I am often asked about ways in which people can improve the energy efficiency of their homes especially in relation to the Green Deal. In my other posts, I discuss the self build low-energy house, but what can a homeowner do to improve their existing dwelling? My first response would usually be that it is unlikely any retrofitted solution would be as efficient as a new build, low-energy dwelling. Having said that, there are numerous measures that will improve the energy efficiency of your home. These include; double glazing, improving air tightness & installing energy efficient appliances. In this post, I am going to look at retrofitted wall insulation as currently there is a push from the government to increase the uptake of these solutions.

As every property is different, this post is intended to act as a guide to the general principles behind retrofitting insulation. The advice of a qualified installer should be sought on the suitability of the different approaches to each individual dwelling. There are two basic types of external wall, cavity & non-cavity (single skin). If you are unsure of what wall type you have, this guide can help you.

Non Cavity Walls (typically homes built before 1920)

Although its is problematic, there are some options for increasing the energy efficiency of legacy housing with a non-cavity wall construction. The practicalities of these methods are open to question. These methods are:

Internal Insulation

Will reduce the internal floor area of a property by about 100 mm to the internal face of the external walls that it is applied to
Requires skirting boards, door frames and external fittings to be removed and reattached
Can make it hard to fix heavy items to inside walls without special fixings
Requires that any problems with penetrating or rising damp are fixed first

External Insulation

Generally more expensive to install than internal wall insulation
Can be applied without disruption to the interior of the building
Does not reduce the floor area
Improves weatherproofing and sound resistance
Fills cracks and gaps in the brickwork, which can reduce draughts
Reduces condensation on internal walls and can help prevent damp (but will not solve rising or penetration damp)
May mean external rainwater down pipes will have to be altered
Is likely to require planning permission

The main issue I can see with the external insulation option is that it is likely to require a planning application. As single skin properties are likely to be of the Edwardian period or earlier I can’t imagine planning & conservation officers giving approval for our Georgian and Victorian townscapes to be swaddled in insulation (thus obliterating their character). As energy bills rise, the difficulty of insulating energy inefficient heritage housing could potentially affect their property values.

Cavity Walls (typically homes built after 1920)

If a house was built in the last ten years it is likely that the cavities are already filled with insulation. It is advisable to find out whether they are filled already by either asking a registered installer for a boroscope inspection or by checking with the building control department of the local authority. If there is a void present, new insulation can be blown into the cavity. This is achieved by drilling small holes at 1m intervals in the external leaf of the wall and then blowing the insulation into the cavity . The holes are then filled. There are a number of different types of insulation that can be used to fill the cavity (I have excluded loose fill insulation materials such as cellulose or vermiculite as they are not typically used for this application):

Mineral Wool Insulation

Mineral wool insulation manufacturers claim that the product will not allow water to pass between cavities and certain products are also Energy Saving Trust Recommended. However, this material attracts the most complaints of all the blown insulation options (source). Although manufacturers and installers claim otherwise, the Building Research Establishment (BRE) has found that cavity wall insulation can allow rainwater to cross the cavity. These findings can be found in the document: BRE Good Building Guide 44: part 2: "Insulating masonry cavity walls - principal risks and guidance". There are other problems that have been associated with the use of blown mineral wool insulation:

Insulation can settle over time (source) causing voids which may lead to cold spots and dampness on interior walls
Poor installation can also leave voids which again, may lead to cold spots and dampness on interior walls
Insulation can transmit water across the cavity in certain circumstances
Capillary action can transmit water vertically within the cavity

Furthermore, when its is found that this system has failed this article claims that: 'in every case the installers have insisted that moisture transmission problems were due to existing faults in the existing building ... or to "lifestyle" condensation caused by the owner'. It should be stated that in the vast majority of situations in which blown mineral wool insulation has been installed no problems have occurred. However, the consequences of the system failing ( & having it removed) are serious enough to at least give a homeowner pause before having it installed.

Bonded Polystyrene Ball Insulation

Expanded polystyrene beads (EPS) is more expensive than mineral wool and can take 2-3 days to install and is not usually funded by government schemes. Certain polystyrene ball insulation systems are Energy Saving Trust Recommended. The main complaint with bonded polystyrene beads, is that the bonding can be insufficient to hold the beads together. This can have several implications:

The beads can escape through holes in the masonry or junctions with windows & doors.
Voids can cause cold spots and dampness
Voids can cause water to travel across the void to the inner leaf
The technology has only been around for a relatively short time
Will cause problems if you want to add additional wall penetrations in the future

As this product has only been around for a relative short time I was unable to find much information on its long term effectiveness or failure rate. This is option removable should penetration of moisture be a problem

Foam Insulation

Closed Cell Foam Insulation Foam insulation systems can take 2-3 days to install, are more expensive and are not usually funded by government schemes. This system can be more problematic to remove than the first two options as the foam must be manually broken up and the process may involve removing part of the external wall (source page 58). The advantage of this system is that it can strengthen the existing structure and does not cause problems should you wish to form new openings. At the time of writing there are no Energy Saving Trust recommended blown foam insulation products.

Conclusion

The value gained from increasing the energy efficiency and comfort level of a dwelling must be weighed up against the potentials risks of filling the cavity. After all, it was put there for a reason. To summarise, the general problems with retrofitted cavity wall insulation can include:

If the cavity is not fully filled, cold spots can occur on the inner face of the external walls this can cause internal damp patches to form
The insulation within the cavity, in certain circumstances, can allow moisture from rain to pass to the inside of the dwelling
In certain circumstances, capillary action can transport moisture vertically
Insulation can cause structural wall ties to be exposed to moisture and as a result potentially shorten their life

It should be stated that the vast majority of installations of these systems do not have any problems, however, it may be advisable to consider how easily it is to remove each type of insulation on each individual property should the worst case scenario occur

Additionally it should be noted that in certain circumstances cavity wall insulation is not suitable at all. For example, North facing walls which do not receive direct sunlight and a are hit by a lot of driven rain are likely to suffer from an ingress of water to their internal walls. The British Board of Agrément approval for retrofitted cavity wall insulation states that they are unsuitable for areas of high exposure to wind-driven rain. Unfortunately, anecdotal evidence suggests that these requirements can sometimes be ignored.

A lot of the issues with the identified systems that have been highlighted above are due to the quality of the installation. It is worth noting that as a consumer it is often the case that you get what you pay for. Prior to considering retrofitted cavity wall insulation it may be worth ensuring that a dwelling has good quality double glazing, sufficient levels of loft insulation and draft proofing as there is less scope for these solutions to introduce problems.