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How to insulate your loft conversion - Q&A

Posted by Tim Pullen on 4 October 2010 at 9:35 am

Q: The house I bought recently has the loft converted into extra rooms. Should I pay a company to take down the plasterboards on the wall and ceilings and install insulation such as Kingspan boards, as there is only 10cm of insulation according to the blueprints?

A: To answer this question directly, we need to know the type of insulation already installed. If it is Kingspan (generically PUR insulation) or similar then 10cm is probably enough to make removing the plasterboards an expensive option. Especially if there is more insulation in the floor of the attic rooms. If the installed insulation is mineral wool then a bit more is needed.

If it is possible, the easiest and cheapest option would be to fix insulation boards to the existing plasterboards. That will reduce the size of the rooms a little bit, but will avoid removing the existing plasterboards. Kingspan, Celotex etc offer plasterboards with insulation bonded to them which can be simply glued or screwed into place.

To answer the broader question of what to do with loft conversions generally is more tricky as there are quite a few issues to consider. The first is what the loft rooms will be used for and therefore if they are part of the main house. By this I mean if the rooms are bedrooms, say, they will need the same level of insulation as the rest of the house, but they don’t need to be insulated from the rest of the house. Heat can be allowed to travel up from rooms below.

If the rooms are to be used for storage or more occasional use (a library in one case I dealt with) then they need to be insulated but can be kept at a much lower temperature than the rest of the house and therefore need to be insulated from the house.

The question then is whether to insulate at floor/ceiling level (which separates the loft rooms from the house) or at the rafters. To remain in line with building regs you need a U-value of no more than 0.16W/m2, and to get that you will need at least 270mm of mineral wool or 130mm of PUR insulation. That can be split between floor joist and rafters if the rooms are not for habitation but must be at the rafters if they are for habitation (bedrooms etc). Obviously more insulation will reduce the U-value further, reduce fuel bills and reduce CO2 emissions.

The problem is often that there is not enough physical room to get the insulation in. There are insulation materials, generally multifoils, that advertise very high insulative properties in a relatively thin material – 40mm thick multifoil gives the equivalent of 200mm of mineral wool. The accepted wisdom is that if things seem too good to be true then they generally are. When multifoils were subjected to the standard UK testing procedure it was found that a 40mm multifoil gave insulation equivalent to 40mm of mineral wool.

The other problem with loft conversions is draughts, or more accurately draught proofing, and that is where multifoils have a role to play. They are very good at draught proofing. Installing a multifoil to the underside of the rafters, and counter-battening to take 100m of PUR boards solves all the problems. It also maintains the air gap between insulation and roof covering (tiles, slates, etc.), which is essential to remove moisture and prevent rot getting into the roof timbers.

Like a lot of things, insulating for a loft conversion is not rocket science but it does need a bit of thought. And as always with insulating, it is difficult to do too much.

Photo by Stevie Rocco

More information about loft insulation from YouGen

How much loft insulation is enough?

Insulation is king

Wise up when buying solar, double glazing and insulation

Loft insulation information page

Find an insulation installer

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

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


peterjohnsonComment left on: 19 November 2019 at 9:26 am

thanks for the informative article. In my home, the loft conversion work has been done by Strictly Loft Conversions . The cost is fully depend on the type of loft we opt for. There are different version and the cost vary acccording to it. 

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RobsComment left on: 21 September 2012 at 11:10 am

We had a loft conversion done by our heating bills only went up marginally. So in answer to S.Grahams question, no, if your loft is properly insulated then you should not notice too much increase.

The fact that our old loft had very poor insulation must have played a factor in this because we would have expected a much larger increase!

Hope this helps

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petercmfjohnComment left on: 16 September 2011 at 6:40 pm

Great article, loft conversion cost depends on the materials and labour charge. Make sure you are getting warranty for the parts and labour which is very important.

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S.GrahamComment left on: 21 December 2010 at 11:43 am

How cost effective are loft conversions? I have been given a reasonable quote from but I am not sure whether this will greatly increase my heating bulls in the future. I have a 2 bed room terrace. 

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