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Roof insulation in Bromley and West Wickham So it stops condensation, not just heat

Insulation in the wrong place causes more damage than no insulation at all. On flat roofs especially, where the insulation goes decides whether the timber stays dry. We put it where it works.

Roof insulation: Insulation put where it stops condensation, including cold flat roofs converted to warm roofs.

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Rotten boarding and firrings stripped out of a cold flat roof over a bathroom in Croydon, with insulation showing between the joists
Cliffe Road, South Croydon CR2. March 2025

Roof insulation prices

Roof insulationPrices checked 22 October 2026

Priced after a visit, because every job is differentFree quoteno deposit
  • Cold flat roofs converted to warm roofs
  • Insulation above the deck, so no ventilation needed
  • Fixes condensation at the cause
Call us for a free quote Every price, and the estimator
Where roof insulation sits on a house

Where roof insulation goes, and why it matters

In a cold roof, the insulation sits below the deck and the void above it must be ventilated. In a warm roof, the insulation sits on top of the deck, with a vapour control layer under it, and the roof needs no ventilation at all.

What a customer said

"We had Raise the Roofs carry out a wide range of work on our home, including insulating and boarding our loft"

Joe C., five stars on Google, February 2026. Read every review

"The requirement will be met by the ventilation of cold deck roofs, i.e. those roofs where the moisture from the building can permeate the insulation."

HM Government, Approved Document C, paragraph 6.11

The regulations put it the opposite way to most people's instinct: the colder roof is the one that has to breathe. Leave the ventilation out and moisture from the house has nowhere to go but onto the timber.

Illustration of a loft with thick mineral wool insulation laid between and across the ceiling joists
Loft insulation laid between the ceiling joists and over them.

Cold flat roof condensation: the damage you can't see

Get the cold version wrong and the damage happens out of sight. On a Croydon bathroom roof, Steve found the boarding and firrings rotted through from below. His summary: "A 'cold roof' construction with no through ventilation, shower room below, rising hot air has destroyed the boarding and furring pieces."

"However, there is a high risk of harmful interstitial and surface condensation forming on the underside of the deck and dripping onto the upper surface of the insulation."

Martin Fisher, IKO PLC, Technical guidance on BS 6229, flat roof design

That is exactly what Steve found in Bromley: water forming under the deck, out of sight, until the boarding was destroyed. It's why we convert failing cold flat roofs to warm roofs rather than re-covering them and hoping.

Illustration of the underside of a flat roof deck stained with mould and rot, with condensation droplets on the boards
Inside a cold flat roof: the deck rots from below, out of sight.

What better roof insulation does for a room

Sally's 1930s West Wickham extension overheated every summer because its flat roof had almost no insulation. We laid 120 mm insulation board under a new three-layer felt, and the room has stayed noticeably cooler since. The same insulation holds the heat in through the winter.

St Mark's Church in Bromley had the same problem on a bigger scale: they called us about a small leak, but the hall was freezing in winter and too hot in summer. We laid 120 mm rigid insulation board over the old roof under a new three-layer felt, about 120 m² in two weeks. The hall is now warmer in winter and cooler in summer, with what Steve describes as significantly lower heating bills. The St Mark's job.

Illustration of a bright kitchen extension with a flat roof and a glazed roof lantern
A bright, warm room under a properly built flat roof.

Warm roof or cold roof? It's where most flat roof rot starts.

The problem Steve sees more than any other is poor ventilation on flat roofs. A cold roof keeps its insulation below the deck, so it needs air moving through the void, whatever room is underneath. Leave that out and the warm, damp air from the house rises, meets the cold underside of the boards, and condenses there. Over a shower room it happens faster, but it happens over a bedroom too.

The fix is usually a warm roof: the insulation goes on top of the deck, so the timber stays warm and the roof needs no ventilation at all.

"Technical knowledge is very important to ensure a long lasting installation is delivered."

Steve Ball, on a Croydon bathroom roof
Rotten boarding and firrings stripped out of a cold flat roof over a bathroom in Croydon, with insulation showing between the joists
What that looks like underneath. A cold roof over a Croydon bathroom, stripped back: the boarding and firrings had rotted from below, not from rain. How we fixed it

Cold roof

Needs air moving through it. Easy to get wrong.
Cold flat roof in section: waterproofing on the deck, a ventilated void, insulation between the joists, a ceiling, and warm moist air rising from the room below to condense under the deck Waterproofing 1 Deck boards, cold underneath 2 Void: air must move through it 3 Insulation between the joists 4 Ceiling 5 Warm, moist air from a shower 6 condenses under the deck, every day
  1. Waterproofing
  2. Deck boards, cold underneath
  3. Void: air must move through it
  4. Insulation between the joists
  5. Ceiling
  6. Warm, moist air from a shower

Warm roof

No void, no vents. What we fit.
Warm flat roof in section: waterproofing on rigid insulation board, a vapour control layer, then the deck on the joists, with the ceiling below Waterproofing 1 Rigid insulation board, on top 2 Vapour control layer 3 Deck boards, kept warm 4 Ceiling 5
  1. Waterproofing
  2. Rigid insulation board, on top
  3. Vapour control layer
  4. Deck boards, kept warm
  5. Ceiling
The timber stays above dew point, so nothing condenses on it.

Roof insulation jobs we have done

Roof insulation questions

We do roof insulation in West Wickham, Beckenham, Bromley, Chislehurst, Orpington and Petts Wood, Dulwich, Forest Hill, Sydenham, and Westerham.

What is the difference between a warm roof and a cold roof?

A cold roof has insulation below the deck and needs ventilating. A warm roof has insulation above the deck and needs no ventilation, so condensation has nowhere cold to form.

Can you convert my cold flat roof to a warm roof?

Yes. It is the usual fix for a cold roof that is rotting from below. The old deck comes off, a new one goes on with rigid insulation above it, then a new covering.

Will roof insulation make my extension cooler in summer?

It can make a big difference. Sally's flat-roofed West Wickham extension has stayed noticeably cooler since we laid 120 mm of insulation board under a new felt roof.

How much does roof insulation cost?

It's priced on survey, because it depends on the roof and how it's built. The quote visit is free, and we visit within 2 days.

How much insulation goes on a warm flat roof?

On Sally's West Wickham extension we laid 120 mm of rigid insulation board under the new felt. The right thickness depends on the roof, and we'll say what we propose on the quote.

Do you insulate lofts too?

Yes. On one house we insulated and boarded the loft alongside guttering, chimney and flat roof work.

Sources

  1. Approved Document C (2013), paragraph 6.11
  2. IKO PLC: Technical guidance on BS 6229 (Martin Fisher)

Written by , Director, and editorially reviewed by John Ireland, Director. Published , last reviewed .

Editorial integrity: read our editorial standards.

Damp, hot or cold under a flat roof?

Free quotes, and we visit within 2 days. No deposit. If it only needs a repair, we'll tell you.

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