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What is the best way to insulate a loft conversion?

Posted by Trish O'Flynn on 20 February 2017 at 12:15 pm

A loft conversion can be a great way to increase your family’s living space without moving house. However, not all lofts are viable. You’ll need at least 2 metres clear head height and enough floor space to make a useable room.

During a loft conversion you will need to add internal insulation within the structure of the roof. The amount will depend on the type of insulation you use and the breathability of the structure. The Building Regulations set out general standards which your insulation must achieve, measured in U-values. U-values determine how effective a material is at stopping the transfer of heat from A to B: the lower the U-value, the slower the transfer of heat and the better it is at insulating.

Building Regulations in England require that the U-value of insulation on pitched or flat roofs, for example, be 0.18W/m2 or lower. So, to insulate a pitched roof, this can be achieved with any of the following materials installed between the rafters:

  • 250mm wood wool board
  • 240mm layer of fibre or wool insulation
  • 165mm of rigid foam (known as PIR) board insulation
  • 75mm rigid foam board with one layer of reflective foil insulation under the rafters
  • 65mm foil face rigid insulation with one multi-layered quilted product under the rafters

In this video Superhomer Maria Hawton-Mead talks about the eco-conversion of her loft which exceeds current regulations to achieve a high level of insulation.

Installing the insulation well is vital. That will mean filling any gaps between the insulation and the wooden framework you insert it within, plus taping the joints between boards you lay over that insulated framework.

The insulation should be installed under a vapour control layer (VCL) on the room side of the insulation. This will reduce the risk of condensation and mould and stop any moisture getting into the roof structure. Some insulation boards come with a foil VCL already attached.

Converting your loft to living space is a major building job that can affect the appearance, structural stability and fire safety of your property, so you should check:

  • whether you need planning permission, especially if you live in a Conservation Area
  • that the work complies with Building Regulations
  • that you have Party wall agreement from your neighbours if attached
  • if you have bats in your loft you have obtained a licence for the works to proceed

It is possible to improve the insulation of an existing loft conversion or dormer roof but you’ll need to do a cost/benefit analysis because of the inevitable disruption involved.

This blog summarises a longer article. You can read the longer article, sources and detailed guidance on the SuperHomes website.

If you find that you don't have the required 2 metres clear head height and enough floor space to make a useable room you might want to consider boarding your loft. To find out about an easier way to do this above the insulation, see our blog here.

With thanks to Tony Gwynne of Forest of Dean Council for his input and supporting guidance.

More information about Insulation on YouGen.

Find an insulation installer

Need help with any Jargon?

About the author: Trish is a web content writer with the National Energy Foundation.

If you have a question about anything in the above blog, please ask it in the comments section below.

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4 comments - read them below or add one


MarianneGreenComment left on: 16 March 2018 at 10:00 am

Thanks for this informative article. Next month, my brother and I will be converting our loft. We’ll take note of the things you discussed here so we can insulate our room properly. This is our first time doing this, so hopefully, everything goes well! But in case we can’t do it, maybe we’ll ask for some help from the loft conversion professionals that my brother knows. Either way, we really hope our project will proceed as smoothly as possible.

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Sandra Micheals

Sandra MichealsComment left on: 30 August 2017 at 6:48 am

Timber frame loft conversions are also good heat insulators. I think this is one of the best ways to keep your roof warm in winters. Also, it adds a classy look to your home. The main thing is they are eco-friendly and weather resistant. Making a warm roof is really important in winter seasons. 

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LoftZoneComment left on: 1 March 2017 at 1:05 pm

If you're just wanting to board your loft for storage whilst having your insulation in there, then have a look at this article!

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alfComment left on: 28 February 2017 at 5:52 pm

Mine has roof ridge vents, how does this work.

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