Partial conversion of loft, airflow advice needed

[DentySpen]

Hoping someone can advise, moved into the house approx 5years ago and found water ingress issues.
Finlock guttering removed(concrete top bricks and gutter/leading combo) replaced with soffit and guttering, roof tiles re used and old felt replaced with breathable membrane.
Breathable membrane has a tape on it that wasn’t removed and builder left a lot of mess on insulation, removed the tape and currently using foil tape to seal inside as it wasn’t stuck down from outside, going to remove insulation as it’s a mess and has some mould in places, I fitted extra 2x3beams bracketed at 90deg and boarded over so will remove and refit boards while removing insulation and hoovering up any rubbish.
Question is can I fit insulation boards between rafters allowing a 50mm gap between boards and membrane and make it a usable space(not to sleep in) is breathable membrane enough to let moisture out or will in need lap vents too, does the air gap need to be continuous over the ridge? No further damp/water ingress issues since roof was done, and I don’t want to cause any by doing it wrong, will also get at least one velux style window fitted so there’s some natural light. Would be nice to have extra room as it’s not a large house.

 

[Johnpaul Manning]

Good that the leak stopped after the Finlock work. That’s the right base to build on. Clearing the mouldy insulation and vacuuming the mess first is also sensible. Don’t trap that under new boards.

Between the rafters with a clear air path to a modern breathable membrane is a common approach, but a few details matter:

Gap and continuity. Aim for a continuous 50 mm gap (minimum often quoted 25 mm to a breathable membrane, 50 mm is a safer DIY target) from eaves toward the ridge on each slope. The gap needs a path for air to move, typically in at the eaves (soffit or eaves vents, or clear tray paths) and out at the ridge (ridge vent), or across if you have a clear route. If you block the eaves with insulation stuffed tight, or stop the gap halfway up, moisture can sit on the cold face of the PIR. So yes: treat it as a continuous ventilated path, not a sealed pocket behind each board.

Breathable membrane versus lap vents. A properly installed breathable underlay usually replaces the old need for bitumen felt plus extensive lap venting, provided the eaves to ridge (or equivalent) ventilation path above the insulation stays open. Lap or refurbishment vents are still useful where the membrane was poorly detailed, tape was left on, or insulation previously blocked the eaves, which sounds close to what you inherited. Since you’ve been sealing odd bits with foil tape from inside, double check you haven’t accidentally closed a needed air path at the eaves.

Warm side. For a room you actually use, put a vapour control layer on the warm face of the insulation (foil faced PIR with joints taped, or a separate polythene vapour control layer) before lining, and keep the loft hatch or door reasonably airtight so moist house air isn’t pouring into the cold loft void. A Velux helps light and can help ventilation if opened, but it doesn’t replace the roof void airflow above the insulation.

Usable loft note. Even “not for sleeping”, turning it into a proper room can trip Building Regs (structure, fire, insulation standards, means of escape). Worth a quick check with your local Building Control before you spend on lining, especially with a new window.

Short version: yes to PIR between rafters with a maintained ~50 mm ventilated gap to the membrane. Keep eaves and ridge paths clear rather than relying on the membrane alone. Vapour control layer on the room side. Sort mould and debris first.

More on loft and rafter insulation at https://www.insulation4less.co.uk