External Wall Insulation versus Internal Wall insulation

With the re-launch of the GDHIF, there has been a lot of renewed interest in insulating solid walls in properties. There are two ways to insulate this type of wall – either internal wall insulation or external wall insulation.

If your home has cavity walls, they can be injected with insulation. If your home has solid walls – or if you have already had your cavity walls insulated and want further thermal comfort – external wall insulation is a great option.

In this article we are going to look at the pros and cons of both types of solid wall insulation, and hopefully help you in making a decision in which way to go!

External Wall Insulation

External wall insulation involves adding insulation boards (normally PIR boards, but this can be mineral wool) to the external walls of the property and then rendering on top of the insulation to finish.

The pros of external wall insulation

External wall insulation is obviously designed to increase the energy efficiency of the property – 10cm of PIR insulating board added to the walls of a house will take the u-value down to less than 0.28

The current building regulations for walls are not actually as stringent as this, so the energy savings will be substantial. We normally look at about a 35% energy saving if all the external walls are insulated.

External wall insulation also improves the wall’s ability to withstand the elements. The layer of insulation sits on top of the brick, protecting the brick work beneath it. This means that any planned remedial work (like pointing between bricks) can be put on hold – although if there are any structural issues these should obviously be sorted before insulating. The insulation will cover cracks, or any poor rendering– in fact in many cases it improves the external appearance of the property.

Mayday Gardens during EWI install

External insulation should not affect life in your home as much

External wall insulation also prevents penetrating damp entering the home, although it will have no impact on rising damp. In most cases the external wall insulation installers will remove the wall furniture (e.g. guttering) so for many people this provides a good opportunity to check the integrity of it at the same time, resealing any leaks and replacing old pipework, before reattaching it to the wall. This is a nice added benefit of getting external wall insulation installed!

Those who live underneath a flight path or on a busy road will know how a well-insulated house muffles the noise entering the home. External wall insulation is a fantastic way to help reduce the amount of noise entering the home.

External wall insulation also means no disruption to the occupants of the property – all the works are carried out externally so apart from the installers needing the odd cup of tea there should be no impact to those living in the house!

A really key benefit of external solid wall insulation, often overlooked, is that the walls will continue to get warmed by the heating system. This means than when the heating is turned off, this stored warmth will be slowly released back into the home, meaning that your home is kept at a more even temperature. It also means that you won’t ever get condensation on the walls – which is obviously a huge benefit.

We have already mentioned that external wall insulation can give your home a face lift. In most cases the home is rendered to finish it, and a nice feature of this is that you can get the render in any colour you want. If you have always dreamt of the Barbie pink house, now is your chance!

The Cons of external wall insulation

Not all solid wall insulation is made equal. If you get companies round to quote for the works, ensure that the insulation is attached to the wall using both adhesive and mechanical fixings. If not, you risk the insulation coming away from the walls in time, which obviously is not going to be cheap to fix. Make sure that any company installing the insulation provides a 25-year industry warranty – this should give you the comfort you need to know the company you are going with really back their product.

In many cases, the soffit (the bit of the roof that over hangs the wall) is more than 100mm so the external wall insulation can butt right up to it. Unfortunately in some cases it is not and therefore the insulation needs to be capped at the top (to prevent moisture getting down the back) or the roof needs to be extended over the top of the insulation and this can be costly.

In most circumstances you don’t need any sort of planning permission / council permission to do works, however if you are in a conservation area or you live in a listed property, DEFINITELY seek permission before getting it installed. To be honest, we would always recommend calling the council and just flagging it – 99 times out of 100 they won’t have any issues although if you happen to live in the borough of Merton (who don’t seem to have a clue what is going on in their planning department) they might kick up a fuss.  Obviously in certain circumstances planning won’t be granted so you will have no choice but to insulate internally.

When external solid wall insulation is installed, the installer should go no where near the damp course – the insulation should start above it, but that does mean you get some cold bridging there.

You require scaffolding, which can add slightly to the cost, although if you try to do other works at the same time (that require scaffolding) then obviously you just pay once!

Internal Wall Insulation

Internal wall insulation involves attaching insulation to the interior walls of the property, although only walls that are in contact with the outside air need to be insulated – e.g. in a mid terrace house, there is no need to insulate the walls between two properties.

The Pros of internal wall insulation

Like external wall insulation, insulating your home internally will lead to significant energy savings. This is obviously the main reason people would get the works done and we really do feel that insulation is one of the best ways at protecting yourself against energy price rises.

Internal wall insulation also has no impact on the external appearance of the property, so there are no planning concerns (unless you live in a listed property, in which case you will need to seek permission). For a lot of people with solid brick properties, the look of the brick was an important selling point, so internal wall insulation allows you to retain that while still improving the energy saving potential.

internal insulation 1

Internal board insulation

Internal wall insulation is relatively easy to do, you can do one room at a time to minimise disruption. Ideally you coincide the insulating works with when you are decorating the property, since there is no way to insulate under the plasterboard without ripping down what is already there.

Also there are no added scaffolding expenses associated with insulating a property internally.

The Cons of internal wall insulation

For many people, losing internal space is the major issue with internal wall insulation.  It is understandable to be honest. In Westminster for example 1m2 of property costs over £7,500.

In a 5m x 5m room, losing 10cm off all the internal walls would cost the owner £15k. Obviously a stupid example, but it makes the point – many people are unwilling to give up any space!

Unlike external wall insulation, where the amount of insulation that can be added is essentially unlimited, for internal insulation there reaches a point where people will be unhappy to lose the space which obviously puts a limit on the thermal efficiency one can achieve.

This was included in the pros for solid wall insulation, but internal wall insulation can lead to disruption – in some cases only one room will be done at any one time, but if that room happens to be the bathroom then that can be inconvenient.

With internal wall insulation you get cold bridging between the floors unless floorboards are lifted and the insulation is run uninterrupted down the wall. In the majority of cases this won’t be done i.e. the insulation will stop when it hits the ceiling (at ground floor level) and when it hits the floor (at first floor level). This means that heat loss still occurs.

If there are any penetrating damp issues, these must be sorted prior to installing the internal wall insulation otherwise far worse issues can occur down the line. In addition the bricks will still be open to the elements, which potentially means that repointing / general maintenance still needs to take place.

One of the challenges for the installers is that a damp course layer must be used when the internal wall insulation is installed. Now this isn’t an issue for the occupants except if they put a nail into the wall and it goes through the vapour layer. Once this is compromised, it can lead to all sorts of issues that will become very costly to install.

Final Thoughts

Unless you buy a house and are stripping it right back to the bare brick, then we would normally recommend external wall insulation over internal wall insulation. That is not to say it can’t be as good – both solutions can go a long way to massive energy bill reduction – but just due to space constraints, it is far easier to achieve higher thermal efficiency with external wall insulation.

Once you insulate the walls you should be looking to achieve a u-value of 0.30 W/m2K or less (remember the lower the U-value the better the thermal efficiency); achieving this will bring the walls in line with cavity walls built today.

The Green Deal Home Improvement Fund (GDHIF) phase 2 is the ideal scheme to help cover much of the cost of the works – the GDHIF solid wall insulation grant is for £4,000 + you can get an additional £500 if you have lived in the home for less than 12 months. It is not means tested and provided you have a Green Deal Report you can access it. The installer who does the work must be PAS2030 approved and enrolled on the scheme to get the GDHIF. The money is available and we can assure you, it will never be as generous again so we suggest getting in quick!


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