When many old houses were built, there was minimal insulation and little thought given to heat escaping upwards through the roof. Often all that stood between the occupants and the elements was a lath and plaster ceiling and the roof covering itself. Even now, many roof spaces that have been insulated have insufficient or poorly performing insulation that has slumped over time, or has been disturbed by tradespeople.
When insulating a house, the loft is usually the best place to start, as it is easy to do yet around a quarter of the heat in an uninsulated house is lost through the roof.
How to insulate a loft
If you intend to use the loft for storage, then the most cost-effective solution is a ‘cold’ roof, where the insulation is laid on top of the ceilings of the rooms below. This is usually done by laying quilts or batts of insulation between and over the joists.
Usually, several layers of material will be required to achieve the necessary thermal performance with each layer laid at right angles to the last; any gaps between can be packed with loose-fill cellulose fibre.
All pipes in the loft space must be insulated too. The layer of insulation should continue over water tanks but do not insulate directly underneath – warmth from the rooms below can help prevent freezing.
Continue the insulation right to the edge of the ceilings but do not allow it to block the eaves’ ventilation. Ventilation is vital to ensure the necessary air movement to prevent condensation from water vapour, as this can lead to decay of the structural timbers.
You must also avoid laying insulation over recessed light fittings or cables in the loft space that may overheat.
Insulation for loft conversions
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There are a number of options as to where the insulation can be placed. The ideal time to install insulation is when re-roofing, as fitting insulation above the rafters is the best technical solution. The downside of above-rafter insulation is that it raises the height of the roof and may have aesthetic consequences.
Where above-rafter insulation is not possible, the alternative is to insulate between or below the rafters, or a combination of both. A key consideration is headroom, as insulation added below the rafters naturally lowers ceiling heights.
In older buildings, where attic space already exists, installing insulation is likely to mean the loss of historic plasterwork, which is not always appropriate, especially if the building is listed.
Whatever form the insulation takes, it is vital that there is an air gap of at least 5cm between the face of the insulation and the roof covering. To avoid condensation problems, this space must be well ventilated so vents may need to be installed in the roof and at the eaves and ridge.
Quilt, batt and board types of insulation, or a combination, can all be appropriate but boards need to be cut accurately so there are no spaces around the edges. Insulation on its own does not necessarily provide an airtight solution that will stop draughts and heat loss, so it is vital that all joints overlap or are taped.
Which type of insulation?
When choosing insulation think about each material’s compatibility with the building, its thermal performance, acoustic properties, ease of use, cost and what it’s made of.
Some materials are thin and highly insulating so are ideal where space is limited; others are cheaper but a greater thickness is required to achieve the equivalent level of thermal performance.
Permeable insulation works well with old buildings that ‘breathe’. Suitable insulation includes natural fibre-based products made of sheep’s wool or hemp, as well as mineral wools.
The other option is loose fill or blown insulation. This can take various forms but the most common are mineral wool or cellulose fibre produced from recycled newspaper. Loose fill has the advantage that it fills all the gaps but it can be blown around by air movement within very draughty lofts and so becomes uneven.
Rigid insulation boards can be hard to cut exactly to fit into the gaps between joists so are generally not recommended, but if are used must fit tightly together.
To help with comparisons, the thermal conductivity of a material is given as its lambda or K-value. The lower the K, the better the product will hold in heat.
Upgrading existing roof insulation
The amount of insulation installed in the lofts of new buildings has steadily increased over the past half century, from about 25mm in the 1960s to 270mm today. Although the building regulations do not retrospectively apply to old buildings, it makes sense to consider topping up existing insulation to a greater thickness.
Where existing insulation is in a poor condition (for example, it has lost its effectiveness where compressed) or badly fitted, complete replacement is normally advisable.
Natural insulation materials
Natural materials are recommended for older homes as they are more breathable than many manmade insulations, are usually safer to work with and have a reduced environmental impact.
- Sheep’s wool insulation provides a market for unwanted fleeces. Typical K value 0.038 W/mK.
- Hemp quilts such as Isonat combine hemp and recycled cotton. Typical K value 0.039W/mK. Hemp is a great eco plant as it is fast-growing and locks up vast quantities of carbon dioxide.
- Wood-wool insulation is made almost entirely from waste wood products. It is stiffer than hemp quilt or sheep’s wool, making it more suitable for certain applications. Typical K value 0.038 W/mK.
- Recycled newspaper fibres such as Warmcel have a shorter lifespan than most insulation quilts but are much more economical and are often used in large loft spaces. Typical K value 0.035W/mK.
Roof insulation top tips
- Watch out for wildlife: check the roof space for bats and nesting birds. When a protected species is present you will need to abide by the law and may have to modify how or when you undertake work.
- Wear protection: wear a mask, gloves and protective clothing. Loft spaces are frequently dirty and some insulation materials are an irritant to the skin, eyes and respiratory tract.
- Steady yourself: take care when working in roof spaces. Always stand or kneel on the joists rather than the ceiling, as one wrong footing and you could fall through the roof.
- Don't pack it in: squashing insulation to fit the space seriously compromises its effectiveness. Also avoid pushing the insulation into the eaves, to keep ventilation paths at the edge of the roof well aired and roof timbers dry.
- Don't upset electrics: don’t lay insulation material over electrical cables because of the risk of overheating. Where this is impossible, ensure cables are in good condition and, if necessary, have them inspected by an electrician.
- Don't use the wrong materials: don’t use spray foam insulation on the underside of roofs. It can trap moisture around the timber, stops tiles or slates being reused in the future and prevents inspection of the underside of the roof.
Avoiding draughts caused by downlights
Anything that penetrates a ceiling results in air leakage, draughts and the passage of moisture into the space above. For these reasons, recessed downlights are particularly problematic and present the added difficulty that insulation must not be laid around or over them because of the risk of fire from overheating.
- One solution is to fit ‘fire hoods’ over downlights, which are widely available from electrical suppliers and are designed to help prevent the spread of fire and air leakage.
- Another is to replace bulbs with modern LEDs which don’t emit any significant heat and can be safely boxed and insulated on top.
Dealing with condensation in a loft or roof space
If humid indoor air finds its way through any gaps in insulated ceilings, it can condense on cold surfaces hidden behind, resulting in damp and timber decay.
Condensation can also affect areas where the insulation is thin or missing altogether. Common ‘cold’ spots are found around windows and door openings and on the sloping edges of ceilings in bedrooms and bathrooms, which are prone to develop patches of black mould. To prevent this, the insulation must be continuous without any gaps.
The most obvious way to minimise condensation, however, is to reduce the amount of moist air generated. Fit extractor fans in en-suite bathrooms in lofts, and improve controllable ventilation by fitting trickle vents to any windows, or simply opening the windows after showering or bathing.