Ventilation and insulation
Roof Ventilation: Airflow Requirements in a Pitched Void
A pitched roof void needs air moving through it continuously, entering low at the eaves and leaving high at the ridge, because the moisture that household air carries upward has to have somewhere to go. Search the entry for roof ventilation, compare it against what is on your own roof, then book a survey. What it covers: Roof void ventilation · Low-level inlet · High-level outlet.
A pitched roof void needs air moving through it continuously, entering low at the eaves and leaving high at the ridge. Looking along the eaves from inside on a bright day settles half of it in ten minutes.
Four ways the two openings are made
- Soffit vent A ventilated soffit board, or inserted vents, providing the eaves inlet.
- Over-fascia vent A vent at the fascia where the soffit cannot be ventilated.
- Ridge vent A ventilated dry ridge roll providing the high-level outlet.
- Tile vent An in-line unit high on the slope where a ridge vent is unavailable.
Air must move through, so both openings are required: an outlet without an inlet produces no flow at all. A breathable membrane reduces the requirement without removing it, and a habitable loft has no cold void to ventilate and is dealt with differently.
4 related pages cover this in more depth: roofline birmingham, roof insulation upgrade during recovering, roof condensation interstitial moisture and Breathable Membranes Under a Roof.
Why does a roof need ventilating?
Warm moist air rises from the house into the roof void. Without ventilation it meets cold surfaces and condenses, wetting the underlay, the rafters and the insulation. Moving air carries that moisture out before it deposits.
The low-level inlet is subject to a continuous opening at the eaves along both sides; the high-level outlet is subject to an opening at or near the ridge letting moist air escape.
Where does the air get in and out?
Air enters through a continuous opening at the eaves along both sides and leaves at or near the ridge. Both openings are required: an outlet with no inlet produces no airflow at all, and the reverse is equally useless.
The blocked eaves shows insulation pushed into the eaves closing the inlet; the soffit vent is ventilated soffit board or inserted vents providing the eaves inlet.
What blocks it?
Insulation pushed or topped up into the eaves is the commonest blockage, closing the inlet the void depends on. Roofline replacement fitted without ventilated boards, and stored belongings piled against the eaves, do the same.
The over-fascia vent involves a vent at the fascia where the soffit cannot be ventilated; the method is a ventilated dry ridge roll providing the high-level outlet. The tile vent involves an in-line vent unit high on the slope where a ridge vent is unavailable.
How can you check yours?
Enter the void on a bright day with the light off, look along the eaves line on both sides, and check whether daylight is visible at the wall head. A continuous dark line means the inlet is blocked.
- Breathable membrane has an interaction of reduces the ventilation requirement without automatically removing it.
- A habitable loft is ventilated differently from a cold void.
- An air entering low and leaving high creates the cross-flow that removes a moisture.
Does a breathable membrane remove the need?
No, not automatically. A vapour-permeable underlay reduces the ventilation requirement, and membranes differ in whether they pass air as well as vapour. The requirement is assessed for the specific roof rather than assumed removed.
- An insulation pushed into the eaves closes the inlet and stops a cross-flow.
- An outlet without an inlet produces no flow at all.
- A breathable membrane reduces the requirement without removing it.
A converted loft is a different question.
Ventilating a room in the roof
Fitting ridge vents while the eaves stay blocked achieves nothing, because air cannot leave what it cannot enter. Topping up loft insulation over the eaves is the single most frequent cause of blocked ventilation, and it was done to help. A room in the roof has no cold void to ventilate and is dealt with entirely differently.
Getting the airflow path checked
Looking along the eaves from within the void on a bright day shows immediately whether the inlet is open.
Roof void, and 6 more
- roof void
- cross-flow
- eaves inlet
- over-fascia vent
- ridge vent
- tile vent
- room in roof
A cold roof works by moving air, not by sealing. Air enters through a continuous twenty-five millimetre gap at each eaves, crosses the void above the insulation, and leaves through a five millimetre continuous outlet at the ridge. The commonest failure is insulation topped up over the wall plate, which blocks the inlet: the path stops, moisture from the rooms below condenses on the underside of the covering, and the timbers wet up with no leak anywhere in the roof.
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