How many weep vents do you need, and what will they cost?

Count one every 900mm along the damp-proof course, plus at least two over every window and door, no more than 450mm apart. Add vents at 450mm centres along any other cavity trays, and a row at each upper floor level where the wall has cavity trays there. Because homes vary so much in size and number of windows, measure your own walls and use the calculator above.

Every 900mm along a damp-proof tray, under Approved Document C (para 5.5(c)). Over windows and doors, NHBC Standards set a maximum of 450mm apart with at least two per opening. For walls with rising damp, we suggest about every 450mm for mild cases or in every perpend joint, about 225mm, for most cases.

From £1.60 to £2.40 per vent depending on pack size: pack of 10 for £24, the Whole House Pack (50) for £90 and a pack of 100 for £160. Crystal Clear costs 10p more per vent. No VAT is added. UK mainland delivery is £3.99, or free on orders from £50. For 500 or more vents, ask about trade pricing.

Yes. Every rising damp job we have fitted so far has dried out and stayed dry, with the vents doing the work on their own. They cross-ventilate the base of the wall, which dries it, and lower the humidity in the cavity, which breaks the cycle of water evaporating from the damp-proof course or a tray, condensing on cold surfaces such as wall ties and insulation, and running back down. They are fitted from outside in the mortar joints, with no chemicals, no wiring and no running costs. Fit them about every 450mm for mild cases, or in every perpend (about 225mm) for most. We are now running monitored trials, logging moisture, humidity and wall temperature, and will publish the results.

Weep vents help the wall cavity; trickle vents help the rooms. If you see condensation on windows or mould on inside walls, add Active Trickle Vents: we recommend at least two in each window or door frame. Each fits a 16mm hole drilled through the frame, with an inside and an outside part that click together.

Not on tall residential buildings. The vents are plastic, and external wall materials must be Class A1 or A2-s1,d0 on any building with homes or residential rooms and a storey 18m or more above ground (Building Regulations regulation 7(2), England and Wales). The same applies to residential buildings with a storey more than 11m above ground under Approved Document B in England, and by law at 11m in Scotland. Use a non-combustible weep vent there. A fire-rated Active Weep Vent is in development.

A brick perpend (the vertical mortar joint) is chased out and the vent, 10 × 65 × 100mm, is pushed into the slot. The removable insert (patent pending) then lets you check and clear the cavity behind it at any time. See the installation guide for the steps.