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A GreenBuildingAdvisor response advises installing foam board first, then using canned foam to seal openings in concrete-block cores beneath a mud sill or at a rim joist. EPS beads can be poured into the cores for some insulation, but the responder says he has not used them and warns they can cling to nearby surfaces from static. The source does not establish a required method for every wall or foundation.
A response on GreenBuildingAdvisor recommends installing foam board over openings in concrete-block cores first, then using canned foam to close gaps and limit air movement. The answer also describes EPS beads as a possible way to add insulation inside the block cores, while warning that the small beads can cling to surrounding surfaces during installation.
The response, posted under the question “Cores under mudsil, foam board over or fill,” appears to address insulation near a mud sill and rim joist. Its author, identified as Bill, says he usually applies canned foam after the foam board is in place. In his account, doing the sealing afterward is easier because the board covers part of the opening. The stated aim is to seal the remaining hole enough to prevent air from circulating.
Bill says the advice assumes the work is being done above the top of the block while insulating a rim joist. He also mentions EPS beads, small expanded-polystyrene pieces that can be poured into hollow block cores to provide “a bit of insulation inside the wall.” He says he has not used that approach himself, so the suggestion is presented as an option rather than a method he has tested.
The response’s practical distinction is between sealing gaps and adding material inside the cores. Foam board covers part of the opening; canned foam is used around the remaining gaps to reduce air circulation. EPS beads can fill space within the block, but the source does not say they substitute for air sealing or specify how much insulation they provide. It gives no product details, measurements, code guidance or comparison of thermal performance.
Sealing the Block-Core Openings
The answer matters to homeowners and contractors trying to insulate around a rim joist without leaving an air path through openings in hollow concrete block. Its sequence—board first, seal afterward—offers a straightforward response to the specific question of whether to cover or fill the cores. The author’s reason is practical: once the board is in place, some of the opening is already covered, making the remaining gaps easier to foam.
It also separates two tasks that can be confused. Air sealing addresses air movement through gaps, while EPS beads are described as a way to add some insulation inside the cores. The response does not establish that either step alone meets a particular energy target or building requirement. Readers should treat it as one contributor’s field practice, not a universal specification for all foundations, block walls or climates.
canned foam for insulation sealing
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The Question Behind the Advice
The original GreenBuildingAdvisor question asks whether to fill block cores beneath a mud sill or install foam board over them. The answer focuses on a particular setup: insulating a rim joist from above the top of the block. That qualification matters because access, wall assembly and the location of the openings may differ in other construction details.
Bill describes using canned foam after the board is fitted, rather than trying to seal the opening before the board is installed. He separately raises EPS beads as a possible core fill. The source does not include a follow-up from the questioner, an independent assessment, or details about the building’s design, moisture conditions or local requirements. It therefore supports a description of the advice, but not a broad claim that this sequence is right for every project.
“I usually just use some canned foam after the foam board to make sure the hole is sealed up enough that air won’t circulate.”
— Bill, responding on GreenBuildingAdvisor
foam board insulation for concrete block
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Limits of the Available Guidance
The source does not identify the precise wall assembly beyond its reference to a rim joist above the block, or provide dimensions, climate, product specifications or installation details. It does not say whether the foam board is continuous, how the board is attached, or which canned-foam product is suitable for the specific materials and conditions.
It is also unclear how much insulation EPS beads would add in the described cores, whether they would be appropriate in this particular assembly, or how the finished work should be evaluated for moisture and air-control performance. The response offers no code or permit advice and does not claim that its method meets any particular standard. Those points require project-specific information that is absent from the post.
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Project Details Will Guide the Choice
The cited response does not announce a follow-up or report a completed installation. Anyone applying the advice would first need to confirm that their situation matches the one Bill describes: a rim-joist insulation project with access above the top of the block. The appropriate materials and sequence can depend on the existing assembly and local requirements, details not supplied in the source.
For a project-specific answer, homeowners can consult a qualified building professional or ask the original forum for guidance that includes photographs, measurements and information about the wall and foundation. Until those details are available, the source supports only a limited takeaway: the responder prefers installing the foam board before sealing remaining openings with canned foam, while treating EPS beads as an optional core fill with a static-cling handling drawback.
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Key Questions
What does the GreenBuildingAdvisor response recommend?
Bill says he usually installs foam board first and then uses canned foam to seal the remaining opening and limit air circulation. He describes this as his practice for a rim-joist setup above the top of the block.
Should the block cores be filled with EPS beads?
The response says EPS beads can be poured into block cores to add some insulation, but Bill says he has not used them. It does not establish that beads are needed or suitable for every wall assembly.
Why apply canned foam after installing the board?
Bill says the board covers part of the hole, making it easier to seal the remaining gap afterward. He describes the goal as preventing air from circulating through the opening.
What drawback does the response mention about EPS beads?
Bill warns that the small beads can build up static charge and stick to nearby surfaces while being handled, making them messy to work with.
Does this advice apply to every block foundation?
No. The post discusses a particular rim-joist situation and supplies no general specification, code guidance or project details. The right approach for another assembly remains uncertain based on this source alone.
Source: rss
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