. The same is true of bulk water management (roof overhangs, proper overlaps, surrounding grade pitched away, etc.). It's never about just one isolated or monolithic thing. Is the final design accounting for ALL the elements that are thrown at it?
The fundamental 4 layers are still a useful exercise and teaching tool. But in many ways I think that is its primary purpose: a teaching tool. There is no reason it is absolute or isolated from the other elements buildings must manage.
The final task is to prioritize the list and make sure that when you inevitably need to make compromises, it is not at the top of the list. I'm willing to let a few spirits sneak in every now and then (and let's face it, I'm not actually sure how to keep them out).
We often work in zero lot line areas and fire control is required and obviously important.
I've been thinking a lot about that this year, since I followed Lloyd Alter's sage advice to read about the Grenfell fire in 2017. There's a pretty readable book about it ("Show Me the Bodies: How We Let Grenfell Happen" by Peter Apps, here's Alter's review: https://lloydalter.substack.com/p/everyone-in-the-building-industry). The final Phase 2 report of the government inquest was also released a few months ago and is astonishing reading as well, though I believe the seven sections run well over 1200 pages I couldn't stop: https://webarchive.nationalarchives.gov.uk/ukgwa/20250319155650/https://www.grenfelltowerinquiry.org.uk/homepage
It's not all that challenging to meet the IRC fire requirements, but like everyone, watching recent huge fires really has me thinking about what it would take to build truly durable buildings in an environment where firestorms appear to be a real possibility.
I recently attended a two day seminar on the 2024 WUIC - Wildland Urban Interface Code. One of the presenters was instrumental in compiling the California Wildfire Rebuilding Guide - hot off the press in April of this year. Obviously as a result of the Los Angeles area wildfires. A wealth of information - even for new builds in wildfire-prone areas.
https://usgbc-ca.org/wp-content/uploads/2025/04/2025-04-01-USGBC-x-Arup-Rebuilding-V1_compressed3.pdf
We use all of these techniques to build our walls and it isn't cheap to train guys (& gals) to manage all of the control layers especially at openings and wall to roof connections @ second floor ceiling. It seems to me that insulation doubles as a soundproofing agent with other options to deaden sound by adding pads to the interior side of exterior walls but more insulation, ie a wider wall cavity would undoubtedly double as a sound proofing method. I suppose replacing cellulose insulation with mineral wool insulation would be a step in the right direction as a fire safety measure although it won't prevent a home from burning but rather slow the process with some ability to prevent fire spread if the fire is mild. The fireproofing issue has to be split into two categories (indoor and outdoor) and my focus would be on outdoor with recent California wildfires burning billions of dollars worth of residential homes last summer. Aside from stone/brick cladding I know that there's also a soffit vent that prevents embers from entering the attic space. I'm not sure there's much more that can be done to protect a home from fire by adding anything to an exterior wall section. My solution for that would be to mitigate the area around the outside of the home to create a buffer between flammable material like trees and dry bushes/grass and possibly adding some kind of emergency sprinkler system that blankets the house with water in the event of a threat. I don't think there's anything you could build into a wall layering system that would effectively reduce the risk of fire damage besides mineral wool insulation and a stone or brick surface.
The wall layering system we use for building science is complicated enough with managing air and vapor movement specific to the local climate and heating/cooling needs. Here in Michigan we drive heat from inside to outside in cooler months which transports vapor until it meets the cold air somewhere outside of the vapor control layer behind the dew point of the wall. If we don't put exterior insulation outside of our zip sheathing, we risk flashing moisture in the wall. But with added insulation allowing the heat to pass through the wall, that heat vapor flashes outside the zip sheathing posing little to no threat to the integrity of the framed wall and interior insulation. When we seek to make our framed walls thicker and bypass exterior insulation, I am curious as to how and where the moisture release from interior heat coming into contact with exterior cold will happen. Theoretically, it will flash off moisture before it gets to the zip sheathing but the wall thickness should allow for the wall to dry out over time since most cold weather events are temporary. We are currently building our first 10" thick staggered stud wall with 2x4 at 24" o.c on the outside of the 10" wall plates and 2x4 @ 24" oc on the interior edge of the 10" wall plate. If anyone has any comments about that strategy in Michigan climate, Im open to hear options.
Thanks,
Mike
Adaptive Building Solutions
Mike,
You may find these articles useful:
https://www.greenbuildingadvisor.com/article/double-stud-wall-with-reservoir-cladding
https://www.greenbuildingadvisor.com/article/the-mythical-threat-to-double-stud-walls
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