How do I insulate a flat roof

How do I insulate a flat roof

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How do I insulate a flat roof?

You need to insulate a flat roof. But where? And how? The answer is especially important if the room below generates high levels of humidity like a bathroom.

Why insulate a flat roof?

Warm deck flat roof insulation is now the standard way to insulate a flat roof click to enlarge

Flat roofs, for example on extensions, bathrooms and dormer windows, are not the best of ideas. They lose loads of heat, and are prone to leaking, causing the materials to deteriorate. Unfortunately, too many dwellings suffer from this problem.

The best way to deal with this is to add insulation. But where? And how?

Firstly, it is always more economical to insulate a flat roof when you are replacing the roof covering anyway. Secondly, its important to understand how to keep it airtight and prevent condensation.

Cold flat roof insulation

Lots of traditional flat roofs are of the cold roof construction type. This means there is a vapour control layer in the ceiling and cross ventilation at the eaves or the top of the walls. This may help with condensation, but causes such a loss of heat that it has been banned in Scotland.

Warm flat roof insulation

Inverted warm deck flat roof insulation click to enlarge

Nowadays, flat roofs have the warm roof type.

Insulation can either be put below the weather proof membrane, which is called a warm roof deck construction, or above the waterproof membrane, which is called an inverted warm deck.

The only point of putting the insulation on top of the waterproof membrane is when you cant take it off, as when, for example, it is bitumen.

Vapour control layer

How do I insulate a flat roof

This goes on first, above the deck. This vapour check and airtight membrane should always be laid on the warm side of the insulation. It takes the form of reinforced bitumen sheeting or polyethylene laminated with metal foil to reflect the heat back in.

It is appropriate for ventilated or unventilated roofs (all flat roofs are unventilated). A product like pro clima INTELLOplus features humidity-variable diffusion resistance that is effective in most if not all climates. It protects thermal insulation where there is a possibility of moisture entry, e.g. through leaks, damp building materials or diffusion through adjoining structural surfaces.

This means that it is possible to use it in combination with all fibrous, organic insulating materials. You can choose this if you dont want to use mineral wool, polyurethane (PUR), expanded polystyrene (EPS) or rigid polyisocyanurate (PIR). It can be used with roofs covered in bitumen sheeting or metal sheet, or green roofs with diffusion resistant exterior cladding.

It should be lapped up around the insulation and affixed securely to prevent any air infiltration.

Flat roof insulation thickness

The chosen insulation laid on top of this should be able to take the weight of anything put above it on the roof without compression, or protected from compression by further sheeting. It should be of the maximum available thickness.

The typical U-value of an existing, uninsulated flat roof is 1.5W/m2K. The target should be to achieve a U-value of 0.25W/m2K or better. To achieve this, add 100 to 160mm of insulation above the structural deck (dependant upon insulant conductivity).

A typical installation would find 80-100mm of PUR insulation laid over a deck made of plywood (conforming to BS5268 Part 2 and to BS EN 636 Part 3, and of quality grades C-D and C-C) or Orientated Strand Board (manufactured to BS EN300 and be certified by BBA or WIMLAS), with a further 38mm of PIR insulation fitted between the roof joists, immediately below the deck. This would give a U-value of around 0.21W/m2K.


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