Wall Insulation Board

What is Wall Insulation Board?

Wall insulation board is a high-performance thermal shield that transforms a wall assembly into a continuous envelope, significantly reducing heat loss. 

Depending on the core material, it can also offer enhanced moisture resistance, contribute to the rigidity of the wall system, and provide varying levels of fire resistance.

The walls of a building are a significant source of heat transfer, accounting for a substantial portion of heat loss in uninsulated or poorly insulated properties. This heat loss leads to higher heating bills and also contributes to uncomfortable drafts, cold spots, and an increased carbon footprint. 

Wall insulation boards are engineered to combat these issues by creating a thermal barrier that slows down the rate at which heat escapes during colder months and prevents excessive heat gain during warmer periods.

Applications

Wall insulation boards are generally installed:

• Between Leaves - E.g. partial-fill phenolic slabs clipped into a masonry cavity.
• Inside The Frame - A foil-faced PIR layer installed within or against the frame, often serving as the primary vapour control layer on the warm side. (If boards are friction‑fitted between studs they must be cut perfectly tight; more commonly, a rigid board is placed across the studs to eliminate gaps.)
• Outside - An external wall insulation (EWI) system where EPS or mineral-wool boards are adhesively bonded and dowel-fixed to masonry then rendered. (Phenolic or PIR boards may be used only in render systems that achieve at least Euroclass B and are not permitted above 18m on UK buildings that require A2‑s1,d0 or better.)

All three rely on tight-butted joints taped or mastic-sealed to create an air barrier.

Cavity Wall Insulation

This is one of the most frequent uses. Cavity walls have two layers of masonry with a gap (cavity) between them. Cavity insulation boards are placed in this cavity to stop heat from moving across it.

• Installation: Boards are usually cut to fit the cavity and held in place with ties, making sure there's a continuous thermal barrier. Some boards are made specifically for partial-fill or full-fill cavities.
• Benefits: Very effective at preventing heat loss, keeping the cavity dry, and improving the overall U-value of the wall.
• Products: Kingspan Kooltherm K106, Kingspan Kooltherm K108, Celotex CW4000.

Internal Wall Insulation (Dry Lining)

For older properties with solid walls or where insulating the outside isn't possible, internal wall insulation is a good choice. Insulation boards are fixed to the inside surface of the external walls, often finished with plasterboard.

• Installation: Boards can be mechanically fixed, glued, or both. It’s vital to deal with potential cold spots around openings and to include a continuous vapour control layer if needed.
• Benefits: Improves thermal performance without changing the outside appearance, and can be less disruptive than external insulation for some properties.
• Things to Consider: Slightly reduces internal room space; requires careful detailing to prevent condensation.
• Products: Many PIR boards like Celotex GA4000 and XR4000, as well as Kingspan Kooltherm K112, can be used for internal wall applications, especially when a thin, high-performance board is needed.

External Wall Insulation (EWI)

EWI involves attaching insulation boards to the outside of a building’s walls, which are then covered with render or cladding. This method creates a continuous thermal envelope, eliminating cold bridges.

• Installation: Boards are mechanically fixed and/or glued to the existing wall, followed by a basecoat, a reinforcing mesh, and a final render or decorative finish. For rendered EWI, thermally‑broken telescopic tube fasteners with steel pins are preferred over simple screws and penny washers.
• Benefits: Excellent thermal performance, protects the existing building structure, improves the look, and can solve problems like dampness in older buildings.
• Things to Consider: Requires external scaffolding, can change a building’s outside appearance, and might need planning permission for historic buildings or those in conservation areas.
• Products: While not specifically marketed for EWI, some denser PIR and phenolic boards can be used where the facade system is tested to the appropriate fire class; stone‑wool (Euroclass A1) and EPS remain the default boards for most render‑based EWI systems. XPS is generally reserved for below‑ground or behind ventilated cladding due to its Euroclass F rating.

Timber Frame and Steel Frame Insulation

Insulation boards are widely used within the framework of timber and steel-framed buildings.

• Installation: Boards are cut to fit snugly between studs, ensuring a tight fit to minimise air leaks. They can be used as a sheathing board or as infill insulation.
• Benefits: Delivers excellent thermal performance for lightweight frame constructions.
• Products: Kingspan Kooltherm K112 are made specifically for framing uses.

Suitable Fixings

Mechanical fixing is critical: for EWI, use telescopic plastic sleeves with steel or composite pins; for internal linings or sheathing layers, stainless or coated‑steel screws with plastic or steel washers spread the load and prevent pull‑through. 

Fire‑stops or cavity barriers must be introduced at floor lines when foams are used in ventilated facades.

On the interior, insulated plasterboard - 12.5mm gypsum bonded to 25-70 mm PIR or phenolic - offer a neat retrofit solution but need services chased or a thin service batten zone to avoid penetrating the vapour layer.