UWE

Roof  Insulation

Contents

1 Introduction

2 Insulation at Ceiling Level

3 Insulation at Rafters Level

1 Introduction

Insulating a pitched roof space is normally a straightforward measure. It can also result in significant CO2 savings. Heat loss through roofs can account for 20% or more of a dwelling's total heat loss. This can be reduced substantially with little expense. In fact the payback (total cost divided by fuel savings) in insulating a non insulated roof is probably less than couple of years. The materials most commonly used are glass fibre and mineral quilts. Other materials includes sheep's wool, so-called 'earth wool' made from recycled glass bottles and various insulation boards (used mostly when insulating between the rafters). Current Building Regulation standards imply an insulation thickness of about 250mm, although the actual insulation thickness installed to achieve minimum U values will depend on the type of insulation installed. However, although increasing loft insulation is very effective if there is not much there in the first place: increasing it from say 200mm to 300mm will only show a marginal benefit - sometimes only a few pounds.

The chart below shows U values for differing thicknesses of insulation. The table to the right shows typical savings assuming the non insulated roof accounts for a heat loss of 300.  

Until recently most roofs were designed as ventilated roofs, (left hand picture below). To prevent condensation they require a flow of air through the roof void. In the last 10 years or so breathing roofs have become more common (right-hand graphic). These depend on a vapour permeable underlay and normally require a vapour control layer at ceiling level to keep vapour out of the roof. Both can have the insulation 'topped-up' but, with the ventilated roof, be aware of the need to keep the ventilation paths open - otherwise condensation will occur. 

In some houses, usually those with rooms in the roof, insulation is at rafter level (ie sloping). These again, can either be ventilated or breathing types. Both are explained in more detail on a later page. Insulating thus type of roof can be expensive. 

Although insulating most roofs is fairly straightforward there are some potential technical problems. These are explained in the following pages; unfortunately, they are not always explained in the publicity form insulation manufacturers and installers. 

In an occupied and heated house a roof which 'holds' its snows is a sign of good insulation.

2 Insulation at Ceiling Level

The insulation is usually placed at ceiling level in simple domestic pitched roofs. This has the advantage of minimising both the heated volume and the quantity of insulation material needed. 

Ventilated Roof

Perhaps the most significant technical problem associated with insulation in traditionally vented pitched roofs is the possibility of condensation. A vapour control layer could be introduced at ceiling level to limit the amount of vapour getting into the roof space but this is costly unless the ceiling requires replacement and in practice it is likely to be breached by light fittings and roof access hatches. It's therefore vital to ensure that the roof is well ventilated. Eaves to eaves ventilation may only operate effectively when the wind is blowing in the right direction and, when ventilation does occur, there may still be a stagnant area in the upper part of the roof. Eaves to ridge ventilation may be more effective in ventilating the whole of the roof void. However, ridge ventilation alone should not be used. This is because as the wind blows over the roof it may create a vacuum in the roof space which will suck air from the rooms below, increasing the transfer of moisture vapour from the building into the roof space.
To minimise the risk of a cold bridge around the edge of the roof the insulation should extend into the eaves: but doing this may block ventilation paths. But stopping the insulation short of the eaves can create a cold bridge: this may result in condensation and mould growth around the ceiling edges (often mistaken for guttering problems or roof leaks).
A plastic tray which fits in between the rafters can be used to maintain the ventilation gap where insulation is pushed into the eaves. Vents can be provided in the soffit or over the fascia.  An overall insulation thickness of at least 200mm (mineral wool or glass fibre) should be considered when upgrading the thermal insulation of an existing roof.  In most pitched roofs, any existing insulation is likely to be between the ceiling joists. Where the existing insulation reaches the top  of the joists, the new insulation should simply be laid across the joists at right angles. Where the existing insulation is below the joist height, add insulation between the joists, directly on top of the existing insulation.  However, remember that existing cables may overheat if covered with insulation. If possible the cables should run over the insulation. 
All insulation joints should be close butted to avoid unnecessary heat loss. The gap between gable/separating walls and the first joist should be insulated to avoid thermal bridging. Where insulation is laid in two layers, the second layer of insulation should be butted up against the gable and separating walls to avoid thermal bridging.   All tanks and pipes in the loft should also be insulated to prevent freezing. Do not insulate directly under the cold water tank. The loft hatch should also be insulated. Insulation should not normally be laid over existing electrical wiring (unless it's uprated, ie using larger cables) as it may overheat and cause a fire. In practice many installers ignore this. 

Left hand photo shows two alternatives to mineral wool (earth wool and sheep wool)

It's usual to lay additional insulation at right angles to the existing in order to cover any gaps and avoid cold bridging. In practice this makes moving around a loft space precarious as indeed does any insulation thickness which rises above the top level of the joist. One at round this is to create a central boarded section of the roof using semi rigid foam insulation covered with 6mm hardboard. Some companies manufacture insulation boarding specifically designed for laying over ceiling joists. 

 

Breathing Roof

A breathing roof offers a warmer, cleaner and drier loft space. In addition the avoidance of draughts improves the energy efficiency of the ceiling insulation. However, a breathing roof normally requires:

  • a vapour control layer at ceiling level to limit the amount of water vapour entering the loft (and preferably a well sealed ceiling)
  • a permeable underlay (ie 'breathing') to allow dissipation of water vapour from the loft space,
  • 25mm deep counter battens to provide an airspace above the breather membrane for dissipation of moisture vapour,
  • ceiling level insulation that is pushed up tight against the breather membrane to prevent air leakage into the loft at eaves level.
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In addition, many organisations suggest that an eaves vent and a ridge vent should be included to help direct air into, and extract air out of, the ventilated space in between the counter battens.  Some organisations just recommend the ridge vent - contact the felt manufacturers for detailed information. Trade information from the leading tile manufacturers also contains guidance. 

Most breathing roofs will be less than 10 years old and may already have an adequate thickness of insulation. Whatever the thickness, it can easily be topped up if required although if the insulation is 200mm or so thick, extra layers will have a minimal impact and the payback will be very long - over 20 years possibly.

Note: there are many roofs being recovered where the builders are relying on vapour permeable felts alone to prevent condensation. Many of these roofs do not have a ventilated counter battened space.  Roofs like this should be inspected with caution. Roofs with a complex plan shape are particularly at risk; they are likely to contain areas which are difficult to ventilate.  Where vapour permeable underlays are to be used in a cold roof without ventilation a well sealed ceiling must be maintained over the life of the building, the slates and tiles must not be too close fitting (most tiles and slates are probably ok), and they should only be used on dwellings. In addition, check with the underlay manufacturers to find out whether counter battens are required.

3 Insulation at Rafters Level

Placing insulation at rafter level results in the roof space being part of the heated volume - the 'warm attic' solution. It's commonly used in houses where the roof space is used to provide habitable accommodation. The roof can be designed as a traditional vented roof or as a breathing roof. In many cases insulation at rafter level is in board form - these can often be thinner than mineral wool or glass fibre quilts because of their better insulating properties.

Ventilated design 

With this design, there is a 50mm ventilated airspace between the insulation and the underlay. Should the rafter depth be insufficient to accommodate both the required thickness of insulation and the 50mm ventilated airspace, the depth can be increased by adding timber battens to the inner face of the rafters over the appropriate area.  Ventilation openings should be provided at each and every roof void at both low and high level. A vapour control membrane should be applied to the warm side of the insulation. This type of design can use permeable or impermeable felt.

Providing enough space form the required insulation thickness and maintaining a 50mm gap for ventilation is difficult. For this reason many room-in-the-roof designs now feature breathing roofs. There are some multi-foil insulation products which provide very good insulation for a comparatively small thickness. Not all building control departments will accept these - there are some concerns about their insulating ability.    

In this conversion the local building control department approved the use of multi-foil insulation fixed below the rafters. The insulation is stapled loosely to the underside of the rafters (left) and then counter-battened (right) ready to receive the plasterboard. A separate vapour check is not necessary as the insulation is foil faced on both sides. The roof was vented 'traditionally' with a flow of air between the rafters; the photo shows the gap above the multi-foil and below the underlay. Ridge and eaves vents were also fitted.   

Breathing design

Because breathing roofs are comparatively recent there should be no need to increase their thermal performance. In a breathing roof with insulation at rafter level the insulation fully fills the rafter space with no need for an airspace between the insulation and the vapour permeable underlay. Counter battens provide a ventilated void which helps vapour to escape; they also ensure that water finding its way through the tiles can drain to the gutter.
In order to restrict the amount of water vapour passing through the ceiling, a vapour control membrane should be provided on the warm side of the insulation. Where piped services are to be installed, the plasterboard lining may be battened out to provide a suitable duct. The services should be routed on the inside of the vapour control layer to avoid any puncturing.  If this is not possible the vapour control layer should be well sealed where services etc pass through it. As with cold roofs, many organisations suggest that an eaves vent and a ridge vent should be included to help extract air from the ventilated space in between the counter battens.  
Insulation can be added below a breathing roof if required although it will normally require the removal of the existing plasterboard and vapour check first. A foam backed plasterboard can then be nailed to the rafters. Most plasterboard manufacturers produce thermal boards in various thicknesses; they are basically a layer of insulation bonded to plasterboard, and are available with an integral vapour control layer. Note that the vapour check should always be on the warm side of the insulation.   This would be a very expensive solution.

©2006 University of the West of England, Bristol
except where acknowledged