Technical Study: Flat roof insulation and managing moisture

When detailing flat roofs, a key consideration is where to position the insulation in order to provide the required thermal performance and manage moisture.

Adding insulation to the construction of a flat roof is a requirement of the Building Regulations. It reduces unwanted heat losses from a building – but doing so can also increase the risk of unwanted moisture forming on or within the construction if not properly detailed. Sustained unwanted moisture can become a significant problem resulting in mould growth and eventual damage to the integrity of the building fabric.

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Flat roof insulation and managing moisture
Effective insulation and moisture management in flat roofs: safeguarding against moisture-related issues, mould growth, and unnecessary heat loss.

What causes unwanted moisture to form?


The amount of water vapour the air can hold changes depending on its temperature. Cold air holds less water vapour than warm air. When warm, moist air comes into contact with a cold surface, it cools down. The dew point is the temperature at which the air is no longer able to contain the moisture it holds. If the temperature of the water vapour in the air drops below the dew point it condenses into liquid water droplets (condensation) on the cold surface.
01 Dew Point Image

In heated buildings, cold surfaces can cause a condensation risk when they are located on the internal surface of a room or if they are located within the building fabric.

The two types of condensation that can occur in a flat roof build up are:

  1. Surface condensation – when condensation forms on the ceiling of the room
  2. Interstitial condensation – when condensation forms out of sight within the fabric of the roof

To avoid condensation, it is important to avoid creating cold surfaces that are exposed to water vapour. This can be achieved by controlling the indoor humidity and temperature through the ventilation, insulation, and moisture control strategies outlined below.

For further detailed recommendations for the management of moisture in flat roof construction, BS5250 should be referred to.

02 Risk of condensation in flat roof

The importance of limiting excess moisture in the air


To reduce the risk of either surface or interstitial condensation, it is important to first reduce the amount of warm, moist air within the building. This is especially true in rooms where moisture is often generated, such in bathrooms, utility rooms and kitchens.

To reduce the amount of moisture in the air, adequate ventilation should be provided through natural ventilation (such as via opening windows) or mechanical ventilation (such as via extract fans). Ideally, excess moisture from the air should be removed as close to the source as possible – before it is able to spread to other parts of the building.

If the amount of moist air inside the building is limited, there is less risk of it accumulating to the point where it can cause condensation. Any remaining condensation risk should then be designed out as far as possible through implementation of the following strategies.

How can surface condensation be avoided?


To avoid surface condensation, a sufficient and even thickness of thermal insulation should be applied continuously across the roof and thermal bridging should be minimised. This is to ensure that the temperature of the ceiling surface remains higher than the dew point temperature. To achieve this, the thickness of insulation specified to the roof should meet Building Regulations Approved Document Part L guidance.

At the time of writing, the limiting standards for flat roofs noted in Part L1 (for residential projects) are as follows:

  • New flat roof in a new building – U-Value 0.16 W/m2K
  • New flat roof in an existing building – U-Value 0.15 W/m2K
  • Existing flat roof in an existing building – U-Value to be improved to 0.16 W/m2K if it is currently below 0.35 W/m2K

Limiting standards are the very worst u-values that should be targeted. It should be noted that some flat roofs will need to achieve significantly better U-values than these to meet the requirements of project specific SAP calculations. This is because SAP calculations review the dwelling as a whole rather than each building element individually. SAP calculations are carried out to determine compliance with building regulations in all new dwellings and in some extensions / refurbishments.

As a guide, for new elements in residential projects we recommend working to the notional dwelling specification standards outlined in Part L1 as follows:

  • Flat roof notional dwelling specification – U-Value 0.11 W/m2K

SAP calculations should then be obtained to confirm this where required by Part L1.

How can interstitial condensation be avoided?


To avoid interstitial condensation, measures should be taken to minimise the amount of moisture entering the roof. This should be done by restricting the movement of water vapour from inside the building to inside the roof structure using a membrane (vapour control layer). The vapour control layer should be located on the warm side of the insulation, behind the internal surface finish. This is to ensure that the temperature at the membrane is always warmer than dew-point temperature.

In all projects, it is recommended to obtain build-up specific condensation calculations to assess the likelihood of condensation occurring within the construction.

There are several different calculation methods used to calculate moisture risk. The type of assessment suitable for the construction type should be chosen according to the guidance within BS 5250. There are also further requirements of the individual calculation methods outlined in BS EN ISO 13788 and BS EN 15026. Assessing moisture risk can be a complicated subject and it is recommended to contact the technical services department of the chosen insulation or membrane manufacturer for guidance.

The manufacturers can often provide build-up specific calculations free of charge. These should be based on the accurate properties of the materials, in terms of their thermal conductivity and vapour resistivity. BS 5250 suggests that independently certified values from manufacturers should always be used in the preparation of calculations where possible. Energy assessors can also often provide condensation risk calculations too. If a SAP calculation is being prepared for the project it is a good idea to ask the assessor whether they can provide this service.

If the condensation calculations highlight a risk, the design should be reconsidered and the calculations re-run until a comfortably viable solution is found.

The following sections outline the common ways of insulating flat roofs to avoid surface and interstitial condensation.

Providing insulation above the roof deck


The recommended way to add thermal performance to a flat roof construction is to insulate it from above. When insulation is placed above a roof deck in a heated building, surface condensation is prevented from forming on the inner surface of the ceiling by keeping it at a warm temperature. Where the waterproof covering is located above the insulation, a vapour control layer should be included on the warm side of the insulation (immediately above the roof deck) to avoid the risk of interstitial condensation forming between the insulation and the waterproof covering. The vapour control layer should have a vapour resistance at least equal to that of the waterproof covering.
03 Flat roof vapour control layer
04 flat roof verge detail
Some other considerations which are detailed further in BS5250 are:

  • any fixings through the insulation should be designed to avoid thermal bridging as this could allow cold spots to form within the construction
  • the moisture content of the structure should not be too high prior to the waterproof coating being applied as this could trap high levels of moisture within the construction
  • if the waterproof covering is located below the insulation (i.e an inverted roof), there would be no risk of interstitial condensation as the insulation keeps the waterproof covering warm.


Providing insulation below the roof deck (with ventilation)


This construction type is not recommended. In some very specific instances, it may be acceptable to improve the thermal performance of small existing roofs (less than 5m in span) by insulating them from below the roof deck. This may be an option in cases where an existing roof covering or external roof height is required to be retained (e.g for planning purposes) and the roof cannot be insulated from above.
When insulating below the roof deck, it will make the room warmer and the roof structure colder. To avoid interstitial condensation forming on the underside of the structure, it is crucial to provide sufficient cross ventilation between the new insulation and the existing roof deck above. Ventilation voids at least 50mm deep should be provided to every roof void and they should run from one side of the roof to the other. The area of ventilation provided should be in line with the guidance within BS5250. The ventilation is designed to expel any water vapour before it can condense on the structure and cause issues. A vapour control layer should also be provided on the warm side of the insulation to restrict moisture from entering the roof construction.
05 risk of condensation in flat roof
DL129 Cold roof retrofit detail 3d
07 Ventilated flat roof
Insulating a roof from below is not suitable for larger roofs and this approach should not be used in new constructions where it is possible to insulate from above.

Be wary of insulating above and below the roof deck


It is not recommended to locate part of the insulation above the roof deck and part of the insulation below the roof deck. It may seem tempting to adopt this hybrid approach to slim down the depth of the roof construction (by accommodating some of the insulation in between the roof joists). However, this can cause issues and would not be considered good practice.

Splitting the insulation in this way results in a thinner layer of insulation being located above the roof deck which may not provide the thermal performance required to keep the deck warm. Any water vapour entering the construction may condense on the underside of the cold roof structure as there would be no cross ventilation provided to expel it.

If this approach is used, BS5250 suggests that at least two thirds of the thermal performance of the roof should be provided above the deck and that calculations should be carried out to determine the risk of condensation.



In flat roof design, it is very important to consider the temperature of the roof structure and the amount of moisture entering the construction.

To control the temperature of the structure, it is recommended to locate all of the insulation above the roof deck.

Alternative solutions are generally not recommended as they can generate greater condensation risks. If it is necessary to locate some or all of the insulation below the roof deck, great caution should be taken and build-up specific condensation calculations should be obtained to prove that there is no risk of moisture being formed and sustained within the construction.

To control the amount of moisture entering the roof construction it is important to provide adequate ventilation inside the building and to incorporate vapour control layers on the warm side of the insulation where required.

Detailing Flat Roofs

Check out the Detail Library for a variety of construction details of flat roofs demonstrating different insulation options. Some of our recent details include flat roofs with exposed joists, flat roofs with modular green roof systems and flat roofs supported on i-joists.

Download the Details

Many of the details featured here can be downloaded from the Detail Library. For all our Flat roof details follow the link below. They are available to all members, or can be purchased individually.

DL231 – Modular Green Roof System with Masonry Wall Detail
DL233 – Flat roof with exposed joists verge detail
DL228 – I-Joist Flat Roof Detail to Masonry Wall

Recommended Reading

Recommended Reading

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DL Blog PDF - 16 Flat roof insulation and managing moisture


Written by Emma Thackstone. Emma is an architect. At the Detail Library, Emma helps the Detail Library with drawing new details and carrying out technical research.

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