If you are looking to transform your loft space into something more usable, such as storage or to create a new habitable space to ease space pressures on the rest of your home, then you will need to ensure that you are insulating the loft in line with guidance laid out in the Building Regulations.

If the loft is going to be used as living space, then the insulation should be placed at rafter level to create a warm roof. But if you plan to use your loft for storage only, then a conventional cold roof option (where the mineral wool insulation rolls are placed inbetween the ceiling joists above the top floor).

Where Should I Fit the Insulation When Insulating my Loft?

You have three options when insulating your loft – you can fit the insulation above, in between or below the rafters. In order to meet requirements, you may need a combination of these three methods.

Between the Rafters

If insulation is placed exclusively between the rafters, they will have to be very deep to ensure you can fit the required depth of insulation (the very best phenolic foam insulation will have to be 125mm deep to meet Building Regs requirements. With this installation method, there is a danger of minor condensation caused by cold bridges at the position of the rafters. This moisture could lead to discolouration of the internal decoration.

Over the Rafters

Fitting a continuous layer of insulation over the rafters  will solve the issue of cold bridging, but is only an option on new builds or renovations where the roof tiles are being stripped and relaid. Even then, the depth of insulation now required to meet the Regulations means that very large fascia boards and wide soffits are required.

Beneath the Rafters

This loft insulation method also solves the issue of cold bridging, but can restrict headroom. This problem can usually be designed out in a new build, but may cause an issue when an existing roof is to be converted.

What is the Most Practical Solution?

When insulating a loft, the most practical solution is to place most of the insulation between the rafters, with a smaller additional layer fixed to the underside of the rafters. Insulated plasterboard is useful for this purpose. Plasterboard is available with a wide variety of rigid insulation products bonded to it.

For applications where the roof is not being completely stripped, a combination of between- and under-rafter insulation with a 50mm ventilated airspace between the upper surface of the between-rafter insulation and the traditional sarking felt is best.

What Material Should I Choose for Insulating a Loft?

When it comes to choosing the best insulation material for your loft, you have numerous options:

Polyurethanes

If space is at a premium, there are a number of plastic foam, polyurethane-type materials that are gas based. The best known are Celotex and Kingspan. While they tend to be expensive, these are considerably better than air-based insulants such as mineral wool.

Extruded Polystyrene

This is denser than the expanded version and is widely specified under floors and where there is contact with the ground. Suppliers include Dow Styrofoam and Knauf Polyfoam.

Boards, Rolls or Slabs

The wool-type insulations (often sold in rolls) are the cheapest. The polyurethanes and polystyrenes are usually sold as boards: these can be used in any application, but are particularly good under floors. Boards are often fitted in sloping roofs but this requires a lot of cutting. Walls can be insulated with boards or slabs, a semi-rigid format which stiffens wool-based insulation.

Multifoils

Multifoils use thin rolls of aluminium foil alternated with layers of foam padding. They promise good insulation levels, way better than expected by their 25mm width. They are a popular way of insulating sloping roofs.

Natural Materials

The use of natural insulation materials is becoming popular. Sheep’s wool is a wonderful material to use in place of the synthetic wools (that are made of glass fibre and mineral wool which are cheap but not pleasant to work with). Look out for Thermafleece. We are also beginning to see wood fibres and hemp being used in both roll and slab format, useful for insulating timber walls and roof spaces. Look for products such as Homatherm and Thermo-Hemp.

Blown-in

Insulation doesn’t have to be dry fixed. Wool, fibres and plastics can all be blown into position via a nozzle. This is a widely used technique under roofs. Warmcel is a popular form of insulation, used mainly in timber frame houses and lofts. It is comprised of cellulose fibre, made from recycled newspaper.

Insulating the Loft Floor and Party Walls

Insulating the floor can be achieved by a mineral fibre quilt laid between the joists. Use the heavier, denser sound insulation quilt.

For many houses, it is often necessary to insulate party walls — both against heat loss and noise. Introducing timber studwork with mineral fibre insulation will allow you to achieve both and it can be covered with sound-rated plasterboard.

Insulating a Loft Between the Rafters: How To Guide

Partially-filled rafters:

  • To maintain the required void above the insulation and to ensure the boards are flush with the bottom of the rafters, sidenail battens to the rafters in the appropriate position to provide a stop
  • The void must be 50mm deep and ventilated for constructions not using a breathable sarking membrane. Ventilation should be provided in accordance with Approved Document F, F2 (Condensation in Roofs) of the Building Regulations or Technical Standard K (Ventilation of Buildings, Regulation 23) of the Building Standards (Scotland)
Installing between-rafter insulation
  1. Measure the space between the rafters before cutting the boards, as spacings will vary slightly
  2. Cut the insulation boards to size using a sharp knife or fin-toothed saw. We used Kingspan Thermapitch TP10 zero ODP which is a 100mm-thick rigid board
  3. Install the insulation flush with the bottom of the rafters, making sure that it does not fill the entire rafter depth. Ensure that the insulation boards fit tightly between the rafters
  4. Fill any gaps with expanding urethane sealant

Fully-filled rafters:

  • Measure the space between the rafters before cutting the boards, as spacings vary
  • Cut the insulation boards to size using a sharp knife or fine-toothed saw
  • Install the insulation so that it is flush with the bottom of the rafters but does not fill the rafter depth
  • In all cases, ensure that insulation boards between rafters are fitted tightly
  • Fill any gaps with expanding urethane sealant
  • Measure up before cutting the boards
Installing loft insulation in fully filled rafters
  1. Cut the insulation boards to size using a sharp knife or fine-toothed saw.We used Kingspan Thermawall TW56 zero ODP
  2. Where board dimensions allow, fix the boards at right angles to the underside of the rafters
  3. Boards should be fixed with galvanised clout nails or drywall timber screws, long enough to allow 25mm penetration of the timber. These should be placed at 150mm centres and not less than 10mm from the edges of the board along all supporting edges

Remember

  • Ensure accurate trimming to achieve close butting joints and continuity of insulation
  • Ensure the continuity of the insulation at the ridge
  • To prevent a cold bridge, tightly pack flexible insulation material between the rafters and the cavity closer

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