One of the most frequently asked questions we receive here at EWI Store is whether it is possible to install External Wall Insulation (EWI) on just one or two walls, or half the house, rather than the whole property.
You can install EWI on just one or two walls! However, EWI must be installed on the whole wall from top to bottom, not just on one elevation.
EWI is a great way to insulate your property against heat loss, all without losing internal floor space. By slowing the heat transfer rate through the walls, EWI keeps your home cooler in the summer and warmer in the winter and prevents issues such as condensation and dampness.
When EWI is installed onto the whole building, you can save hundreds on your annual heating bill.
Is it cost-effective to install EWI onto just one wall?
EWI isn’t inexpensive, and installing it onto just one wall may not be cost-effective as you won’t achieve all of the benefits, particularly insulation against heat transfer and noise pollution.
When looking to get the most for your money when installing EWI, the general rule is that the more of your house you insulate and the thicker the insulation, the more money you will save on energy bills. However, the depth of your insulation is also dictated by Building Regulations. You must achieve a certain U-value for retrofits, with the general value for walls being 0.30(W/m²K).
Also, in most cases, the more of your property you insulate with EWI, the less you pay per sq metre. As you pay for installers to come out in the first place, you may as well get the whole job done immediately. We've worked out the rough costs for an EWI system per square metre in our recent blog!
Although the upfront cost may be higher, the payback period is acceptable, and the savings are crucial. Installing external wall insulation also raises the value of your property. The upfront costs and payback naturally vary depending on your home type. The table below demonstrates the typical costs associated with installing EWI:
| Type of home | Size (sqm) | Bedrooms | Cost | Savings | Payback |
| Detached | 147 | 4 | £15,000 - £20,000 | £1,000 | 20 years |
| Semi-detached | 96 | 3 | £8,000 - £10,000 | £700 | 15 years |
| Mid-terraced | 64-100 | 2-3 | £6,000 - £8,000 | £500 - £700 | 12-15 years |
| Bungalow | 77 | 1-2 | £6,000 - £8,000 | £700 | 12-15 years |
A downside to installing EWI on half your house
Another thing you may need to consider when installing EWI onto just one wall is whether or not you will need to re-render the whole property to achieve a seamless finish.
When installing EWI, the insulation boards are covered over by a render of your choice. Depending upon your property and its current render situation, installing EWI onto just one wall could create a visible imbalance in the external appearance of your property.
What this means is that just one wall with EWI - a visible, newly rendered wall could exacerbate the appearance of issues on other external walls, such as organic growth, weathering and cracking. This creates a sharp contrast between the old and new walls. Therefore, you may spend even more money re-rendering your entire property to address the imbalance. Re-rendering your whole property will create a much more pleasing appearance to your home but does not rack up the total cost of solving the problems of just one wall.
Creating a seamless thermal envelope is crucial in any passive house design. If you choose to insulate only a portion of your home, you create thermal bridges, which are essentially heat highways. Heat will always look for the path of least resistance. Therefore, if half of your home is not insulated, much of the heat will easily escape.
Damp on one wall. Will EWI resolve it?
Finally, some homeowners look to install EWI onto just one wall due to a greater level of dampness and mould building up on particular external walls. EWI could be a solution for preventing problems with damp on these walls in the future, but any direct causes of damp must first be ascertained and resolved. So why is there damp on your walls? Below, we have listed some possible causes of dampness in your home. We would recommend that before going ahead with EWI, you hire a professional surveyor to check for any of these issues:
- DPC (Damp Proof Course) is broken or bridged, or you simply do not have one.
- The surface of your external wall is cracked and broken, allowing moisture to seep inside.
- The property has a lack of light on certain walls
- The property has poor drainage
- The house has poor ventilation
In some cases, however, damp is just a recurring issue that cannot be easily fixed. Take, for example, a house with an external wall positioned very close to another house, thereby creating an alleyway with little light. In these situations, EWI could provide a long-term solution.
Choosing render to combat damp
We suggest that when installing EWI onto walls susceptible to damp, you consider breathable materials such as our Silicone render systems. These allow water vapour to travel across the surface, ensuring the building can breathe. You can also try out lime render if you have an older property with a heritage facade. Lime render is inherently breathable and has self-healing properties. As it is made entirely from natural resources, it takes a while to cure and set. The self-healing property can cover micro-cracks that occur due to expansion.
If EWI is something you are considering for your home, we always advise that you seek expert advice. Here at EWI Store, we can offer the best materials on the market for the job. You can get a free quote on materials using our EWI Calculator. We are always happy to answer any enquiries from our customers, so give us a call or drop us an email!