Envelope and protection · Insulation
Cellular Glass Insulation Board
Explains what an all-glass foamed board is, why it is met in loaded and repeatedly wetted positions, and why bedding, cutting and edge treatment are the subjects rather than fitting tolerance.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What cellular glass is
Cellular glass is made by grinding glass to a powder, mixing it with a cellulating agent and heating the mixture until it foams and the cells close. What comes out of the process is a block of glass with sealed voids through it, which is then cut into boards. There is no binder, no facing paper and no fibre in the body of the material: the cell walls are glass and nothing else, which is what makes it different from every fibrous and polymeric insulation in the library.
It behaves quite unlike a quilt or a plastic foam in the hand. It is rigid, it does not spring back, it is brittle at arrises, and it chips and dusts when cut. Boards are commonly cut with a saw and bedded in a continuous adhesive or in hot bitumen so the board and its bed act together, rather than being pushed into friction between joists or studs. Some grades arrive with a bitumen or mineral facing already applied and others are treated on site as part of the build-up.
The reason it is met at all is that it is the inorganic answer to positions where insulation is loaded, wetted, or both: beneath a slab or a plant base, in an inverted roof where the insulation sits above the waterproofing and is repeatedly soaked, at a column or wall base, or in a below-ground build-up. How the material actually behaves in any one of those positions is a matter for the manufacturer's documentation for the exact grade and for the professional who has designed the build-up, not something a material description can settle.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Foamed glass insulation board
- Cellular glass board
- Foam glass board
- Cellular glass slab
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Insulation beneath a ground-bearing or structural slab where the layer is loaded by what is built above it.
- Inverted and protected-membrane roofs where the insulation sits above the waterproofing and is repeatedly wetted.
- Below-ground and perimeter positions on retaining and basement walls as part of a designed build-up.
- Load-transferring positions such as column bases, plant plinths and bearing details where a professional has specified an inorganic board.
- Roof and terrace build-ups where the insulation is bedded into a bituminous system rather than mechanically fixed.
- Tank, vessel and service-related insulation within buildings where an inorganic rigid board has been specified.
Consideration
Where it is commonly considered
- Where the insulation layer is loaded and a professional has ruled out products that would creep under sustained load.
- Where the position is expected to be wetted repeatedly in service and the designer wants an inorganic body.
- Where an insulation layer has to be bedded continuously into a bituminous or adhesive system rather than fixed through.
- Where the build-up has been designed as a whole and the bedding, facings and upstands have been drawn.
- Where the installer is familiar with cutting, bedding and dressing this material rather than fitting quilts and foams.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- In any loaded position, since what the layer can carry is an engineering judgement about the whole detail and not a property of the board on its own.
- Where the board would be expected to flex, to be fitted into an irregular gap, or to take a point fixing.
- Where the bed, adhesive or bitumen it is to be laid into has not been named by the manufacturer for this substrate.
- Where cutting dust and handling cannot be controlled, particularly in occupied or finished spaces.
- Where the moisture behaviour of the whole build-up has not been assessed by a qualified professional.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The closed glass cells are the reason the material is raised for wetted positions, but cut faces, chipped arrises and the bed beneath the board are where moisture behaviour is actually decided. How the specific grade behaves, and how cut edges are treated, belongs to the manufacturer's documentation and to the professional who designed the build-up.
- Thermal behaviour
- Thermal behaviour is discussed here only as a topic. No value, class or comparison is stated, and any figure belongs to the manufacturer's data for the exact grade and to a calculation prepared for the actual construction.
- Installation dependency
- Boards are cut rather than compressed to fit, and are commonly bedded in a continuous adhesive or hot bitumen. Setting out, joint tightness and the treatment of the bed are the installation subjects; there is no friction fit to take up an inaccurate cut.
- Substrate dependency
- Because the board is bedded rather than fixed through, the flatness, cleanliness and dryness of what it is bedded onto carries directly into the result. Substrate preparation is commonly set out by the manufacturer for each bedding method.
- Repairability
- A chipped or fractured board is not repaired by filling. Damaged boards are generally cut out and replaced before the layer above goes on, which is why handling and storage on site matter more than they do for a compressible quilt.
- Fire-related questions
- No fire characteristic is stated or implied here. Fire behaviour belongs to the complete assembly including facings, adhesives and bedding materials, and is a matter for the manufacturer's documentation, a qualified professional and the relevant authority.
- Documentation
- Ask for the data sheet for the exact grade, the manufacturer's guidance on bedding and on treating cut edges, and written confirmation that the adhesive or bitumen proposed is one the manufacturer accepts with this board and this substrate.
Exposure
Exposure and environmental conditions
- Whether the position is expected to be wetted in normal service, and whether that has been designed for rather than assumed.
- Whether the layer sits above or below the waterproofing, since that changes what the insulation is asked to do.
- What loading the layer will see once the construction above it is complete, and who has assessed it.
- Whether groundwater, standing water or a high water table affects the position, and whether that has been investigated.
- How the material and its bed will be protected from weather and from site traffic between delivery and covering.
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The flatness, cleanliness and dryness of the surface the boards are bedded onto, established before delivery rather than discovered on the day.
- The bedding method named by the manufacturer, whether a cold adhesive, a hot bituminous bed or a dry-laid arrangement.
- What sits directly above the boards, since a loaded layer commonly needs a spreading or protection layer designed with it.
- How the layer is terminated at upstands, thresholds, penetrations and perimeters, which is usually detailed rather than improvised.
- Which waterproofing, vapour-control or drainage layers the build-up carries, where they sit and who has designed that sequence.
Appearance
Appearance and finish considerations
- The material is concealed in service in almost every position, so nothing about the finished appearance of the building depends on the board itself.
- Where the board is visible during construction its dark, open-cut face shows every chip, which makes handling damage easy to see and worth photographing before cover.
- Boards are cut to a line rather than squeezed into an opening, so the setting-out is visible in the quality of the joints.
- Facings supplied on some grades change what the surface looks like and what can be applied over it, so the grade and its facing are worth confirming.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- How the board and its bed are expected to behave together over time in this particular position.
- What would happen at a chipped or fractured arris that was covered without being noticed.
- How movement in the structure around the layer has been allowed for, given that the board itself does not accommodate movement.
- Whether anything will later be fixed through the layer, and how that has been designed rather than left to the day.
- How water arriving at the layer is intended to leave it, and where it goes.
Upkeep
Maintenance and repair considerations
- What would have to be removed to reach the layer if a defect appeared above or below it.
- Whether the same grade and facing would still be obtainable if part of the layer had to be replaced.
- How a local area would be cut out and re-bedded so that the bed remains continuous.
- What record exists of the grade, the facing and the bedding material actually used, since none of it is visible afterwards.
- Who would be asked to assess a suspected problem in a buried or loaded insulation layer.
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- The manufacturer's cutting, handling and dust-control guidance for the grade supplied.
- The bedding material named by the manufacturer for this substrate, and the conditions under which it can be used.
- Protection of boards on site, since the material chips and boards are commonly damaged before they are ever laid.
- Setting out and joint tightness, because there is no compression in the board to take up an inaccurate cut.
- Sequencing with the waterproofing and with whatever is placed over the layer, so that boards are not left exposed or trafficked.
- Weather conditions during bedding, particularly where a hot bituminous bed is used.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The technical data sheet for the exact grade and facing supplied.
- Manufacturer guidance on bedding methods, acceptable adhesives and the treatment of cut edges.
- Written confirmation that the proposed bedding material and substrate are ones the manufacturer accepts.
- Handling, cutting and dust-control guidance for site use.
- Any system documentation where the board forms part of a wider roofing or waterproofing arrangement.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is it about this position that makes an inorganic, bedded board the answer rather than a conventional foam or quilt?
- Who has assessed the loading on this layer, and what does that assessment depend on?
- How is the layer bedded, and has the manufacturer confirmed that bed for this substrate in writing?
- Where does the insulation sit relative to the waterproofing, and why that way round?
- How are cut edges, upstands, penetrations and perimeters detailed, and who has drawn them?
- What protects the boards between delivery and cover, and who is responsible for that on site?
- How would a problem in this layer be found and reached once the construction above it is complete?
- Which parts of this build-up need confirming against local requirements, and who is doing that?
Blind spots
Commonly overlooked points
- It is bedded, not fitted, so the bed is as much a part of the specification as the board and is far easier to get wrong.
- The material is brittle at arrises, and a large part of the damage it suffers happens on site before it is ever laid.
- Nothing about it is compressible, so an inaccurate cut stays an open joint rather than closing up under pressure.
- Cutting produces a glass dust that is a genuine handling consideration in occupied or finished spaces.
- Because the layer is normally buried or loaded, a record of the grade, facing and bedding used is unusually valuable later.
What this page does not do
- This entry states no thermal, moisture or fire value and makes no performance claim of any kind.
- Whether any insulation layer can carry load in a given construction is an engineering judgement about the whole detail, and nothing here should be read as such a judgement.
- Build Design Hub does not test, approve, certify, specify or recommend any cellular glass product, grade, facing or bedding method.
Component forms
Building components commonly formed from this material
Elements this material is commonly used to make, named by the position they occupy in an assembly rather than by what they are bought as. This states common practice only — it is not a statement that this material suits any of these components in a particular building.
Related entries
Alternatives to consider
Entries that may be considered instead of this one for a similar job. Not a statement that they are interchangeable, or equal in any respect.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Insulation Materials
Reference entries for thermal insulation materials — mineral and glass wool, rigid foam boards, natural-fibre insulations, blown and sprayed products and reflective systems.
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