Seen as the hardest to improve part of the hardest to heat properties, traditional solid walls are often considered too difficult to deal with. To break the deadlock, we need to take a different angle, striking a balance between saving energy and preserving the building fabric.
Internal Wall Insulation – Losses & Savings
Most of us are aware of U-values. The U-value tells us the rate of energy loss for a given area and temperature difference either side of a building element. It provides a simple target to follow and most of us follow it. Building Regulations (England) provides two limiting values for existing solid walls, the improved value of 0.30 and the threshold value of 0.70. We tend to focus on 0.30 because on the face of it, it’s more than twice as good as 0.70.
A U-value of 0.30 or 0.70 tells us how much energy we continue to lose after we insulate but it doesn’t tell us how much energy we save which is the main purpose of insulating. To do this we need to know how much energy is lost from the wall before it is insulated and deduct the energy loss after insulating. When we consider things from this perspective, our U-values of 0.30 and 0.70 compare very differently.
If the U-value of the uninsulated solid wall is 2.20 then the energy savings for U-values of 0.30 and 0.70 are 1.90 and 1.50 respectively. Put into pounds and pence, 0.30 saves around £5.00 per sq.m of wall per year and 0.70 saves around £4.00. Both represent a significant saving against the uninsulated wall. Whereas there is very much a place for insulating to the highest standard including super low energy retrofit, this is not possible in many situations. If the bar is too high, it is worth doing something. When it comes to insulation and energy savings, it’s very much worth doing something rather than nothing.
Heat & Humidity
Heat and humidity interact closely. Air feels damp when it gets cold which eventually leads to condensation particularly around cold surfaces. It stands to reason if we alter a heat profile, we risk altering the humidity profile which could lead to condensation.
With internal wall insulation, the inside wall surface gets colder and the more we insulate. The colder the surface gets and the colder it gets the greater the risk of condensation. We therefore need to weigh up saving energy with balancing humidity. This means that more isn’t always better when it comes to insulation thickness and requires us to think about breathable materials.
Types of Insulation
There are three broad types of insulation;
- non-breathable such as PIR which have high vapour resistance and are non-sorptive;
- vapour open such as mineral fibre that has low vapour resistance and is non-sorptive
- and breathable such as sheep’s wool that is both vapour open and sorptive. In order to appreciate the importance of vapour openness and sorption we need to understand humidity.
It’s hard to imagine how water can be an airborne gas when we experience it as a liquid most of the time. Air is a gas because the molecules like oxygen and nitrogen buzz around with such energy that they keep colliding and sending each other in all directions. Occasionally, water molecules jump into the air, and other gas molecules immediately knock them about in all directions. They then behave like a gas as long as other molecules keep knocking them about.
Warm air has the energy required to knock more water molecules about but when the air is cold there is less energy and we begin to see the water molecules turn to condensation. Vapour open materials are gas permeable so when water is acting like a gas, it can easily pass through the material. High vapour resistance materials are not porous so water can’t pass through them as a gas.
Sorption
Sorption is a measure of how materials can bind water. All natural fibres are sorptive so they can bind water molecules like tiny magnets preventing the water from forming a liquid. That’s why natural fibres can contain nearly 30% of their weight in water feel completely dry. When the temperature drops and the air can’t hold as much water, sorptive materials pluck water molecules from the air making less available to form droplets of condensation. This is very useful at cold spots such as the wall-insulation interface.
It is important to note that breathability is not a substitute for ventilation. It should be seen as a separate component that contributes to a safer and healthier moisture balance within the building and its fabric. Breathability should really be seen as a way of balancing and harnessing residual moisture within the building fabric.
Striking the Balance
There is no ‘one size fits all’ approach to retrofit: but if the aim of IWI is to optimise energy savings whilst maintaining a safe moisture balance. Then the ‘Goldilocks’ zone lies between 40-80mm of natural fibre insulation. That’s the point where the likelihood of a moisture imbalance is at its lowest and where the highest proportion of energy savings are gained.
Visit the ASBP website to find out more https://asbp.org.uk/events/iwi-natural-fibre
26/01/2023