Seal air leaks with continuous plaster, clay slip, lime or fibre-and-clay chinking, because moving air carries heat past insulation: a 52 cubic metre room leaking 2 air changes per hour loses about 686 watts at a 20 degree temperature difference, while 0.5 changes lose only 172 watts. Never seal a room that has a fire or stove without a safe flue and a separate air supply, because trapped combustion gas is lethal.
Part of Passive Solar Building Design
Closing cracks, joints and openings in a building with plaster, clay, lime and fibre so insulation is not bypassed by moving air.
Gaps Matter More Than Thickness
The basic insulation page says gaps matter more than thickness (see Thermal Insulation Basics). Air moves through a house because of wind pressure and the stack effect: warm air rises and leaks out near the roof, drawing cold air in at the base. Every leak carries heat. The heat lost this way is about 0.33 x n x V x dT watts, where n is air changes per hour, V is volume in cubic metres, dT is the temperature difference in degrees, and 0.33 is air’s heat capacity in watt-hours per cubic metre per degree (1.2 kg/m3 x 1,005 J/kg K / 3,600 = 0.335).
What Seals Air
- Plaster — earth plaster or lime plaster applied continuously over walls is the air barrier. In a plastered bale wall, the plaster, not the bale, stops the air. See Plaster and Lime Rendering and Lime Mortar.
- Clay slip — thin clay and water brushed into cracks and over surfaces.
- Wattle and daub infill — daub pressed from both sides. See Wattle and Daub.
- Chinking — clay and straw or moss, wool or flax packed into the gaps between logs or boards.
- Mortar — lime or clay mortar in all masonry joints.
- Gaskets — strips of felt or hide around door and shutter edges.
What You Need
- Clay or lime, fibre and water — mixed to a workable paste.
- A thin strip of cloth or light thread — to find draughts (see below).
- Time on a windy day — to find the leaks.
Method: Find the Leaks, Then Close Them
Step 1: Find them on a windy day Hold a light thread, a feather or a strip of cloth near joints, edges and openings and see which move. Use a wet finger on the back of the hand. Do not use an open flame near dry straw, thatch or insulation.
Step 2: Close from the outside in Fill the largest gaps with fibre-and-clay chinking, then skim plaster over the whole surface so that no joint stays open.
Step 3: Seal the places where materials meet Wall and floor, wall and roof, around doors, windows and any pipe or flue. These are the leaks plaster alone misses.
Step 4: Leave a controlled opening Keep deliberate, closable ventilation openings, and a flue and air supply for any fire.
Worked Example
A room 5 m x 4 m x 2.6 m high has V = 52 m3. Indoor-outdoor difference 20 K.
- At 2 air changes per hour: heat loss = 0.33 x 2 x 52 x 20 = 686 W.
- At 0.5 air changes per hour: 0.33 x 0.5 x 52 x 20 = 172 W.
- Saving: 686 - 172 = 514 W, which over a 12 hour cold night is 6.2 kWh.
That is roughly 6 kWh a night that would otherwise be paid for in fuel, saved by plaster and chinking.
Common Mistakes
- Sealing without ventilation — humidity condenses inside, rotting timber and growing mould.
- Insulating a leaky wall — the air goes round the insulation.
- Cracks reopening — earth plaster cracks as it dries and as the building settles; re-skim in the first seasons.
Safety
A tight house with a fire in it can kill
Burning wood, charcoal or oil uses up oxygen and makes carbon monoxide, which has no smell or colour. Never close up a room with an open fire, charcoal brazier or unvented stove. Every combustion appliance needs a working flue to the outside and a separate way for fresh air to get in, and sleeping rooms must never have an unvented fire. See Indoor Use and Ventilation.
What’s Next
- Learn next: Ventilation & Summer Cooling — the controlled openings
- Combine with: Insulation Strategies — the other half of the wrap