Insulating Your Loft

How to Install Loft Insulation With Minimal Loss of Storage Space

Many people want to improve the insulation in their loft but can't afford to lose their boarded loft floor - often an important storage area.

There are ways to improve the insulation in your loft and keep your storage space and this article discusses some of the options.

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One of the most commonly faced difficulties is how to add insulating material to a loft space when the floor has already been boarded and the loft is used for storage.

When you are standing in the loft, the depth of the void formed between the plasterboard of the ceiling below and the top of the ceiling joists is usually about 100mm (4"). Given that current building regulations in the UK require 270mm of insulating glass wool, if a home owner wishes to retain a boarded loft and does not want to use a "spray-on" insulation product up between the roof rafters then there is a choice to be made about which direction to take. The main choices are between:

1. Building the joists up with additional "packing joists" to create a 270mm void which can then be filled with insulating material and then re-boarded over.

2. Using a solid insulating board cut to fit in the void that exists between the joists without raising the level of the loft floor.

3. Insulating between the roof rafters with either insulating board, spray on insulation products or insulation blanket products.

This article focuses on the first two potential methods.

Pros and cons of the first two potential methods:

Building the floor level up: Pros

1. If you run the additional "packing joists" across the existing joists at 90 degrees, you are able to run wool insulation over the top of the original ceiling joists) which reduces potential "thermal bridging".

2. The materials are, in total, probably less expensive that using insulating board

3. You retain most of your loft storage area.

Building the floor level up: Cons

1. It's a big DIY challenge

2. The wood required for the "packing joists" may be heavy and difficult to manoeuvre into position

3. You lose some height in the loft because of the floor being raised

4. Insulating with insulation board between the existing joists: Pros

5. Your floor stays the same height and therefore no loft height is lost.

6. You retain your loft storage space

7. There are no heavy timbers to work with

Insulating with insulation board between the existing joists: Cons

1. Its time consuming and fiddly to cut insulation board to fit between existing joists (which are invariably, even within one loft, are different widths apart). The cutting to size of the thermal boards usually needs to be done before they are taken into the loft space as access is usually restricted but it is also important that any dust resulting from the cutting does not contaminate the property's living space

2. Insulation board can be costly (up to £50 -£60 a sheet depending on the gauge) and care is needed to cut them safely as the resulting dust acts as an irritant

3. Because you are insulating between the ceiling joists you are (by definition) not insulating over the ceiling joists. This means that where the joists run, your insulation is limited to the thermal properties of the joist timber only and your insulation will be "bridged" by the timber i.e. cold will be able to be transferred through the timber joists.

4. Where a home has been built with traditional 4" x 2" timber ceiling joists you are unlikely to get much more than 90mm of insulation board in between the joists (this will vary from property to property). It might be that if there is any old fashioned electrical conduit that needs to be worked around, you get even less than 90mm in.

Issues to worry about include thermal "bridging" when building up the joists to get the required height or when filling between the joists and re-boarding straight over the original joists.

From a pure insulation, building regulations and "required effort" point of view the best solution is getting a professional contractor in to carry out the required work.

In conclusion:

If retaining storage space in your loft is important then you can improve your level of insulation but you may have to compromise on the level of insulation you ultimately have and you have to be realistic about the difficulty and complexity of the DIY task involved.


There are a number of dangers associated with carrying out work in your loft and great care must be exercised. Follow all safety instructions available from the suppliers of the materials you decide to use and if you have any doubts about your capability to carry out insulation work in your loft safely please consult a qualified builder or insulation contractor.


About The Author

Alex Perry is a founder of a site dedicated to making it easy for people to save energy and cut their personal contribution to Climate Change by giving them information and putting them in touch with companies that can help.

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"Packing Joists Could you provide more information as to keep the cost of packing joists down? As you say, most people have 4x2 ceiling joists and so they will need to pack the joists by 5 inches. If you use solid 5 inch timber, this will be a great waste of timber and a great weight. 2x2 timber laid at right angles to the ceiling joists, would be 3 inches short of the total 11 inches height required. have you any ideas how to solve this problem economically? A.Georgi"


"To the person recomending Actis Triso-Super10, reading the specs, this is not fire retardent and therefore not safe in your deployment. From their specs "Actis insulation is designed to be installed behind a fire safe surface such as plasterboard.""


"I'm getting conflicting information over the wisdom of adding 5" to the height of the joists, because of the danger of the load-bearing structures being unable to take the extra weight. I really want to go for Option 1, so does anyone have the definite answer or, failing that, know who I should ask?"

Trish Stableford

"it would help if they said if you can insulate loft on top of floor boarding."

b mullins

"Interesting article but the author did not say what thickness of insulating board is equivalent to 270mm of glass wool."

J Jones

"Another option which I am considering is to insulate the rafters with thin multifoil insulation like Actis Triso-Super10 fixed onto the rafters thereby creating a warm loft. Looks a bit like a wadded foil blanket and you fix it across the rafters. There is some debate about its insulating properties. The manufacturer claims it is the equivalent of 210mm of rockwool but competitors dispute this claim in laboratory 'hot box' settings. The advantage is that you don't have to reboard the loft or carry out joist raising joinery work. the pipework and electrical cabling can stay and does not have to be moved or insulated. Disadvantage as far as I can make out is that you are moving the insulation from the living space to the loft space so to speak."

"It is a useful article, my problem is the loft is 50% boarded and being a bungalow, all the water and CH pipework and all the cabling is situated in the loft space. Original insulation is almost worthless and I intend to re-insulate. Offers to install the 270mm thick government grant insulation is not practical, they are not prepared make consideration for the pipework, cabling and worst of all about 20 downlighters fitted in the ceilings. I guess insulation boards with suitable cut-out is the only way unless someone can advise differently."

Mike S

"I think that's a really useful article - it is for me anyway as that's just the issue I've got with my loft"

Steve G