It may sound pretty far-fetched, but silicone render can save energy on your heating bills. That is even without insulation being installed. The reason behind this is the phenomenon of wet walls losing heat more easily and quickly. Therefore, any waterproof render should offset this. But how does this all work in practice? And is re-rendering a suitable solution when you already have some insulation? In today’s blog, we’ll discuss how silicone render can save energy on your heating builds.
Thermal conductivity of water vs air
Water has a higher thermal conductivity than air, meaning it can transfer heat more effectively. Air is a poor conductor of heat and acts as an insulator in wall cavities and porous building materials. However, when water infiltrates these materials (due to rain, leaks, or condensation), it displaces the air pockets that would otherwise provide insulation.
- Thermal conductivity of air: 0.024 W/mK (watts per meter Kelvin)
- Thermal conductivity of water: 0.58 W/mK
This means water conducts heat approximately 24 times more efficiently than air. When water fills a material’s pores, heat flows through it much faster than when it contains air pockets.
How does silicone render save energy on your heating bills in regards to thermal conductivity?
A material like silicone render, which actively repels water away, would prevent this higher thermal conductivity. Consequently, your internal heating system has to work less hard to heat your home as your walls retain that heat more efficiently.
Material saturation, moisture wicking, and effect on building insulation
Capillary action can cause porous materials like bricks, concrete, and drywall to absorb moisture. When these materials get wet, their thermal resistance (R-value) decreases. For example, a well-insulated brick wall might have a good R-value when dry. However, its insulation properties can drop significantly as it absorbs water, leading to more heat loss. Plaster and insulation materials, such as fibreglass or cellulose, are also prone to moisture absorption. When these materials get wet, they become less effective at trapping heat within the building, allowing more warmth to escape.
In walls with insulating layers, such as foam or fibreglass insulation, moisture reduces the effectiveness of these materials. Insulation works by trapping small pockets of air, which reduces heat transfer. When water infiltrates these materials, it replaces the air pockets, leading to greater thermal bridging—where heat moves more easily through the structure.
- Water-logged insulation can lose up to 50% or more of its insulating properties, depending on the material and the extent of the moisture.
- Fibreglass insulation: When dry, fibreglass has a thermal resistance (R-value) of approximately R-3.0 to R-3.7 per inch of thickness.
- Wet fibreglass: When saturated with water, the R-value of fibreglass insulation can drop by as much as 50% or more, bringing it down to R-1.5 or lower. This drastically reduces the material’s ability to slow down heat transfer.
- Brick wall: Dry brick has a thermal conductivity of around 0.6–1.0 W/mK. When wet, this value can increase by up to 50%, allowing heat to escape from the building.
How does silicone render save energy on your heating bills in regards to building insulation and saturation?
The silicone render acts as a protective barrier for the insulation system. Therefore, the air pockets that are replaced with water in a non-waterproof system remain clear and can slow the rate of heat loss more effectively.
Latent heat and evaporation
When the water in the wall evaporates, it takes heat from its surroundings. This is known as evaporative cooling. As water turns into vapour, it absorbs heat energy from the wall and the surrounding air, cooling the area and allowing more heat to escape from the interior of the building. This effect is more pronounced in materials that can trap moisture, like wood, which can hold water in its fibres and release it slowly, causing prolonged heat loss.
- To evaporate 1 litre of water, approximately 2260 kilojoules (kJ) of heat energy is required. This latent heat is drawn from the surrounding wall and air, further lowering wall temperatures and increasing heat loss from the interior of the building.
- The Building Research Establishment (BRE) in the UK showed that a wall containing wet insulation loses 35% to 50% more heat than a dry wall.
- The National Renewable Energy Laboratory (NREL) found that water absorption in building materials like concrete and wood can increase heat flow by as much as 30-40%, depending on the moisture content level.
How does silicone render save energy on your heating bills in regards to evaporation?
Silicone renders impact materials that trap moisture, like wood, by preventing water passage into the material. The water cannot get past the waterproof facade, so it cannot seep into the material behind it. Therefore, when the water evaporates, it does so at a lower rate and less heat is lost.
Condensation and thermal bridges
Wet walls can also exacerbate thermal bridging, where heat moves more quickly through certain parts of a wall or structure due to the presence of conductive materials, such as metal studs or waterlogged wood. Thermal bridges create pathways for heat to escape, bypassing the insulation. Condensation within walls, especially caused by poor ventilation, can further accelerate heat loss by perpetually damping the wall. This moisture can also lead to structural damage, mould growth, and long-term degradation of materials, reducing their thermal performance.
How does silicone render save energy on your heating bills in regards to thermal bridges?
Silicone render prevents any water from seeping in and causing structural damage. With the risk of structural damage significantly reduced, the risk of thermal bridges is also significantly reduced. This allows insulation systems to work far more effectively and ultimately reduce energy usage. With functional and sealed insulation, your heating needs to be on for shorter periods as your walls act as heat storage units. Less heat is lost through them, and the increased thermal mass allows them to retain more heat.
Long-term effects of wet walls
Prolonged exposure to moisture can damage the structural integrity of building materials, causing cracks, gaps, and further weakening of the wall. These physical defects can create more pathways for heat to escape. As walls become wetter and lose their insulating properties, homeowners and building managers often have to use more energy to maintain indoor temperatures, leading to higher heating bills.
According to the US Department of Energy, every 1°F reduction in indoor temperature typically results in around a 1% reduction in heating energy use. Conversely, if wet walls cause heat loss and the indoor temperature drops by just 5°F, heating systems may need to work 5-10% harder to maintain the desired comfort level, increasing energy consumption and costs.
If your building’s insulation loses 50% of its R-value, the heating system will need more energy, potentially leading to a 20-40% increase in energy bills, depending on the climate and severity of moisture problems. Increasing moisture levels inside walls in a typical home can also raise indoor humidity. For every 1% increase in relative humidity, the perceived temperature feels warmer by about 0.5°C. If the relative humidity rises due to wet walls, you may have to reduce the indoor temperature further to maintain comfort, increasing energy use.
Real-world example of wet wall impact
Suppose you have a 100 m² wall with an R-value of R-20 when dry. If moisture reduces this R-value to R-10 (a 50% reduction), heat loss through the wall can be calculated using the equation for heat transfer:
- Q is the rate of heat loss (in watts)
- A is the area of the wall (100m2)
- 𝝙T is the temperature difference between the inside and outside (assume 20℃)
- R is the thermal resistance of the wall
- For a dry wall with R = 20:
- For a wet wall with R = 10:
In this example, the heat loss doubles when the wall is wet. Ultimately, this will require your heating system to work harder for longer to compensate for the increased heat loss.
Not so far-fetched after all! If you have any questions about how silicone render works alongside insulation to save energy and ultimately reduce energy bills, pop it down in the comments!
