Insulating Solid Walls

Fitting insulation to the existing walls of your house to increase its thermal effi­ciency is usually regarded as quite easy. Simply call in a contractor to fill the cavities with foam or polystyrene balls. However, many renovators find themselves faced with the problem of insulating walls constructed in a different way — perhaps of solid masonry or brickwork, or even single skin brickwork or timber – with no cavity to fill with insulation.

According to David Olivier of Energy Advis­ory Associates, H&R’s energy specialist, the first thing to look at is external insulation. “It is much more durable than internal insulation, keeps the walls extremely warm and you do not use up valuable living space as you can easily do installing internal insula­tion,” he says. With houses that are clad with weatherboard, hanging tiles or a render, he rec­om­mends remov­ing the outer covering, installing insula­tion in the form of rigid batts or boards, then replacing the cladding.

When this is not feasible, or is unaccept­able from a visual point of view, most archi­tects would recommend a standard dry lining system from one of the ‘Big Three’: British Gypsum, Lafarge or Knauf. However, the answer is often not that simple, because this might well ignore the nature of the building. If, for example, it is a conversion project and the walls are of high quality, the last thing many renovators want is to lose the building’s character. In any case, should the building be listed or in a conservation area, the owner might well be required not to interfere with the appearance of the outer walls. To conform to current insu­lation requirements it might there­fore be neces­sary to add insulation in other parts of the build­ing, such as the floors and roof.

It is important to remember that there is a difference between conversion and renova­tion. Renovators are mostly effecting repairs and this work does not therefore require Building Regulations approv­al and so upgrading insu­lation is option­al. If you are conver­ting a build­ing, how­ever, you must conform to the current thermal insu­la­tion requirements: 0.35 W/m2°C for the outer walls as from 1st April this year.

Even then matters are not that straight­forward. Mark Lovett, who is converting an unlisted 19th century grain storage barn in Gloucestershire, was not made to upgrade the wall because of the high quality of the stone he wished to preserve. Instead, he was told to add insulation in the roof. On the other hand, at a conversion project on another unlisted barn in rural Monmouthshire, where the stone external walls were consid­ered to be of no great quality, David Olivier advised using 50mm polyurethene foam against the wall and 75m Rockwool batts inside the sections of timber studs fixed to the floor and the ceiling. A polyethylene air vapour barrier is fixed on the inside, with a small service cavity between that and the plaster­board.

Standard dry lining systems will not always suit barn walls or the walls of old buildings that have very irregular walls. To get further insulation into walls like this you may well need to install your own system using timber studs from floor to ceiling with rigid insulation behind. The controversy will always rage on whether these materials will allow breathability in the same way as sheep’s wool or other natural insulation products. They do, however, have the advantage of compactness, though if you choose to use a board material like Kingspan’s Kooltherm or Celotex double-R, you will pay more than using mineral fibre.

One advantage of a ‘DIY’ system is that it should be sufficiently adaptable to be used in one section of a building and not another, so that where you choose to retain the inside of the original walls in full view internally, you can do so. The only problem of an approach like this, where the stud partitioning does not cover all the external walls, is that you may encounter problems in concealing services.

Norfolk-based building control officer Colin Williams, who likes to experiment with new techniques, created his own internal insulation system using a method akin to this in his new brick house, built with solid external walls. Internal insulation comprised a 25mm venti­lated cavity containing a 15mm layer of Bitvent vapour permeable insulation board. Against this firm layer he positioned 100mm Warmcell recycled newspaper insula­tion, then a polythene vapour control barrier and on the inner layer 25mm of Heraclith woodwool board. The internal finish was a lime plaster as plasterboard is too brittle for his liking.

Useful Contacts

• Royal Institution of Chartered Surveyors (RICS): 020 7222 7000
• Energy Advisory Associates: 01568 780868
• British Gypsum: 08705 456123
• Lafarge: 0800 373636
• Knauf: 0800 521050
• Bitvent: Filcrete: 01482 223405
• Kingspan: 0800 610061
• Celotex: 01473 822093
• Warmcell: Excell Industries Ltd. 01495 350655
• Heraclith: 01628 784330
• Rockwool: 01656 862621
• Construction Resources: 020 7450 2211
• Natural Building Technologies: 01491 638911