The main non-structural function of masonry elements is as a cladding to buildings and the key function of such elements is to maintain habitable conditions within the building. It is therefore important to know how effective masonry wall systems are at preventing heat loss in winter and maintaining comfortable conditions in summer, preventing ingress of wind and rain, reducing noise transmission and limiting the spread of fire should it break out.
Another key concern that is becoming increasingly important is the effect of a product on the environment – the sustainability or green credentials of a product. Increasingly, materials will be chosen not just for their inherent performance, appearance or economy but also taking into account their energy cost and other effects on the global environment such as emission of green house gases and land destruction due to quarrying.
Non-structural Physical Properties of Masonry
The rate of heat flow through a given material is controlled by the thermal conductivity. Metals generally have higher conductivities and very-low-density porous materials (containing a lot of air or other gas) have lower values.
Masonry materials fall in a band between the two extremes. The property is important in that it affects the winter heat loss from a building through the walls and thus the energy efficiency of the structure. Surprisingly, although it is often of lower density and quite porous, normal bedding mortar is frequently a poorer insulator than many bricks and blocks, as shown by Fig. 1.
This has led to the development of insulating mortars containing low-density aggregate particles and thin-joint mortar. The latter type reduces heat loss because of its much smaller area proportion of the wall face.
Because porosity is a key parameter the thermal conductivity (k) is bound to be partially related to material density, and general equations for dry solid porous building materials tend to be a function of density (r) with regression equations of the form:
k = 0.0536 + 0.000213ρ − 0.0000002ρ2
The presence of moisture increases the conductivity of porous materials because evaporation and condensation heat-transfer mechanisms become possible. Hollow and perforated products give some improvement over plain solid products, although the potential gain from the trapped air pockets is compromised by the convection of the air within them.
Units with many small perforations perform better than hollow units because the smaller size of the holes reduces convection and the smaller solid cross-section reduces conduction. There is also an improvement if the holes are staggered such that the direct conduction path through the solid material is as long as possible.
Figure 2 illustrates the effects of different perforation patterns on thermal resistance. If convection is prevented by filling the hollows with foamed plastic materials such as urea–formaldehyde there is a further substantial improvement. The properties of walls used as thermal barriers are normally expressed in the form of the ‘U value’, the overall heat transfer coefficient, which is a synthesis of the k value of the actual materials and the heat transfer coefficients at the hot and cold sides. More detail on thermal insulation is given in Diamant (1986), BRE Digest 273 (1983), BRE Digest 355 (1990), BR 262 (2002 edition), Good Repair Guide 26 (1999) and Good Building Guides 44-1 and 44-2.
Resistance to Damp and Rain Penetration
From the earliest use of built housing one of the primary requirements has been that the walls will keep the occupants dry and thus most masonry forming the perimeter walls of houses and other buildings is called upon to resist the ingress of rain. All masonry component materials are porous, however, and there are always some imperfections in the bond and seal of the mortar joints (de Vekey et al., 1997) and the workmanship (Newman and Whiteside, 1981; Newman, 1988) that will admit some water so no solid masonry wall is likely to be absolutely watertight (de Vekey et al., 1997).
Dampness rising through porous masonry with no damp proof course (DPC) has never been shown to occur under laboratory conditions, and the current view is that cases diagnosed as such are really falling or horizontal damp from faults or retained soil (Howell, 2008). Paradoxically, it is normally easier under UK conditions to make a rain-resistant wall from porous bricks. This is because some leakage always occurs at the joints, which is mopped up by high-absorption units but allowed free passage by low-absorption units. Provided the rain does not persist until the units are saturated, they can dry out in the intervening dry spell and do not actually leak significant amounts of moisture to the inner face.
Under similar conditions, some modern, low-absorption facings may leak quite seriously during a moderate storm. As would be expected, resistance is greater for thicker walls or if a water-resistant coating is applied over the exterior. Typical coatings are renders and paints or overcladding systems such as vertical tiling.
The commonest technique for avoiding rain penetration in the UK is the cavity wall. This is a twin layer wall with an enclosed air space between the two leaves. Some leakage through the outer layer (leaf) is anticipated and any such water is directed back out through weepholes before it reaches the inner leaf by the use of damp-proof membranes and cavity trays. This is sometimes thought of as a very recent wall form but it was probably used in ancient Greece and has been in use in the damper parts of the UK for nearly 200 years.
It has given remarkably good service and is quite tolerant of workmanship variations. The main problems have been leakage of rainwater, which affects a small percentage of cases, and the corrosion of the steel ties used to ensure shared structural action of the two leaves. Useful references are Newman et al. (1982a, 1982b), BRE Digest 380 (1993), Good Repair Guide 5 (1997) and Good Building Guide 33.
Figure 3 illustrates some routes for moisture and dampness to penetrate firstly the outer leaf (or a solid masonry wall) and then to reach the inner leaf, usually because of bad design or workmanship in the construction of the cavity.
Moisture Vapour Permeability
Vapour permeability is important since vapour trapped within cold walls will form condensation on surfaces leading to mould and rot and causing health risks. Vapour absorbed within the wall will condense and cause damp walls. Permeable materials will allow the damp to escape provided there is ventilation to carry it away. It is measured in accordance with BS EN 772-15 (2000) for AAC or BS EN ISO 12572 (2001) for other materials.
Sound transmission is another parameter that is very dependent on density, because it is the mass of the wall that is critical. Generally, the greater the mass, m, of a wall the more effective it is at attenuating (absorbing) the sound passing through it. A typical equation for the resistance, R, in decibels, dB, given in BRE Digest 273 (1983) is:
R = 14.3 log m + 1.4dB
Any holes will allow the sound to bypass the wall so that wet plastered walls where any minor perforations or imperfections are repaired by the plaster layer are more effective than dry-lined walls (walls covered with plasterboard).
There are additional techniques to try to cut out sound such as air gaps and cavities with fireproof blankets hung in them, which will damp out some frequencies. More details on principles and basic values are given in Diamant (1986), BRE Digest 337 (1988), and BRE Digest 338 (1989).
Fire resistance of masonry is an important characteristic since it has long been recognised that it is a very effective material for resisting and preventing the spread of fire. In the UK this is now enshrined in various building regulations dating from the Great Fire of London. Masonry’s effectiveness in this role is due to the following characteristics:
- a relatively low thermal conductivity, which prevents the fire spreading by inhibiting the rise in temperature of the side of a wall away from the fire
- a relatively high heat capacity, which also inhibits the rise in temperature of the side of a wall away from the fire (this is especially true of concretebased products that contain a lot of loosely bound water, which absorbs heat while being boiled away)
- zero flammability and surface spread of flame
- refractory properties that mean that it retains its strength and integrity up to very high temperatures, approaching 1000°C in some cases.
These properties mean that it does not catch fire itself, it inhibits the spread of fire by conduction and radiation and it is not easily breached by the fire. Fire-resistant insulating finishes such as vermiculite plaster improve performance still further. It has been shown to resist fire for between half an hour and six hours depending on material, thickness and finishes.
The classic data on masonry walls are contained in Davey and Ashton (1953). Relevant Codes of Practice are BS EN1996-1-2 (2005) and BS5628: Part 3: 2005 and BRE Digest 487-3 (2004).