Module 3: Pitched roof insulation

3.1 Overview

Pitched roof insulation can be placed in two different locations. It can run over the top of a horizontal ceiling or it can be placed at rafter level. In this section we’ll be dealing only with insulation at rafter level and forming a warm roof construction.

Wood fibre roof insulation is usually made up of two different products. The first is the flexible wood fibre insulation, such as UdiFLEX or PavaFlex, and is designed to go between the rafters, fit snugly and give maximum insulation to the interior. The second would normally be an external sarking board such as UdiTOP, Isolair or Pavatherm Plus, which are designed to provide a weatherproof external layer over which no breather membrane is required.

The typical roof construction would be as follows:-

counter battens
Wood fibre sarking board (UdiTOP, Isolair, Pavatherm Plus, etc.)
rafters fully filled with flexible wood fibre insulation (SteicoFlex, PavaFlex, etc.)
OSB3, taped for air tightness and to provide a VCL
counter battened service void (optional)
skimmed plasterboard

3.2 The strengths of flexible wood fibre insulation

This construction gives the best thermal and acoustic insulation and maximum decrement delay (as mentioned in the overheating section). From a building physics or heat and moisture movement perspective, this is also the most robust and durable build-up as any small amounts of condensation that do form will do so in the sarking board.

Due to the high density and ability to store and release moisture, the sarking boards can store significant amounts of moisture without any detriment to their conductivity and also without causing any damage to the boards themselves. This helps keep roof timbers dry and preserve structural timbers, both new and historic.

The sarking boards tend to be long and thin in their sizing to enable fast installation and are suitable for all common UK rafter spacings as their T&G profiled edges mean that they do not need to join on rafters. Their densities vary widely from 140kg/m3 (Pavatherm Plus and Beltermo Ultra) up to 270kg/m3 (UdiTOP), depending on manufacturer and cost. If acoustic absorption and decrement delay are important then the denser boards will be more suitable. However, if U-value and space is more important then the lower density boards, which have lower thermal conductivities, will be more suitable.

For pitched roofs below 20 degrees with a slate or tile covering, the roof build-up is the same as for the steeper pitches but all of the joints of the wood fibre sarking boards must be taped or sealed to prevent water penetrating the joints. Alternatively, a breather membrane with all of the joints well sealed can be used to cover the wood fibre sarking boards prior to application of the counter battens.

Flexible wood fibre products, such as SteicoFlex, are typically cut for rafters at 600mm centres but some manufacturers also stock insulation cut for rafters at 400mm centres. They are also only designed for solid timber so if you’re using I-joists for the roof then the web of the joist will need to be insulated to the full width and depth of the joist prior to installing the flexible wood fibre insulation. 

When sizing the insulation for the joists it is important not to leave any air gaps in the structure as this allows heat to escape through air movement, reducing the effectiveness of the insulation. For example, if you’re using a 145mm rafter then you should install 150mm flexible wood fibre insulation and not 140mm, to ensure there are no air gaps.

Flexible wood fibre insulation is much more dense than mineral or glass wool or sheep’s wool and so when you press it between the rafters it will not tend to leave depressions where you have pressed it in.

However, it should also not be squashed hard down, such as by walking on it, as it will tend not to come back to the same shape quite as easily as glass or mineral wool.

3.3 Installing the insulation

All the wood fibre insulation should be stored on a flat surface and kept dry. Any wet insulation should be allowed to dry thoroughly prior to installation.

Once the OSB has been installed to the underside of the rafters, the flexible wood fibre can be installed from above. Assuming the rafters are at 600mm or 400mm centres then the batts should just press in easily. Whilst the insulation can be installed from below, it is not easy or pleasant as the fibres do tend to fall on you. Whilst this is not hazardous it is uncomfortable and so full face mask and dust masks should be worn.

Cutting the flexible wood fibre batts is easily done with the correct tools. For thicknesses up to 80mm, a power saw can easily cut through the material and give a neat, accurate cut. The insulation should be cut 10mm wider than the cavity you’re filling to ensure a good, snug, airtight fit. Above 80mm use an Alligator saw (from Bosch or DeWalt) with new timber cutting blades for a fast, neat cut.

Once installed, the flexible wood fibre insulation should be covered with a tarpaulin if there is any chance of rain. Should it get wet it does dry quickly but should be allowed to dry fully before being covered with the wood fibre sarking boards.

Next, the sarking boards can be applied. Starting at eaves level with the tongue of the boards pointing up the slope of the roof, the first row of boards are fully interlocked and laid along a string line or timber batten fixed to the surface of the rafters, to ensure the row is straight. The overhang of the last board is cut off of the first row and used to start the second row, ensuring that the vertical joints are staggered by a minimum of 200mm but ideally at least one rafter spacing. Boards are easily cut with a circular saw.

This process continues up to the ridge where, if the roof has a dual pitch, the off-cuts from the top row of boards can be used to start the first row of insulation on the other side of the roof. This process ensures minimal waste and the fastest installation speeds.

Roofs with an overhang are usually insulated with the sarking boards right down to the ends of the rafters. This ensures the simplest passage for any water which penetrates the roof covering and it also reduces the risk of mould development on the underside of the eaves by reducing night time cooling.

Each board can be temporarily fixed with broad head nails/screws but is permanently fixed by the counter-battens. The number and type of fixings used for the counter-battens should be determined by an engineer but for most areas in the UK it would be 3-5 fixings per metre depending on the pitch) on a 38 x 50mm counter-batten.

The counter-battens should be added in short sections, followed by the battens, to provide a surface on which to walk and allow the installation to proceed up the roof. Generally, boards up to 52mm can be walked on lightly directly above the rafters but they are not designed to take a load in between the rafters as this can cause them to bow and potentially leak. Above 60mm they can usually take a person’s weight across the whole surface of the board if on rafters at 600mm centres.

As the wood fibre sarking boards are being installed the cut joints, such as ridges, hips and valleys, or any areas where the tongue and groove has been damaged should be sealed with the recommended system primer and tape. All penetrations should be carefully detailed and sealed to ensure any leakage through the roof covering cannot run down the boards and penetrate the structure. Counter-battens must be applied over the tapes and not vice versa.

For external applications Aluminium faced butyl tapes, such UdiSTEAM Alubutyl or Pavatape should be used to ensure the seal has a similar life time to the boards. The use of polypropylene tapes with acrylate adhesives is not recommended as the lifetime in an external location is not as long as the wood fibre boards. This creates the possibility of leaks on the assumption that as the wood fibre boards are fully intact the tapes should be too.

With regards to taping edges of the roof, this depends on the advice from the system manufacturer but as a general rule if you’re using boards which are only water repellent on the upper surface (UdiTOP 60, 80, 100mm or Pavatherm Plus) then these edges should be taped too.

As with using a breather membrane, the counter battens stop short of the fascia board and allow an eaves tray to be bonded to the surface of the sarking boards. This ensures any water able to penetrate the roof covering runs down the surface of the sarking boards and back out into the gutter.

Once sealed the roof can be left uncovered for 2 – 3 months, depending on the system, whilst the roof covering is applied. Occasionally small amounts of lignin can leach out of the surface of the boards discolouring the surface but this does not affect their performance.

These are a few possible sections for roof build-ups. If you can’t find what you’re looking for please contact us.