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One of the most commonly faced difficulties is how to add insulating material to an attic space when the attic space is required for storage.

The problem is that with regular fiberglass or rockwool, if you simply throw old boxes or Christmas decorations on top of the insulation you will compress it. When the material is squashed down it loses its insulating abilities. When you are standing in the attic, 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 require 300mm of insulating fibreglass, it is then impossible to store material in the attic without compromising the effectiveness of the insulation. There are 3 main solutions:

1. Raising the joists up with extra “packing joists” to create a 300mm void which can then be filled with fiberglass and  then re-boarded over. This will give a finished u value of 0.16 and will eliminate any thermal bridging

2. Using a solid insulating board (such as xtratherm or kingspan) cut to fit in the void that exists between the joists without raising the level of the loft floor. This method still allows for heat loss through the timber as the material is only between the joists.  

3. Using a solid insulating board to fit over the existing joists (such as xtratherm or kingspan), thus raising the flooring by a minimal amount but eliminating cold bridging. Fiberglass is used to fill in between the ceiling joists. The required U value of 0.16 is easily achievable with 80mm board and 100mm fibreglass

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

This article focuses on the first three potential methods, and looks at their advantages & disadvantages.

Before reading the article, please note that 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.

RAISING THE JOISTS

Advantages

• 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”.
• The materials are less expensive than the other options below.
• You retain most of your loft storage area.

Disadvantages

• It’s a big DIY challenge
• The wood required for the “packing joists” may be heavy and difficult to get  into the attic
• You will lose around 150mm (6”) in height in the attic  because of the floor being raised

Using solid Insulating board between joists

 Advantages

•  Your floor stays the same height and therefore no attic height is lost.
• You retain your loft storage space
• There are no heavy timbers to work with

 Disadvantages

• 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
• Insulation board can be very costly  and care is needed to cut them safely as the resulting dust acts as an irritant
• 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.
• 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.

Using solid insulation over the joists

Advantages

• Because the solid insulation has a better thermal value than fiberglass, less of it can be used to achieve the same overall u value. Therefore loss of height is kept to a minimum.
• Ordinary fiberglass can be rolled between the existing joists to cut down on cost & speed up the job
• As the insulation board has a compressive strength, the flooring can be layed directly on top without the need for additional joists
• Storage space is maintained
• Thermal bridging will be eliminated as the joists are covered

Disadvantages

• There is still some height lost (approx 80mm)
• The material is expensive
• The sheets may have to be cut outside of the attic if the trap door is too smal

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.

To sum up, 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.