Vapour Permeability in Older Buildings
'“Old buildings need to breathe…”
It’s likely you’ve heard someone utter this phrase over the years. Traditionally, air circulation is used to remove humidity and prevent mould growth; but it’s only a half-truth, and mis-understanding the concept can lead to draughty, inefficient and uncomfortable houses.
Heat energy moves in three main ways; convection, conduction and radiation. In low-energy buildings we want to eliminate convection to ‘unconditioned’ spaces (outdoors), or the efficiency of our insulation will be severely reduced. We do this by air-sealing the building to eliminate draughts, essentially creating a sealed bubble within the structure. Our insulation can then perform at its best to reduce conduction of heat to the outside, removing cold spots within the building where condensation and mould might form. Excess humidity in the air is then removed by mechanical ventilation and heat recovery to retain the heat you’ve paid for, while bringing fresh air into your home.
The ability of a building to ‘breathe’ is really about water vapour flow (moisture vapour). This moisture vapour needs to be able to flow through the walls (assembly) so that it dries adequately. The more insulation we add in order to retain heat, the more more important this vapour flow becomes. However, just increasing airflow across a surface is risky, as potentially humid air can add its own moisture into the equation, and with it the possibility of interstitial condensation and freeze-thaw damage.
Insulation can be broadly categorised in one of two ways; vapour-permeable and vapour-impermeable. A vapour-permeable material will allow moisture vapour to dry to either side of an wall, whilst vapour-impermeable insulation will create a barrier to moisture vapour.
Vapour-Permeable
Wood-fibre insulation
Sheep’s wool insulation
Mineral wool insulation
Open-cell spray foam
Recycled denim insulation
Blown-in cellulose
Aerogel
Vapour-Impermeable
Polyisocyanurate insulation (PIR)
Expanded polystyrene insulation (EPS)
Extruded polystyrene insulation (XPS)
Closed-cell spray foam
In EnerPHit and other types of retrofits, it’s not uncommon to wish to avoid external insulation strategies in order to retain period characteristics on the façade, and instead use internal insulation. At this point, you might say “Yes, but with all the heat kept inside the building, how will my solid walls stay dry?”. This a reasonable question, as masonry does tend to soak up moisture during the wetter months. Masonry is what is known as a ‘reservoir cladding’, and will safely hold moisture up to a point. This is then either dried by warmth conducting through the brickwork from inside or by the sun’s radiation drying the moisture inward. To determine whether the proposed insulation strategy is suitable for the property, we would carry out a hygrothermal analysis.
We would first look at where the moisture is coming from. Sufficient roof overhangs and maintained gutters/downpipes deal with the bulk of the rain, but wind-driven moisture is still a factor; but by how much? Using a device called a Karsten Tube, we perform a test to see how absorbent the brickwork and mortar joints are. This data, along with site exposure and the local micro-climate is analysed to predict just how much moisture the masonry might be expected to face, and whether or not this might be a problem with the drying potential of the proposed assembly. If highly absorbent masonry is causing concern during the analysis, a ‘brick-cream’ coating may be considered. These specialised creams retain the original look of the brickwork or stone, but increase the water resistance of the wall, thereby reducing the potential wetting of the brickwork.
If internal space is at a premium, you may wish to use external insulation, either all over the property or just at the rear, preserving the street-facing façade. External insulation opens up a greater variety of insulation types that can be used, in conjunction with water-resistant external finishes including render and thin brick slips, to retain the brick finish externally, hiding the insulation beneath.
Of course, day-to-day occupation of a property will result in moisture vapour being created, be it through breathing, cooking or bathing, and this needs to be dealt with safely so as to not risk condensation on cold surfaces. To this end, we would always recommend installing mechanical ventilation with heat recovery (MVHR). An MVHR is a box that extracts stale, humid air from bathrooms and kitchens and exhausts it outside, but not before pulling the heat out of the air. It then uses that heat to warm up fresh, filtered air from outside before delivering it into the living room and bedrooms. By removing the humidity from the air we reduce the potential for condensation which might have led to mould growth.
Insulating and ventilating an older building is a careful balance, which should not be taken lightly, but can lead to a much more efficient, healthy environment for everyone.
To discuss your retrofit options, contact Locke Passive House Consultants today.
