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Insulating a Roof: All You Need to Know

man applying insulation to roof
(Image credit: Shutterstock)

When it comes to insulating a roof, your choices will depend on:

  • your roof type (flat or pitched)
  • its structure (depth of the rafters)
  • your willingness to invest

In most cases, extra investment made in beating the Building Regulations on insulation is money well spent — it gives a lot back in returns.

Insulating a roof is perhaps the easiest of all the energy-saving home improvements. Mineral wool rolls are ubiquitous and cheap, meaning a whole roof on a standard home could be insulated to the recommended 270mm with a spend of little more than £200.

It’s also within the capability of most DIYers, although not the most pleasant of tasks. And the effect it will have on your energy bills depends on what you’re replacing and how big your roof is. It’s also much more important to do if you don’t have any loft insulation at all.

(MORE: What is the Green Homes Grant?)

Building Regulations for Insulating a Roof

Heat rises, and therefore the Building Regulations requirements for heat loss through roofs are more stringent than those for walls.

The limiting U value (the maximise U value which cannot be exceeded) required under current Building Regulations for the roof is 0.20W/m².

How to Insulate a Roof

The first decision is whether to insulate a pitched roof at ceiling or rafter level. The current trend is for a ‘warm roof’ where insulation is installed between the rafters, thereby keeping the roof timbers warm.

On new build or replacement pitched roofs, the ideal build up would be insulation fitted over and then between the rafters — giving a big boost for airtightness

On new build or replacement pitched roofs, the ideal build up would be insulation fitted over and then between the rafters — giving a big boost for airtightness (Image credit: Kingspan)

With a flat roof, this issue does not arise, but the U value constraint and the amount of insulation needed is just the same.

Achieving a U value of 0.20W/m² (meaning your roof will be better insulated than your walls) will need 120mm thickness of rigid foam or 200mm of mineral wool or natural insulation. Hitting amazing levels of 0.13W/m² will need 300mm of mineral wool or 180mm of rigid foam.

Insulation boards installed

For retrofit projects, Kingspan Kooltherm K14 Insulated Plasterboard goes ‘under’ the rafters. There will also be Kooltherm K7 Pitched Roof board between the rafters (Image credit: Kingspan)

With a warm roof the usual process is to split the insulation into two layers. If 180mm is to be installed then 100mm board might be introduced between the rafters (there has to be a minimum 25mm air gap between the insulation and the underside of the tiles or slates) with an 80mm board running across the rafters.

The proportions do not matter but this arrangement helps to eliminate any draughts caused by gaps between the insulation and the rafter.

Alternative Options For Insulating Old Roofs

One modern option for existing buildings is sprayed foam. There is a good deal of controversy around these systems. They are often advertised as a DIY option, but those who have done it suggest that it requires skill and experience to do well.

The other issue is more technical and concerns the effect of enclosing roof timbers in a material that can retain moisture. What cannot be argued against is that the sprayed foam sticks very well to whatever it touches — roofing felt, slates or tiles and rafters. If it becomes necessary to remove any of these, the sprayed foam will make it a difficult and expensive job to rectify and is generally considered bad news by many.