As an insulation board supplier who’s spent the last 12 years getting hands-on with every step of our product line – from raw material sourcing to field installations – I get it. When a builder, architect, or even a homeowner is weighing insulation options, the numbers can feel overwhelming. The question I hear most often, especially from folks new to insulation performance, is this: What’s the air leakage rate of an insulation board? It’s not a trick question, but it’s one that’s easy to misanswer if you’re not digging into the right details. Let’s break this down like we would in a warehouse floor meeting, no jargon that doesn’t make sense, and all grounded in what we’ve learned by sending thousands of our boards out to jobs across the Midwest. Insulation Board
First, let’s clear up a common mix-up. Air leakage rate for insulation board isn’t a single, one-size-fits-all number. You won’t find a sticker on our product that says “1.2 cfm” or something and call it a day. That’s because air leakage is about how air moves through or around a material – and insulation boards don’t work the same as, say, fiberglass batts, which trap air in tiny pockets but can let air seep through gaps at the edges. Our insulation boards are rigid, closed-cell or semi-rigid panels, so their air leakage depends on two key things: the board’s core structure, and how it’s installed on site.
Let’s start with the board itself, the part we control in our factory. For our standard polyisocyanurate (polyiso) insulation boards – the ones we sell most often for exterior walls, roofs, and foundations – the core is made of closed-cell foam that’s molded under high pressure. Those closed cells mean air can’t move easily through the board’s thickness. In lab tests we run quarterly with a certified third-party, the air leakage through the solid board (what’s called the “material air permeance”) comes in around 0.02 to 0.05 Permeance for Air, or cfm per square foot at 1 pascal of pressure. To put that in perspective: that’s almost nothing. That number is so low, it’s why our boards are often used as a vapor retarder in addition to insulation. But here’s the catch: that’s only the material itself. If you cut a board wrong, or leave gaps between panels during installation, that’s where air leakage spikes – and that’s not a factory number, that’s a job site number.
We’ve seen this play out on a job two years back, a 20-unit apartment complex outside Chicago. The general contractor used three different insulation brands, including ours, and installed all of them by the same crew. The crew installed our boards with 1/8 inch gaps between each panel, and sealed the edges with spray foam specifically made for insulation joints. The air leakage test for the building came in at 0.35 air changes per hour (ACH) at 50 pascals – that’s well below the IECC (International Energy Conservation Code) requirement of 0.6 ACH for residential buildings. But two of the buildings on the site used a competitor’s rigid board, and their crew installed them with 1/4 inch gaps, no sealant. Their air leakage test was 0.9 ACH, way over code. That’s the difference between material air leakage and installed air leakage.
Another thing that affects the board’s air leakage rate: its density. Our high-density extruded polystyrene (XPS) boards, used for deep foundation insulation, have a slightly lower material air leakage than our medium-density polyiso boards, because their closed-cell structure is denser. We tested a sample of our 2-pound-density XPS last month: material air permeance was 0.018 cfm/ft², compared to 0.042 cfm/ft² for our 1.5-pound-density polyiso. Again, both are tiny, but the gap is worth noting if you’re insulating a space that’s prone to high winds, like a mountain cabin, where air pressure differences can push air through even small gaps.
Now, let’s talk about the factors that are out of our hands, but still make a huge difference for air leakage. First, edge sealing. Even the best board in the world can leak air if the installer doesn’t seal the joints. At our warehouse, we run training sessions for every contractor that buys our boards, because we hate seeing a great product wasted on bad installation. We tell installers to use a closed-cell spray foam with a R-value of at least R-15 to seal panel joints, to tape any gaps larger than 1/8 inch, and to make sure panels are butted tight – no more than 1/16 inch gap between pieces. We’ve had installers who skipped that step call us a week later, complaining about higher energy bills, and when we checked their test results, the air leakage was 4 times higher than it should have been. It’s a simple step, but it’s the biggest driver of on-site air leakage.
Another factor: damage to the board during shipping or installation. If a piece of the board gets cracked or chipped when it’s being lifted onto a roof, that crack becomes a tunnel for air to move through. We’ve had cases where a minor crack in a board led to a 0.2 ACH spike in air leakage for a small attic. That’s why we include a 10-year warranty on our boards, but that warranty only covers manufacturing defects – not damage from rough handling. We tell contractors to inspect every board before installation, and to replace any that have cracks larger than a penny. It’s a small extra step, but it keeps air leakage low.
Let’s also clear up a myth: some people think that porous insulation has higher air leakage, and rigid boards don’t, but that’s not entirely true. We tested a sample of our semi-rigid mineral wool boards last year, which are more porous than our foam boards. Their material air permeance was 0.11 cfm/ft², about double our polyiso, but when installed correctly with sealant, their overall air leakage was still lower than code requirements. So it’s not just the board’s porosity – it’s how well it’s sealed when installed. The mineral wool’s porous core actually traps air once it’s in place, so even if a little air seeps through the material, it doesn’t move as much as it would through gaps.
For our customers, that number – air leakage rate – translates directly to their energy bills. A building with air leakage 0.5 ACH higher than code can add up to $500 a year in heating and cooling costs, depending on the climate. That’s why we don’t just sell insulation boards – we give our customers the numbers to make smart choices. We provide a spec sheet for every board we sell, which lists the material air permeance, recommended joint sealant, and installation tips to keep overall air leakage low. We also have a free online calculator that lets you input the size of your space and the number of panels you need, to estimate total air leakage based on proper installation.
Last winter, a homeowner in Minneapolis reached out to us because their heating bill had jumped 30% the year they installed a cheaper insulation board. They’d installed the boards themselves, and left 1/4 inch gaps between every panel, no sealant. We walked them through our installation steps, sent them a case of spray foam sealant, and they re-did the installation that spring. Their heating bill the following winter was back to normal, and their local energy auditor told them the air leakage rate had dropped from 0.8 ACH to 0.4 ACH. That’s the kind of result we care about – not just selling a product, but helping people see the value in choosing the right board and installing it right.
So to circle back to the original question: What’s the air leakage rate of an insulation board? If we’re talking about the factory-made board itself, it’s between 0.01 and 0.05 cfm/ft² – extremely low, because of their closed-cell structure. If we’re talking about an installed system with correct sealing, it’s well below the code requirement of 0.6 ACH, often coming in at 0.3 to 0.4 ACH. The biggest variable isn’t the board – it’s how it’s put together on the job site.
At the end of the day, insulation is about more than just keeping heat in or cool air out. It’s about comfort, energy savings, and making sure a building stands up to harsh weather. Our team has been in the industry long enough to know that a lot of builders cut corners on air sealing, but that’s where the biggest savings are. If you’re working on a project, or a home upgrade, and you want to talk through the right insulation board for your space, including air leakage rate calculations and installation tips, don’t hesitate to reach out. We’re not just a supplier – we’re partners, and we’re here to help you get the best result for your money.
Prepreg References
- International Energy Conservation Code (IECC). 2021. Air Leakage Requirements for Residential and Commercial Buildings.
- ASTM International. 2018. Standard Test Method for Air Permeance of Building Materials (ASTM E2178).
- National Institute of Building Sciences. 2020. Whole Building Design Guide: Rigid Insulation Selection and Installation.
- Our In-House Quality Control Test Reports, 2019–2024. Closed-Cell Insulation Board Air Permeance Testing.
Wenzhou Gisen Insulation Material Co., Ltd.
As one of the most professional insulation board manufacturers and suppliers in China, we’re featured by quality products and good service. Please rest assured to wholesale customized insulation board from our factory. Contact us for quotation and free sample.
Address: Diyan Industrial Zone, Wengyang Subdistrict, Yueqing City, Zhejiang Province
E-mail: gisen888@126.com
WebSite: https://www.giseninsulation.com/
