How do I tell if my roof space is properly ventilated?

Warm moist air always leaks into a roof space from the house below, however carefully the ceiling is sealed. Ventilation is what carries it away before it condenses on cold timbers and membrane. Since loft insulation got deeper and houses got more airtight, roof spaces run colder and the ventilation matters more than it did when the standard was 100mm and a draughty hatch.

Safety

What you'll need

• Binoculars
• Head torch
• Crawl boards
• Rigid eaves trays (optional)
• Mirror on a stick (optional)

Step by step

1. Work out whether you have a cold roof or a warm roof.Insulation at ceiling level with an unheated space above is a cold roof and needs ventilating. Insulation between and under the rafters, with the space below heated, is a warm roof and follows a different design entirely.
2. Look at the soffits from the ground.A continuous slotted strip, a run of round vents, or an over-fascia vent at the top of the fascia. If the soffits are solid boarding with nothing in them, the eaves are not ventilated.
3. Check the soffit vents are not blocked.Paint, cobwebs, wasp nests, birds' nesting material and insect mesh clogged with dust all close them off. A soffit replaced during a fascia job without vents is very common.
4. Go into the loft and look at the eaves from inside.You should be able to see light or feel air at the eaves. Insulation pushed hard into the angle blocks the path completely, and this is the most frequent single cause of an unventilated loft.
5. Fit eaves trays if the insulation would otherwise choke the gap.Rigid plastic trays clip between the rafters at the eaves and hold a clear air path over the top of the insulation. They are cheap and they are the correct detail for a deep-insulated loft.
6. Look for a high-level outlet.Air needs to leave as well as enter. A ventilated dry ridge, ridge vents, tile vents high on the slope, or gable end vents. A loft with eaves vents and no high-level outlet ventilates poorly because there is nothing driving the flow.
7. Check the hipped ends and the corners.Hipped roofs and dead corners ventilate worst because there is no gable and the air path is longest. If the condensation is at one end, that is usually why, and a tile vent high on that slope is the usual remedy.
8. Test it on a cold, still morning.Go up before the day warms. Beads or frost on the underside of the membrane and on nail points mean the moisture is not being carried away, whatever vents you have counted.
9. Fix the moisture side too.Ventilation removes what arrives. Sealing the hatch, sealing cable and pipe penetrations, and making sure no extract duct discharges into the loft reduce what arrives in the first place. Do both.

Tips

• A roof with old bitumen felt depends completely on ventilation because the felt itself cannot pass vapour. A breather membrane is more forgiving but is not a substitute for the vents.
• If a loft conversion or a room-in-roof has been formed, the ventilation design changes completely and any remaining void needs its own path. Do not assume the original eaves vents still serve it.
• Gable vents alone give poor cross-flow in a long loft. Eaves plus ridge is the arrangement that actually moves air.

Common mistakes

• Insulating right out to the eaves to avoid a cold strip — It blocks the only air inlet the roof has. The cold strip at the very edge of a ceiling is a small penalty compared with a roof space that cannot dry.
• Assuming a modern breather membrane means no ventilation is needed — Membrane specifications state the ventilation they assume. Some allow reduced ventilation, none allow none, and the assumption in the specification is rarely what got built.

Questions people ask