Insulating External Walls

Options for Insulating External Walls in Buildings

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In the previous article I outlined various means by which one can attain the required U values as set out in the current building regulations (TGD Part L 2008) for pitched roofs and horizontal ceilings in residential buildings.

The following article will provide a range of methods by which one may attain the required U values for walls in timberframe and block constructions. It is important to highlight that there are many other means by which one can attain the equivalent U value which is outside scope of this article. I will provide some examples by which this can be achieved.
Table 1 in the previous article (Insulation of Roofs) outlined the required elemental U values to comply with Part L of Technical Guidance Documents 2008.
When the revised building regulations are launched at the end of this year it is expected that the elemental U values will have to attain levels as set out on the following table. It should be understood that these values are based on a draft of the building regulations 2010.

The latest Technical Guidance Documents L are available on the website:

Table 1 Proposed Maximum elemental U-value (W/m2K) for New Dwellings Draft TGD Part L 2010

Fabric elements elemental U-value
Roofs-Insulation at ceiling

-Insulation on slope

Flat roof




Walls 0.21
Ground floors 0.21
Other exposed floors 0.21
External doors, windows and rooflights 1.60

While there are many means of constructing a building which have their own particular advantages, ranging from Insulated Concrete Formwork, Steel-frame or Hempcrete to name a few, masonry and timber-frame are still the predominant forms of construction in Ireland today. I will therefore limit the guidance in this piece to timber-frame and masonry construction

Masonry Construction

The most common form of construction in Ireland today is with a twin leaf wall construction. This constitutes building with an inner leaf concrete block which is connected to outer leaf block work with wall ties. The size of the cavity is determined by the amount of insulation which is to be installed in this location. As building regulations have become more demanding, this has consequently meant that the cavity in the walls has increased in order to accommodate thicker layers of insulation. A rigid foam insulation board (Polyurethane or Expanded Polystyrene for example), is commonly applied in this area.

Masonry Fig 1

Figure 1: Twin-leaf block wall insulated with rigid cavity insulation boards

It has become more common in recent years to inject a beaded or a fibrous insulation into the cavity. Whichever system is used it is essential to strictly follow the manufacturer’s guidelines and to ensure the material is professionally and properly installed. It is important that the installer is approved, that the system is guaranteed and adequately certified.

It can be exceptionally difficult to eliminate non repeating thermal bridges (e.g. at window surrounds, roof to wall junctions, wall to floor junctions) in this form of construction. An effective means of overcoming this issue to introduce continuous insulation on the inside (see figure 2 below) or outside the construction

Masonry Fig 2

Figure 2: Twin-leaf block wall insulated with full fill cavity beaded insulation and internally insulated with Holzflex 040 woodfibre

Externally insulating is the most effective means to offset thermal bridges. There is a vast array of external insulation systems on the market today. Some systems are supplied with a render system which has been tested and guaranteed by the insulation manufacturer (e.g GUTEX Thermowall PLUS).

External insulation involves fixing insulating materials such as mineral wool, wood fibre or expanded polystyrene slabs to the outer surface of the wall. The insulation is then covered with a suitable render to provide weather resistance. A steel or fibreglass mesh is embedded in this render to provide strength and impact resistance. These systems should be installed by reputable trained and approved installers. While the twin leaf construction described above is very common, the use of a single leaf layer of blockwork on the flat is becoming more widespread

Masonry Fig 3

Figure 3: Single-leaf brick wall externally insulated with GUTEX Thermowall PLUS

Where a single leaf block is specified one should strongly consider specifying a block with a superior thermal value. These blocks are termed “light weight” or “aerated” blocks which feature a far lower thermal conductivity and therefore are more resistant to heat transfer. While these blocks are more expensive than a conventional concrete block, they feature far superior thermal values. For example, a conventional concrete block has a thermal conductivity (λ value) of 1.13W/mK; while a light weight concrete block can have a λ value as low as 0.11W/mK, which is nearly ten times more thermally efficient! This can have a dramatic effect on the depth of insulation required for attain a target U value.

For example, as table 2 illustrates, to attain a U value of 0.21W/m2K for a single leaf wall using light weight blocks 120mm of GUTEX THERMOWALL external insulation is required. If conventional blocks are used then 180mm of the equivalent external insulation is required to attain the same U value! Therefore, provided a lightweight concrete block meets the structural requirements of the project in hand, the use of light weight concrete blocks should be strongly considered

Table 2: Comparison of construction with lightweight and conventional concrete blocks



U Value

215mm Lightweight Concrete Block λ 0.11W/mK



215mm Standard Concrete Block λ 1.13W/mK



Timber frame construction

While the proportion of timber frame constructions in Ireland is still relatively low compared to masonry construction, timber frame is the most popular method of building in the modern world. More than 70% of homes in developed countries such as Sweden, Canada, The United States and Germany are built in timber frame construction. Even in damp climates, such as Scotland, timber frame holds 60% of the market for new, private dwellings.

As the name implies, Timber Frame Construction is a method of building which utilises a timber frame as its basic means of structural support. Stiffened by sheathing material such as Panelvent or more commonly OSB or Plywood, the frame is capable of withstanding lateral loads and of supporting multi-storey and wide span structures. The frame itself is generally constructed with timber members at either 400mm or 600mm centres.

An insulation material is applied between each of the timber members. An appropriate vapour control and airtightness layer is applied on the inner side of the frame. Best practice dictates that a service batten may be applied on the inside of the frame which in turn minimises penetrations of the external wall. Then the internal lining is applied. A standard internal lining maybe gypsum plasterboard. Figure 4 illustrates this build up from the inside and the outside. The wall may have a brick or block outer leaf, similar to that of masonry construction or it may have a cladding such as timber and fibre cement board

Timber Frame 4a Timber Frame 4b

Figure 4: An internal and external view of a timber frame wall

As building regulations become more demanding and as homeowners become more energy conscious, external insulation becomes a much more attractive means of improving the thermal performance of these walls. As highlighted previously, external insulation is the most effective means of improving thermal performance and minimising thermal bridging. Figure 5 illustrates an external insulation system with a rendered finish (GUTEX THERMOWALL PLUS) affixed to a timber-frame wall. This system combines a high performance breathable wood fibre insulation board with a complimentary rendering system.

In order to demonstrate the impact of applying external insulation on a timber-frame wall consider the following U value calculation comparison based on figure 4.

This timber-frame wall, which has the following make up, attains a U value of 0.22W/m2K:

From the inside:

Gypsum Plasterboard

50mm insulated service cavity

Pro clima INTELLO PLUS Intelligent airtightness layer

140mm THERMO HEMP natural insulation within the timber frame

9.2mm Panelvent wall lining sheathing board

Solitex WA wall lining breathable membrane

50mm vented cavity

External brick (102mm)

If one were to replace the 50mm vented cavity and the external brick, which have a total depth of 152mm, with 110mm of GUTEX Thermowall external insulation with GUTEX plaster, the U value is reduced to 0.14W/m2K, potentially a Passiv Haus Wall.

While the overall thickness of the wall is 38mm thinner, a vastly superior U value is attained. When one considers that externally insulating the wall provides a thermal bridge free construction, it is very clear that in the future, this form of construction will become common practice.

Before considering external insulation, the following are key questions one should ask:

Will the product be suitable for use on my property?

What thickness of the recommended insulation is required to achieve the target U-value of say 0.21W/m2 K or less than 0.15W/m2K for Passiv Haus standards?

Will the system result in dampness in the walls?

Will the system be at risk due to moisture absorbed where it comes into contact with the ground? How will this danger be avoided?

How will the insulation affect the breathability of the walls? Is there any risk of interstitial condensation?

Will I need planning permission because of any change in appearance of the home?

Will this system restrict my ability to paint or fix any external fittings to the outside of my home?

Is the system adequately approved?

Is there a warranty? Has it been tried and tested?

Has it be used in the past in the locality?

Does it require maintenance over time?


As we approach the end of the year, the reviewed Part L of the building regulations will be introduced. While this demands higher levels of insulation and more detailed consideration of thermal bridges and improved airtightness, there will be even more demanding requirements introduced in the not too distant future as we move towards Passiv Haus/zero carbon construction.

Passiv Haus construction is becoming more common in Ireland and there are even some buildings constructed in Ireland today which are energy positive! When one is considering how much insulation should be applied on a building, the Technical Guidance Documents should not be viewed as a target, but rather as a base level. The target should be to attain Passiv Haus/Zero carbon levels.

It is clear that whether constructions are built using timber frame or masonry methods, where it is suitable, external insulation is the most effective means of attaining the most thermally efficient specification. Externally insulating buildings also significantly reduces interstitial condensation risk and creates a thermal bridge free envelope. External insulation is still relatively new in Ireland though. It is essential that any system which is specified should be applied by a trained and approved installer. It is also important that the external insulation supplier provide a warranty for the system and that it is adequately certified.

By Niall Crosson
Technical Engineer BTech, MEng Sc, MIEI

Pictures and graphics provided courtesy of Ecological Building Systems