More Product Guide
Continuous exterior insulation (CEI) has a lot of potential benefits on walls and roofs. When done right CEI can increase thermal performance by mitigating thermal bridging and increasing durability by reducing the risk of condensation and moisture accumulation on the sheathing. Paired with a ventilated rainscreen on walls, many high-performance building professionals find CEI to be a path to an efficient and durable assembly. There are many materials and a few common assemblies that designers and builders use for CEI, from strapping rigid insulation to the wall with furring strips, to variations on Larsen trusses.
There’s a new product available from InSoFast, the X-Bracket System, that is meant to give designers and builders an off the shelf solution for a CEI assembly. The X-Bracket is a plastic standoff that supports furring strips for siding attachment and creates space between the structural wall (or roof) sheathing and the siding (or the nailing deck on a roof). The space can be filled with insulation and can include a drainage plane and ventilation gap, depending on the details. The company hopes the product will be an efficient way for designers and builders to detail continuous insulation and simplify some CEI details in both new construction and retrofit applications. However, before exploring how it works, let’s review the benefits of CEI.
Why continuous insulation?
Common insulation methods, such as filling wall cavities between studs with fiberglass or foam, fail to address thermal bridging—heat transfer through structural elements like studs and fasteners. According to the U.S. Department of Energy, this unwanted heat transfer can reduce the effectiveness of cavity insulation by up to 25% in wood-frame construction. Continuous insulation mitigates thermal bridging by creating an uninterrupted thermal barrier. This improves energy efficiency. To reduce the impact of thermal bridging, continuous insulation can be…
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
This article is only available to GBA Prime Members
Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.
Start Free TrialAlready a member? Log in
25 Comments
This article talks about eliminating the thermal bridging from nails connecting furring strips to the framing when using rigid foam as continuous insulation. But what about the area of spray foam lost due to the volume of each x bracket? It's not obvious to me which would cause the greatest energy penalty overall.
I will ask the manufacturer and get back to you. I'll also invite them to subscribe and answer the questions personally.
From Ed Scherrer with InSoFast:
"When properly installed, the X-Bracket is actually a net positive “penalty”. The material of the X-Bracket is 352 times less conductive than steel so the X-Brackets have less thermal bridging than a 16g finish nail. We also recommend mounding up spray foam around the bracket up to the bottom of the furring strip thereby fully encapsulating the X-Bracket.
The recommended 2x4 furring strips and insulation offer a thermal insulating buffer for the X-Brackets. "
Ed says he will supply some more info later.
Josh: the volume of spray foam that is displaced by the bracket is equivalent to 1/2" over the area of the bracket. The thickness of the spray foam is typically 3/4 to 1" less than the bracket height to allow a gap for cross flow ventilation behind the furring strips. around each bracket extra foam is applied so that it mounded up and is in contact with the backside of the furring strip providing 3/4 to 1" more foam around each bracket. This creates a net positive performance around each bracket!
This is an interesting product concept.
I remember a few years back, after seeing the high-dollar extruded aluminum rainscreen bracket systems for commercial cement board cladding, and thinking about people putting loooong screws through ComfortBoard.
I thought "Why doesn't someone make an engineered H or I or Z profile from pultruded fiberglass, cut to short lengths to act like a small Maine Deck Bracket for furring strips"?
The X-Bracket is like an injection molded version of that concept. I somehow feel validated.
Do you mean something like these:
- Cascadia Clip: https://www.cascadiawindows.com/products/cascadia-clip
- SMARTci GreenGirt: https://greengirt.com/smartci-building-enclosures/
If so, they exist! I've only seen them used in commercial construction where the weight of the cladding makes the long screws method common in residential construction less attractive.
Fernando,
Kudos for the very thorough and comprehensive write-up.
It should also be simple to detail both walls and roofs so you could use batts rather than foam by moving the ventilation gap outboard of the strapping.
Thank you Malcolm.
The article doesn't directly mention it, but the installation docs state that the brackets can be used with horizontal furring for vertical siding.
Check. Thanks for pointing this out.
I very much appreciate the discussion about fire safety with this product. I imagine that rockwool insulation plus a fire resistant siding would create a decent fire resistant exterior, but what about the plastic/composite brackets themselves? If fire gets through to the brackets, when they melt, the whole exoskeleton could collapse, and then it wouldn’t matter what kind of insulation would be on the inside, because it will be in a pile on the ground. I’m interested to know the fire resistance of the brackets compared to other materials, such as dimensional lumber (which is obviously combustible too, but less combustible than most plastics).
Using spray foam would be basically like wrapping the house in several inches of gasoline, with a completely unbroken chimney effect from the foundation to the top of the roofline.
Foam insulations are treated with fire retardants that significantly reduce the likelihood of ignition and slow down the spread of flames if they do catch fire; however, if exposed to a direct heat source like an open flame, they can still burn. The analogy with gasoline is farfetched.
I was borrowing the "gasoline" analogy from an analysis by The Atlantic https://www.theatlantic.com/science/archive/2025/01/los-angeles-fire-smoke-plastic-toxic/681318/, which was looking at the combustibility of common home furnishings like couches, rugs, curtains, etc. The article was written as the Los Angeles fires were (are) still burning. The gist of the article, for those who don't have full access, is that everything plastic burns soon and quickly. Plastic is in fact made from petroleum, which is the same source substance from which gasoline is made. In that sense it isn't so much an analogy as it is a recognition of the flammability of all petroleum-based products, including spray foam.
I'm aware that flame retardants are added, and that you can coat spray foam in additional flame retardants after installation. I'm also aware that it doesn't take a lot to break through those safeguards, and once that happens, it's just petroleum going up in flames.
I do actually like the idea behind this product, quite a lot. In fact, I may end up using it, though not with foam.
Ed Scherrer from InSoFast replies as follows:
"The wood in the wall assembly will combust before the X-Bracket is a concern. The windows and other openings in the wall have lower resistance to fire."
He mentioned adding more information later today, but I don't know if it will be about insulation or fire.
You May want to watch this video of spray foam in a fire
https://www.youtube.com/watch?v=2ZBa1ijiHwY&pp=ygUYYnVybmluZyBzcHJheSBmb2FtIGpvbmVz
It's encouraging to see those results, though I'm sure it will depend on the brand, type of foam, and other factors. Maybe I should ask a broader question: if you were going to use the X-brackets (which I think are a brilliant idea, by the way) in the most fire-resistant assembly, what would that look like?
For the highest level of fire safety, I would wrap the building in mineral wool or for marginally less safety, borate-treated wood fiber or cellulose.
The rigid forms of mineral wool and wood fiber are expensive so I would use the brackets with batts or blown insulation. Steven Baczek has a great outrigger system (like a Larson truss but with manufactured trusses) where he has loose insulation simply dumped in the exterior cavities. That would work with these brackets, wood battens and a high-quality WRB. Or you could dense-pack it.
I am no expert in the formulation of spray foam and welcome any responses if I am wrong. Canned spray foam is very flammable for the first few months after it is installed because it is formulated with propane or butane, it becomes more fire retardant as it ages.
The biggest fire concern with 2-part spray foam is when it is being installed. When the A and B components are mixed it forms an exothermic reaction that gives off a lot of heat. The fire danger comes with an inexperienced installer that applies the foam many times thicker than the typical 2-3” so that heat builds up to more than 800 degrees F where it spontaneously combusts.
https://www.greenbuildingadvisor.com/article/another-spray-foam-fire
Don't you want more than 2 or 3 inches with the X-Brackets though? Especially with the larger ones that the website says are coming later this year? Or is it your recommendation to use something other than foam for the thicker walls?
Paul,
That's the recommendation for most spray foam installations, no matter how much you want in total, or where they are going. Each pass needs to be limited in depth, and left to cool.
I suspect it is in part why some spray foam installers make false claims about the R-value, which allows them to use one pass.
I think the most fire-resistant assembly would be a thick monolithic layer of rockwool that blankets your walls and roof. Install the X-Brackets on the walls and roof then fill with 4# blown-in dense packed rockwool behind a class A fire rated WRB. Fasten steel hat channel or fire-retardant wood furring to the X-Brackets so you form a vented rainscreen along with ember proof vents. The furring supplies solid attachment for noncombustible siding or fire rated plywood for the roof sheathing.
Fire Resistant Building is a system not just picking one product, it must be a system. Even an all-concrete home will burn up from the inside if a window breaks or a roof ignites. https://passivehousenetwork.org/phn-presents/
Thermoset plastic has a high-temperature tolerance, from 204°C to 260°C (400°F to 500°F). The "set" in thermoset implies it does not melt but may char or burn at very high temperatures. The ignition temperature of framing lumber is generally around 250°C (482°F). I will pass your question onto the manufactur
Josh: the volume of spray foam that is displaced by the bracket is equivalent to 1/2" over the area of the bracket. The thickness of the spray foam is typically 3/4 to 1" less than the bracket height to allow a gap for cross flow ventilation behind the furring strips. around each bracket extra foam is applied so that it mounded up and is in contact with the backside of the furring strip providing 3/4 to 1" more foam around each bracket. This creates a net positive performance around each bracket!
Log in or become a member to post a comment.
Sign up Log in