Attic and roof insulation basics
Applies to: Assumes a pitched roof over a top-floor ceiling, with the roof space either unused or partly converted.
Last worked on 2014-11-19
The first decision in a roof is not how much to add but where the boundary sits: at the ceiling or at the slope. Everything else follows from that.
Cold roof or warm roof
A cold roof puts the insulation on the top-floor ceiling and leaves the roof space outside the heated volume, ventilated and cold. It is simpler, cheaper and easier to inspect, and it is the right answer whenever the roof space is not being used.
A warm roof puts the insulation in or on the slope and brings the roof space inside. It is necessary if the space is habitable, and it is far less forgiving: the timbers are now inside the insulated envelope and any moisture that reaches them has fewer routes out.
Ventilation is part of the assembly
A cold roof only works if the space above the insulation stays ventilated. Adding insulation and simultaneously blocking the eaves ventilation is a common and damaging combination: the space gets colder because it is no longer warmed from below, and it stops drying because the air has stopped moving.
Insulation pushed hard into the eaves is the usual cause. Keeping a clear path at the eaves, above the insulation, is not optional in a cold roof.
The hatch and the boarding
The loft hatch is a hole in the insulation layer and in the air barrier at the same time, at the highest point of the house, which is where the pressure difference is greatest. An insulated, draught-sealed hatch is worth a surprising amount relative to its size.
Boarding a loft for storage directly onto the joists compresses the insulation to joist depth and creates a bridged, colder platform. If the space must be boarded, the boarding needs to be raised clear of the insulation on a cross-batten layer.
| Consideration | Cold roof | Warm roof |
|---|---|---|
| Insulation line | At the top-floor ceiling | At the roof slope |
| Roof space | Outside the heated envelope | Inside the heated envelope |
| Ventilation | Required above the insulation | Required only if the build-up calls for it |
| Timbers | Cold and ventilated | Within the insulated assembly |
| Use of space | Storage only | Habitable |
| Tolerance of error | High - faults are visible and reachable | Low - faults are concealed |
Scroll the table sideways for the remaining columns.
Roof insulation is the cheapest heat you will ever save, which is why it is usually done first and usually done carelessly. The care is nearly all at the edges: the eaves, the hatch, and wherever the layer has to stop.