Installing loft insulation between sloping rafters

No sealant required use insulation tape too seal any discrepancies and ideally board over the rafters with ply board or plasterboard all the best

Remember to leave an air gap between your insulation and underfelt or you will create a major condensation issue leading to dampness.

Definitely leave an air gap of about 30 to 40mm. You can use pir board between and over the top and seal the sides and joints with foil tape👍

I'd say you're up against it, trying to install PIR board with any kind of integrity, without either stripping the plasterboard or stripping the roof. There'll undoubtedly be loads of debris and rubbish down those 'channels', as you put it, on an 'old style house' too. Not sure that the other replies, really understand that you're intending to try and push it down from within the loft space???? So you can't possibly use any kind of foil tape to seal any joints or discrepancies.

✅ Answers to Your Questions: 1) Can 100mm Kingspan boards be pushed down a 112” channel if cut tight to fit? Yes, but with caution. Rigid foam like Kingspan can be cut and slid into place, but a tight fit along a 9–10 ft channel may get stuck or snag on nails/screws/uneven timber. Instead of one long 112" piece, cut the boards into smaller sections, e.g. 600–800mm (2–2.5 ft) each. Then push/slide them down one at a time. Bevel the edges slightly to allow a snug fit. Use wooden battens or thin sticks to push and guide them from the loft. ⚠️ Watch for: Any roofing nails sticking through — these can puncture the foil backing. Leave a ventilation gap of 50mm between Kingspan and the roof membrane/felt (especially if it’s bitumen or non-breathable). 2) Do you need to seal between the Kingspan and rafters for airtightness? Yes – ideally. Even small air gaps around rigid board edges can allow warm, moist air to bypass the insulation and cause condensation. Use low-expansion PU foam (like Soudal, Insta-Stik, or similar) or foil tape around the edges. Also seal joints between boards with foil tape. ➡️ Airtightness = better performance and prevents condensation issues. 3) Has anyone done this successfully? Yes, many have insulated rafter channels this way — it's a common DIY approach when access is limited. Here’s what usually works well: Cut 100mm Kingspan boards slightly narrower than rafter spacing (by ~3–5mm) Slide boards down in segments from loft access Maintain 50mm ventilation gap above Seal gaps with foam or foil tape Optionally, add a vapour control layer (VCL) over the inside of the rafters before reboarding the sloped ceiling