Learning from practical project experience

This page contains information that may be helpful to know prior to a site visit and quotation from an installer. Because it comes from only a few practical projects, it will not cover every house design or insulation requirement and should be viewed only as a learning zone.

Important Note. This article is intended to provide insight into insulation (educational awareness) not as a guide for installing insulation. There are important health and safety considerations when installing fibrous materials and in particular there is a risk of encountering extremely hazardous materials such as asbestos in some older properties. If in doubt seek professional advice before touching or removing old insulation.

Are there damp walls or mould in the property at present? If so then address these first. A leaking roof or a structural problem must always be fixed before an insulation project is started. Insulating without understanding the cause can hide the problem and contribute to a worse outcome if it is allowed to continue. An independent professional view from someone not selling an insulation project may be worthwhile in this case.

1. Loft Insulation

Loft insulation is cheap and very effective (heat rises). We need to cover the whole area. Pay attention to the loft hatch – you may need extra insulation here so it doesn’t become the thermal weak point of your loft (a fire-resistant insulation is suitable because openings are weak points for fire spread) . The first layer is layered between the beams and the second layer is layered across the beams. If you need to use the space in the loft for storage then you cannot squash the insulation or it will lose its effectiveness. The boarding for storage will need to be lifted up on stilts or equivalent.

For an uninhabited roof space the cheapest option is a cold roof with extra thick insulation between the room ceilings and the attic (if money allows, you could consider a warm roof option with under rafter insulation too – see next section). Do not let the insulation touch the roofing felt or membrane (the material under the tiles) at the edges of the roof as the roof needs to stay ventilated to prevent damp and condensation. Dripping condensation will weaken the thermal effectiveness of the loft insulation (wet timbers must also be avoided as that will eventually damage the wood and lead to structural problems).

If you have a water tank in the roof then do not insulate under the tank. The heat from the house is needed to keep the tank ice free. You can, however, insulate around and above your water tank with a thick water tank jacket. To be extra safe, the overflow lines from the tank should be insulated with pipe insulation. This is to prevent an overflow in winter freezing and causing a major issue.

The thermally weak zone on most UK houses is in fact the very corner of the roof where there is no space for thick insulation (remember that the loft insulation should not squeeze up against the roof felt or membrane). Wall insulation does not reach that far up either. So we have a thermal gap in the upstairs external corners of the rooms. You can check this with an infra-red thermometer – the room wall and ceiling on all the high external edges will be colder! One way of addressing this is to use a thinner more-performant insulation over this zone that maintains the necessary gaps between the insulation and the roofing felt but allows the room corners to be insulated. This is rarely done in the UK.

It is generally not acceptable to put insulation on top of electrical cables in the loft (e.g. ceiling light cables). Try and keep the light cables above the insulation. If you have spot lights, the insulation should be separated from the lights by a cage or fire enclosure. This is to eliminate the risk of the light overheating.

Typical loft insulation materials include rolls of fibreglass, plastic fibres, mineral fibres or quilts. There are some natural fibre alternatives.

2. Under Rafter Insulation

If you have an inhabited roof space then there are warm roof insulation approaches. Insulation can be applied above or between and below the rafters. Generally above rafter insulation is an option if the property is being re-roofed. More common is to retrofit installation between and below the rafters. A good technique is to install rigid insulation panels between the rafters (leaving a 50mm gap between the insulation panels and the roofing felt or membrane to prevent condensation – the manufacturers of the insulation products will detail this in their technical notes) and to complete it with a multi-foil insulation that provides insulation and a vapour barrier (again leaving a 25mm gap between the rigid panels and the multi-foil as most multi-foils need a gap on both sides to be really thermally effective). Insulated plasterboards may be useful to improve the U-value or thermal effectiveness even further (see end of article for U-value definition). If not using a multi-foil then an insulated plasterboard is essential to address thermal bridging (cold spots) for heat conducted through the structural timbers.

Pay attention to dormer windows and dormer cheeks. There are plenty of angles and tight spaces where insulation may be badly fitting (potentially bringing cold air to the back of the plasterboard) or may be missing completely. The most effective approach may be a high performance rigid panel insulation snugly fit between the timbers and finished with a multi-foil insulation quilt taped and sealed around the whole dormer.

3. External Wall Insulation

For older properties with solid walls, external wall insulation is generally an effective insulation approach. Unfortunately it changes the external appearance of the property. For properties already with external render, that change in appearance may not be that obvious, but for properties with external brickwork the change in appearance to a rendered finish is very noticeable. If the exterior of a property needs re-rendering then installing external wall insulation or increasing its thickness at that time is ideal. External wall insulation is usually panels of grey (treated) expanded polystyrene, rendered with a suitable acrylic waterproof render (external wall render needs to comply with section B4 of the Building Regulations on external fire spread). The quality of the installation is evident during the process (panels are tightly and neatly arranged and special plastic screws attach them to the house, any gaps should be filled with suitable expanding foam to get best performance). There are alternative materials that some contractors may offer, including mineral wool and other fibreous materials (mineral wool gets an A1 classification to BS EN 13501-1 meaning it is not combustible however its thermal performance is not the best). Key with the fibre-based materials is to keep them dry to maximise their thermal performance. Check the U-values for the different options if you are considering other materials. You may require different thicknesses the get the same level of heat insulation.

4. Internal Wall Insulation

Internal wall insulation may be appropriate if you don’t want to change the external appearance of the property and are happy sacrificing a few centimetres of space in rooms with external walls. This is most easily done by installing insulated plasterboard instead of the existing plasterboard. Insulated plasterboard is standard plasterboard with a relatively thin high-performance rigid insulation bonded to the back of it. An alternative is to use a multi-foil insulation stapled to wooden battens behind a standard plasterboard. Repositioning of electrical switches and sockets and redecoration would be necessary with internal wall insulation. It is often convenient to consider it if a property or a room is being comprehensively refurbished.

Ask your installer or independent professional about interstitial condensation. This is not usually an issue but there are some constructions that could be affected by thick internal wall insulation where vapour control is not adequate.

5. Cavity Wall Insulation

Old properties up until the beginning of the 20th Century in the UK invariably had solid walls. Cavity walls were introduced in Britain as a defence against damp. By separating two layers of brick or block, even if the outer exposed layer is saturated with water, the cavity prevents water getting to the inner layer and then into the property. Water in an empty cavity will just drip straight down. The two layers are structurally connected through ties – these are many small slender metal pieces attached at regular intervals between the mortar of the outer brick layer and the mortar of the inner block layer. The cavity may be ventilated (evident if there are air bricks in the outer brickwork) or non-ventilated. New build houses have insulation panels inserted during construction. To maintain the water-tightness of the cavity, it must be difficult to get water across the cavity. With foil-faced rigid panels, water will still drip down even if the cavity is substantially filled. Cavity wall insulation with foil-faced panels on new build is highly desirable. Using a high performance panel in a large cavity (100mm+) can give excellent thermal insulation and a very low U value.

Retrofit cavity wall insulation has become popular in the UK in recent years. Fitting panels into an existing cavity is impossible, so retrofit cavity wall insulation is generally blown through drilled holes in the external masonry. Quality control is difficult and the homeowner may not be able to inspect the coverage and evenness of the installation. Further, the method will fill the entire cavity and water penetration through the fully-filled cavity may now be possible. Most installations are free at point of installation with the energy company being billed for the work so that ultimately the costs are paid through everyone’s energy bills. The auditing and inspection of cavity wall insulation installations is, in our view, lacking. A poor installation may not be easy to spot and may not reduce the real heat loss of the home. If water starts to penetrate through the installation, then the heat loss could actually increase and ultimately damp problems may start.

Remember the ties? Not all ties used in 20th Century constructions were rust proof. In a dry ventilated cavity those ties would last relatively well. However, once in a damp cavity or worse still in a saturated or water-logged cavity, the corrosion process will accelerate. If the ties rust through they no longer provide the structural integrity for the wall.

It is acknowledged by the industry that not all areas of the UK are suited to retrofit cavity wall insulation. Western parts exposed to wind-driven rain are explicitly not suited for these retrofit installations. Met Office climate change predictions for the UK suggest that more rain is possible with greater likelihood of storms and abnormal rain events. This may mean that a substantial part of the UK is not ultimately suited to retrofit cavity wall insulation. By a similar logic, fibre type cavity wall insulation installed during new build has a risk of water saturation too. Perhaps leaving a small cavity for water draining is a sensible option even for new build?

Interstitial condensation is also a risk with retrofit cavity wall installation. Ask an independent professional. They should also check the state of your external masonry and wall ties and assess the risk.

Is external wall insulation a remedy for a property with retrofitted cavity wall insulation? External wall insulation should come with a waterproof render and this could stop the ingress of water with side-driven rain. So it might work although we’ve not heard of anyone actually doing this. If the cavity wall insulation is causing issues then the best remedy is probably a complete removal. A wet, damp or saturated cavity really needs to dry right out. If water is entering the cavity via a roof issue then a waterproof external render will not fix this issue either.

6. Natural Materials and Historic Properties

Improving the insulation on listed or historic properties must be done in consultation with the appropriate professionals. To learn more about insulation approaches we recommend the excellent Historic England guides available at HistoricEngland.org.uk/energyefficiency.

Click here to learn about U-values and how to calculate them.