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Insulation not only saves energy but also improves the comfort of your home. However, inappropriate insulation techniques in old homes can cause other issues such as damp and condensation. It is especially essential in a home with solid stone walls, to use a vapour permeable insulation material to avoid these issues.
A typical example of incorrect insulation, is using insulated plasterboard internally. With nowhere to go, moisture reaches its dew point on the impermeable material, creating damp patches on the wall.
Likewise, covering the exterior of a home in a non-permeable render can prevent internal moisture from escaping, again creating internal damp.
Traditional Homes Need to Breathe
Most traditional buildings are made of permeable materials and do not incorporate the barriers to external moisture such as cavities, rain-screens, damp-proof courses, vapour barriers and membranes which are standard in modern construction. As a result, the permeable fabric in older buildings tends to absorb more moisture, which is then released by internal and external evaporation.
When traditional buildings are working as they were designed to, the evaporation will keep dampness levels in the building fabric below the levels at which decay can start to develop. This is often referred to as a ‘breathing’ building.
Materials used for insulation and maintenance must be selected with care to preserve this permeability. In the case of your traditionally built stone house, it is important to remove any impermeable materials such as cement concrete or foam boards off the walls to allow moisture to pass through freely.
As temperature and therefore pressure is generally higher inside of the home vs outside, water vapour is pushed outside of the home. Permeable materials such as light-weight insulation renders and plasters act as a buffer for moisture, absorbing it from the internal air, and releasing it to the outside. Modern construction methods rely on mechanical extraction to remove water vapour formed by the activities of occupants.
What are Vapour Permeable Materials?
Breathable, thermal renders effectively control dampness passing through walls. Being ‘breathable’ means that the render allows balanced moisture movement through the construction fabric, allowing the stone wall to dry naturally.
When choosing a breathable render or plaster, look for these features:
- Lightweight materials that won’t put unnecessary strain and weight on the structure
- Renders containing low-density materials such as perlite (a low-cost volcanic material that is antimicrobial and non-toxic)
- Materials that control humidity levels efficiently
What are the U Value Requirements for a Wall?
Part L of the Building Regulations is on the thermal efficiency of buildings. Walls, roofs, floors and windows are all given what is known as a U value — a measure of how effective they are as an insulator. The lower the U value, the slower the rate at which heat can be loft throught that material, meaning it provides higher levels of thermal efficiency.
According to Part L1B, part 5.12, which looks at the upgrade of insulation in renovation projects, where economically, technically and functionally feasible, internal and external wall insulation should result in a U value of 0.3W/m2K. Where it is not economically feasible (ie. the cost of insulating would have a payback time of more than 15 years), a target U value of 0.7W/m2K should be achieved.
The Insulation Process
Assuming solid stone wall at 600mm thickness with as-built U value of 1.5W/m², the recommended options would be:
External and internal insulation, 50mm or 70mm thermal render application:
- 1st day: wet the substrate and apply breathable key coat
- 2nd day: thermal render (such as Bauwer Light) application at 50mm in one layer in one day or 70mm in two layers in two days
- 3rd or 4th day: finishing with a finish product at 3mm thickness. Embed rendering mesh.
- 4th or 5th day: apply highly breathable coloured render coating or wait a few days and paint with a vapour permeable paint.
Remember to use a vapour permeable paint both internally and externally, or you will create an impermeable layer on top of your breathable plaster or render.
Improved U value would be 0.70 W/m² with 50mm Bauwer Light application option and 0.58 W/m² with 70mm option correspondingly.
How Much Will it Cost to Insulate my Walls?
Lightweight Bauwer Light (yield 1m² at 25mm) and Bauwer Finish (8m² at 3mm) are competitively priced at £10 per bag each.
Bauwer insulation works out at £20/m² material cost for the 50mm option and £25/m² for 70mm application option or £40/m² to £55/m² including plastering labour.
Alternative external insulation solutions could cost up to £85/m².
Based on Historic England guidance notes