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Insulating Older Buildings

Historic buildings were typically built using materials to hand, with solid walls of stone or brick bound with a lime or earth mortar. Without a...

Historic buildings were typically built using materials to hand, with solid walls of stone or brick bound with a lime or earth mortar. Without a cavity, as the mortar was porous the walls were simply made as thick as necessary to stop water coming through. Lack of insulation and poor thermal properties of the materials themselves meant they retained very little heat, and even with insulated doors, windows and ceilings fitted, older properties are still noticeably colder than more modern buildings.

While insulating the walls of a historic building will make it significantly warmer and reduce heating costs, it is important to use the right materials – For a porous (breathable) solid wall any coating you apply must be breathable or your walls will become saturated.

Why use Cornerstone?

Modern closed-cell systems are very good insulators but often have poor fire-retardant properties and have been designed specifically for cavity-wall construction. As they are impervious they will trap moisture within the wall and ultimately cause severe damage or even failure.

Fibre-board systems should also be avoided, having zero fire resistance (class E) and poor long term water resistance. Anything over 20% moisture content will cause them to swell, and there is a very high risk of failure as ultimately they will become wet and rot, swell and crumble.

Insulating renders are therefore the most appropriate overall, but again not all renders are created equal – many contain cements and/or polymers that are inappropriate for modern buildings, or contain vermiculite which crushes and packs flat when rendering to give very poor thermal properties.

Cornerstone Insulating Render has been designed specifically for building conservation work, is made with pure St. Astier Natural Hydraulic Lime and is highly breathable, conservation friendly, and fully compatible with historic masonry. It contains a specialist expanded aggregate that maintains its structural integrity when applied and will not crush or pack flat.

Its closest competitor on the market is nearly 50% more expensive per bag and 50% less effective an insulation material, making Cornerstone Insulating Render twice as cost-effective overall.
It can be applied as either a single coat or two coats, depending on the level of insulation required, at up to 25mm per coat without the need for a stipple-coat.

It can also be applied as part of a boarded system, using Celenit wood-wool boards as a render carrier to double the level of insulation. Celenit boards have a Class B rating (will char but cannot catch fire), and will not swell, crumble or rot when wet

Thermal Comfort and the benefits of insulation

There is a real need to improve the efficiency of our homes, as around one-fifth of the UK’s total energy consumption is used towards space heating. The benefits of doing so however go well beyond the purely financial.

  • It improves the thermal comfort of your home, reducing temperature fluctuation and temperature variation between heated and unheated parts of the house
  • With older buildings in particular, applying a breathable insulating render also reduces humidity, mould and condensation.
  • And finally it reduces your heating bills and allows you to increase the temperature at which you can afford to keep your house

Thermal Comfort

Thermal Comfort is a standardised measure used by heating engineers, based on at least 8 out of 10 surveyed occupants being satisfied.

Measured under the standards ASHRAE-55 or EN-15251, for a home environment it is mainly dependent on four main factors – activity levels and clothing, room temperature and the temperature of the walls themselves.
Activity levels and clothing levels are highly personal and adaptive, and the biggest impact insulation will have on thermal comfort obviously is on the room temperature (what you set the thermostat to) and the temperature of the walls themselves.

The greater the heat loss through the walls, the colder the wall itself will be. Insulating your house not only allows you to increase the air temperature, it also keeps the walls themselves warmer – reducing temperature differences across the house and reducing the rate at which your house loses heat.

World Health Organisation guidelines recommend an internal temperature of no less than 16 degrees, particularly with people with allergies or respiratory problems, and no less than 20 degrees for the sick, disabled, very young or very old. The generally agreed ideal is 21 degrees. But while recommendations are all well and good, heating is expensive. Older buildings take considerably more energy to heat than more modern ones, and as such are typically kept colder.

Energy Savings

For a typical solid wall construction kept at 18.5 degrees, applying a single coat of Cornerstone Insulating Render would allow you to raise the temperature to 21 degrees for same spend on heating. By applying two coats or a boarded system, energy savings become considerable.

Working out the U-Value of a property is complicated as there are so many variables. To use a general example: If we were to take a average detached uninsulated house with solid walls with a U-value of 2.10 (W/m2K), a 25mm single coat of insulating render would reduce this to 1.47 (W/m2K) while two coats would give 1.13 (W/m2K). We have more examples in the attached PDF or contact us for more figures and test data.

Factoring in contractors’ costs and tax incentives, a single coat of insulating render also costs less to apply than a standard render – if you are already planning to render the inside of your house, use an insulating render.

Moisture Buffering and interstitial condensation

Closed-cell insulation systems such as expanded polystyrene moisture in the building, leading to condensation. In a building without adequate ventilation, interstitial condensation starts to form at an internal Relative Humidity of over 95%, increasing exponentially as humidity increases – the higher the humidity the worse it gets.

Cornerstone Insulating Render, which is fully breathable, is able to overcome this problem and reduce both the length of time for which condensation can occur and the severity of any condensation. While moisture will still be trapped in the building, the insulation is able to absorb and disperse this over time through a process known as buffering. Simply put, it acts in much the same way as a sponge or towel by absorbing moisture during peak periods and then slowly drying, allowing it to pass through the wall without causing any build-up.

With expanded polystyrene, the Relative Humidity internally can be over 95% from October to July, and remains at over 99% for more than half the year. With Cornerstone, Relative Humidity will still rise above 95% as the weather starts to turn, but then drops rapidly.

Heat loss across a building

Heat loss varies hugely with every building, but as a rough rule of thumb in an uninsulated home 35% percent of heat loss is through the walls, 25% percent is lost through the roof, and the remaining 40% lost through doors, windows, floors and draughts.

Draughts and roof spaces are the easiest to address and should be dealt with first. Replacing old doors and windows is more costly but well worth doing, and if possible it’s worth also improving your underfloor insulation.
Insulating the walls as well will significantly reduce the overall heat loss, noticeably improving the warmth, comfort and efficiency of your home.

Properties and Specification

  • Composition – 1:3 NHL2 : lightweight aggregate by volume
  • Thermal Conductivity (independently measured) – 0.123 W/mK (see associated report)
  • Dry Mortar Density – 610 Kg/m3
  • Compressive Strength @ 28 Days (dry bar) – 3.60 N/mm2
  • Vapour Permeability @ 28 Days – 0.71 g/h.m2.mmHg
  • Water Vapour Diffusion Coefficient – 5/20µ (tabulated)
  • Reaction to Fire – Euroclass A1
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