Do I need to tape PIR insulation board joints?

Ask on any building forum whether you need to tape the joints on Celotex, Kingspan or other foil-faced PIR boards and you'll get several different answers. That's because the honest answer is: it depends what job the boards are doing.

The short answer

• If the boards are acting as your vapour control layer (VCL) or air barrier, every joint, edge and penetration needs sealing. Foil tape is the usual way to do it.
• If there's a separate VCL, such as a polythene sheet or a foil-backed plasterboard on the warm side, taping the board joints is good practice for airtightness and thermal performance but isn't what keeps the vapour out.

Either way, tape doesn't make up for poor fitting. Gaps between boards are cold bridges, and tape over a gap just hides it.

Why the foil facing matters

The foil on PIR boards has a very high resistance to water vapour. Butt the boards tight and seal the joints, and the face of the insulation becomes a continuous vapour control layer. Leave the joints open, and warm, moist air from the room can get through them into the cold parts of the wall or roof, where it condenses.

Insulation manufacturers accept taped board joints as one way of forming the air and vapour barrier, alongside a separate membrane or sealed plasterboard. Always check the installation guidance for your specific board and build-up, as it varies by product.

Warm side only

A vapour control layer belongs on the warm side of the insulation, the side facing the heated room. Putting a second vapour-tight layer on the cold side can trap moisture between the two. If your build-up has insulation in more than one layer, or you're not sure which side is which, get a condensation risk analysis (to BS 5250) done before you start. Your insulation supplier or building control can help.

How to tape PIR board joints properly

1. Fit the boards tight. Cut accurately and butt the boards together. Fill any gaps over a few millimetres with low-expansion PU foam, let it cure, and trim it flush.
2. Clean the foil. Brush off dust and wipe away any grease. Tape won't stick to a dusty surface.
3. Work in the warm. Foil tape bonds best in a dry room above about 5°C. Keep the roll indoors in cold weather.
4. Centre the tape over the joint. Lay it on without stretching it, so equal amounts land on each board.
5. Press it down hard. Rub it down firmly with a plastic squeegee or a seam roller to push out air and get full contact.
6. Seal the edges and penetrations too. Board edges at walls, floors and ceilings, and every pipe and cable that passes through, need sealing for the layer to be continuous.
7. Repair any damage. Patch tears and dents in the foil facing with tape as well.

What width of tape?

50mm tape covers a straight butt joint between boards. 100mm is better where you're spanning boards fixed over studs or rafters, covering wider joints, sealing board edges into corners, or patching larger areas of damaged foil. It lands on both sides in one pass.

Foil tape, not duct tape

Cloth duct tape is no good here. Its rubber adhesive dries out and lets go, often within months. Use an aluminium foil tape with an acrylic adhesive, which is made to stay put through heat, cold and condensation.

Supraseal® Premium HVAC Aluminium Foil Tape 100mm × 50m is a reinforced, high-tack foil tape made for insulation board joints and ductwork. It's arriving soon: use the Enquire now button to register your interest. There's also a 50mm version for standard joints. Read more in our foil tape guide.