A breathing wall is one that allows air, moisture, odours, and the toxins carried in the air, to move from the inside of the house to the outside. Prior to the 18th century, virtually all buildings were breathable as they were made from natural materials, but they were largely uninsulated.

As we moved to brick and cement construction, and the levels of insulation we needed increased, rigid foams, glass, and mineral fibre quilts were developed to provide insulation at a reasonable price.

What to Look For

Breathable insulation is made from natural materials — hemp, sheep’s wool, cotton, wood fibre, cellulose and flax. It comes in many shapes and sizes and the different materials have different properties.

Natural insulation is available as rigid boards, semi-rigid batts, flexible quilts and loose beads. It is a matter of selecting the right material for a particular application, and in that respect, it is the same as conventional insulation.

Hemp batts

Hemp batts or rolls offer good carbon sequestration

Why People Use Natural Insulation

People use natural insulation for many reasons. Conventional insulation materials may or may not gas-off (give off toxins), may or may not affect allergies and asthma, and may or may not degenerate quickly. But there is no question that natural materials will do any of these things. They also come from a renewable source (making them sustainable), and some can even sequester CO2.

What breathable insulation does, is maintain the breathability of the wall as a whole. That in turn allows the insulation to wick moisture away, helping to prevent internal condensation and mould growth, and thus helping to extend the life of the timber in the surrounding construction.

The smell or ‘atmosphere’ of homes built using natural insulation is often perceived as more pleasant than a conventionally insulated equivalent.

Airtightness

The air that gets into and out of a house comes by way of poorly sealed joints, cracks and the like — the things we work on to achieve good airtightness levels. The maximum natural air pressure across a wall is some 30 Pascals. To get any appreciable oxygen increase requires a pressure difference in excess of 700 Pascals.

The rules around maintaining airtightness (and thereby good thermal performance) remain the same, irrespective of the insulation material. Cracks, joints and penetrations still need to be sealed. A vapour control layer is still needed but for a breathable wall, it needs to be an air permeable layer too — Intello Plus, or similar. This allows air to move outward, carrying vapour, toxins, etc., but maintains the airtightness.

Using breathable insulation will not necessarily give you a breathable wall. To allow the transport of vapour, toxins, and odours, the complete wall needs to be constructed entirely of breathable materials — from the internal paint to the external finish (such as lime render).

Warmcell being wet-blown into a wall cavity

Paper based insulation like Warmcell can be wet blown into a cavity

Know Your Values

U value — the rate at which heat passes through a fabric. The U value for a wall includes everything that makes up that wall. Expressed in W/m²/°C, the lower the value, the less heat will escape through the wall.

R value — a material’s thermal resistance. This is related to the thickness of the material, so in this case, the higher the figure the better the insulation. Each element of a wall will have an R value and those come together to enable the calculation of the U value.

K value — the thermal conductivity of the material. This is an absolute figure for the material, irrespective of thickness, and allows direct comparison of materials.

Qualities of Insulation

This table gives a comparison of breathable and non-breathable insulation materials based on performance and probable cost. Don’t assume natural insulation is expensive — it is price comparable to conventional alternatives. Achieving the same U value will require a slightly thicker wall, and that will have a cost implication.

Material K value Cost per m² at 100mm thick (1) Thickness needed for U value 0.2W/m² (2)
Breathable:
Sheep’s wool 0.038 to 0.043 £7 to £8 180mm
Wood fibre 0.038 to 0.048 £4 to £10.50 180mm
Cellulose 0.038 to 0.40 £2 to £8 175mm
Hemp 0.038 to 0.040 £6.50 to £9.50 175mm
Non-breathable:
Mineral Wool 0.033 to 0.40 £4.50 170mm
Expanded polystyrene 0.032 to 0.036 £4.50 150mm
Polyurethane 0.020 to 0.033 £13 to £15 105mm
Polyisocyanurate 0.020 to 0.033 £13 to £15 105mm

(1) Price variation is for different manufacturers and product types, ie. batts, boards, roll, etc. (2) Assumes timber frame construction

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