An approach to improving the thermal performance of old timber-framed buildings by Daniel Hurst April 2019.

External walls insulated plastered and decorated

Recently I have been considering how to improve the thermal performance of older properties, particularly those that are built of a traditional timber frame. The walls of these properties cannot be treated with Government Subsidised Cavity Wall Insulation so I decided to carry out an experiment on my own home, starting a couple of weeks before Christmas with my bedroom – much to my wife’s joy!

A traditional timber framed house is built out of timber studs (in my case these are made of solid oak) faced with timber planks, wired then cement and lime rendered. This is usually painted. Internally the studs are faced with wooden laths and then plastered with traditional horsehair and lime based plaster. The internal surface is usually painted but are more commonly found wallpapered. The surface of horsehair plaster dries over the years and becomes too dusty for paint to stick to.

The cavity formed between the external wall and internal lath and plaster is about 150 mm so is a pretty large air gap. The air in this cools quite rapidly and, in turn, cools the internal wall. Draughts can also occur at wall and floor junctions where skirting boards have moved and are not sealed to the floor. In my case we have exposed the original floor boards – carpet would be better at reducing draughts but we both love the original timber floors. They certainly keep the character of the house.

My approach to improving the thermal performance of the walls.

My plan was to strip out the lath and plaster internal wall surfaces. This is possibly the most dusty operation that can be performed in a domestic dwelling. Much ventilation is required as is dust extraction.

Luckily I have a very effective dust extractor which works remarkably well. There are some important points to note when using a dust extractor, most importantly where the dust is vented to. A strategically placed gorilla bucket underneath the outlet of the extractor hose half filled with water serves to act as a water dust trap.

First the room needed to be removed of all furniture, , stripped of curtain poles and curtains and the floor sheeted with a suitable protector (I used black high impactproplex from Travis Perkins).

I stripped out the skirting and architrave then removed the horsehair lime plaster with the wooden laths:

I stripped out the skirting and architrave then removed the horsehair lime plaster with the wooden laths:

The studs were de-nailed and vacuumed to remove dust.

Due to the small cavity between the studs and the external cement rendered wall face being 70mm I opted for Actis Super Triso 10 insulation ( This is a multi layer foil based insulation product which is ideal for small gaps and provides a low U-valueequivalent to 250mm fibre glass insulation. This is with a required gap of 35mm so is great for space saving. 

I installed the super triso 10 insulation by stapling it over the studs, holding it tight between each one and foil taping the joints. This was then counter battened to ensure an air gap on each side of the foil and faced with 12mmm square edged plaster board:

External wall insulation in timber framed house Super triso 10
External wall insulation in timber framed house Super triso 10
Ceiling traditionally plastered.

To finish the walls I plaster skimmed them, mist coated and decorated then fitted new skirting and architraves.

The outcome.

As well as looking good (see the first photograph), the room feels much warmer. The radiator only needs to be set to 2 or 3 maximum on the thermostatic valve and I have noticed a decrease in my heating demand proportional to the space heating reduction (supporting data to follow in a later blog post). 

thermal insulation upgrade

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