Why the roof build-up decides how a loft conversion feels

A converted loft is really a room wrapped inside a roof. Everything above your head is a thin sandwich of tiles, membrane, timber and insulation, and that sandwich has to do three jobs at once: keep heat in, keep water out and let any moisture that does get in escape again. Get it right and the room is warm in December and quiet in a hailstorm. Get it wrong and you get cold patches on the slope, damp stains on the plasterboard and a nagging smell of old timber.

The most common mistake is treating insulation as a single product rather than a system. Thickness matters, but so does the air gap above it, the seal around every pipe and cable, and the way the whole build-up handles water vapour. Here is how to get each part right.

Leave an air gap above the insulation

Between the top of your insulation and the underside of the roof covering there needs to be a clear, ventilated void. Where the roof has a traditional bitumen felt or boarding directly beneath the tiles, that void is normally 50mm minimum, running uninterrupted from the eaves to the ridge. If the roof has a modern breathable membrane and counter-battens, the gap can often be reduced to around 25mm, but confirm this with your building control officer before you design the rafters around it.

Why it matters: without that gap, warm moist air migrating through the build-up reaches the cold underside of the felt, condenses, and then drips back into your insulation. A wet mineral wool batt performs far worse than a dry one, and the timbers below it stay damp for months. Ventilation also needs a route — eaves vents at the bottom, ridge or tile vents at the top — so air can move through the void by convection. A sealed cavity with no inlet is not a ventilated cavity.

And please don't compress insulation to make it fit. A 100mm rafter packed with 100mm of mineral wool and then squashed flat by a board loses a surprising amount of its thermal value; the fibres need loft and trapped air to work.

Seal the small holes that undo the big work

Air leakage often costs more than conduction. A 10mm gap around a soil pipe can carry away more heat than a square metre of slightly under-thick insulation, because air moves heat far faster than insulation resists it.

• Pipes and soil stacks passing through the ceiling or the sloping head: seal with a flexible airtight sealant or a purpose-made grommet that still allows for movement.
• Cables: run them through sealed glands, and seal both sides where they pass through the insulation layer.
• Downlights: use sealed, fire-rated units and keep insulation clear unless the fitting is explicitly rated for contact. Covering a non-rated downlight is a fire risk, not just a thermal one.
• Junctions: the wall plate, party wall, chimney breast and eaves are where airtightness usually fails first. Tape and seal these before the plasterboard goes on.
• The loft hatch or door: fit a compression seal and insulate the back of the hatch itself.

Do all of this before boarding. Afterwards you'll be working through a 100mm hole with a torch in your teeth.

Choosing insulation for between and under the rafters

For a converted roof, UK building regulations commonly look for a U-value of around 0.18 W/m²K, which usually means a combination of insulation between the rafters and a continuous layer beneath them to break the cold bridges.

• Rigid PIR boards give the best performance per millimetre, which matters when rafter depth is limited. They only work if cut tight and taped — a 5mm gap between boards is both a cold bridge and an air leak.
• Mineral wool is cheaper and easier to work around hips, valleys and awkward rafters. It needs more depth, and it must not be squashed into place.
• Hybrid build-ups — flexible batts between the rafters with a layer of PIR beneath — are often the easiest way to hit the target without losing headroom.

Whatever you choose, keep a services void below the insulation for cables and pipework. Cables buried in insulation can overheat because they can no longer dissipate heat, and the wiring regulations require them to be derated or separated.

Breathable boards for historic roofs and exposed timbers

If you're converting an older roof — a cottage, a Victorian terrace, a listed building — and you want the rafters left on show, foil-faced PIR pressed against old timber is the wrong choice. It acts as a vapour barrier on the cold side, so any moisture in the timber has nowhere to go. Over a few winters you get condensation, staining and eventually rot.

Instead, look at vapour-open insulation: wood fibre boards, hemp or sheep's wool batts, held in place with a breathable membrane and finished with lime plaster or clay board. These allow the timber to dry inwards and keep the structure healthy. They are thicker for the same U-value, so work out your rafter depth early in the design.

Where headroom or listed status makes that impossible, the honest answer is sometimes a cold roof: insulate at ceiling level, keep the roof space ventilated and leave the timbers visible. You lose the usable floor in the eaves, but the roof structure stays sound.

Moisture, ventilation and the final checks

A roof can only cope with so much vapour. A shower room or kitchen in a loft conversion needs mechanical extract ducted to the outside, not into the roof void — moisture that ends up in the roof space will find the coldest surface it can and condense there. A continuous vapour control layer on the warm side, lapped and taped at every joint, does the rest.

Before the boards go on, photograph every cable run and pipe so you know what's behind the plaster in five years' time. Then run through this list:

• Is the air gap present and continuous from eaves to ridge?
• Is the insulation continuous at junctions, with no gaps and no compressed sections?
• Is every pipe, cable and downlight penetration sealed?
• Is the vapour control layer on the warm side, lapped and taped?
• Is there a services void below the insulation, with cables kept out of it?
• Are extract fans ducted to the outside?

A good loft conversion is mostly detail work that nobody sees once the plaster is painted. If you're unsure, ask building control to inspect the roof build-up before boarding starts. It is far cheaper than taking it apart again.