Air Sealing near you

There is no airtightness target that an existing British home must meet. What there is instead is a pair of duties that point the same way. Regulation 4(3) of the Building Regulations 2010 requires that where a building is extended or materially altered, the work leaves it complying with the applicable requirements of Schedule 1 or, where it did not comply already, no more unsatisfactory in relation to that requirement than before. Ventilation is one of those requirements, so work that reduces infiltration cannot lawfully leave a dwelling's ventilation worse than it found it.

The retrofit standards are more prescriptive, and they are worth following even when you are paying privately. The design guidance published for PAS 2035:2023 requires the retrofit designer to assess the adequacy of the existing ventilation system and, where necessary, to specify an upgrade, in either of two circumstances: where the package of energy efficiency measures falls into the relevant categories in Approved Document F Volume 1, or where the retrofit assessment records evidence of condensation or mould growth in the dwelling. It adds that where there is evidence of surface condensation or mould but no insulation or airtightness measures are proposed, and the ventilation does not comply with Approved Document F's checklist for existing dwellings, the ventilation system must still be upgraded.

The same guidance describes what a complete ventilation system has to include before it can be considered adequate: extract ventilation removing moist air from wet rooms, background ventilation letting fresh air into habitable rooms, undercuts beneath internal doors so air can move through the dwelling, and purge ventilation available in each room through an opening window or door. Where any of those are missing, not working, or not to the right specification, the system is treated as inadequate. Approved Document F's own method for counting energy efficiency measures and grading the result is covered in the draught-proofing guide.

The government's 2021 guide to retrofit internal wall insulation is unusually specific about this, and its wording is the most useful thing a homeowner can take into a conversation with a contractor. It requires an air barrier to be installed on the warm side of every internal wall insulation system, and states plainly that this is not the same as a vapour control layer, although some vapour control layers can double as air barriers. It also requires the air barrier to be designed to continue around junctions, edges and corners, with proprietary airtightness tapes, adhesives or paints appropriate to the situation used where different materials meet.

It then lists the products that are commonly used for this and do not form robust and reliable air barriers: duct tape, gaffer tape, aluminium foil tape, expanding foam, silicone sealant, decorator's caulk, dot-and-dab plasterboard, skim finish, vinyl, plywood, flooring and masking tape. That list is worth reading twice, because it covers most of what gets used on a typical job. A continuous barrier made of materials chosen for the purpose is a different thing from a day spent going round with a gun of sealant.

The practical method follows from that. Decide where the barrier line runs before anything is fitted, draw it as a continuous line around the heated volume, and identify every point where it is crossed by a pipe, a cable, a joist or a change of material. Those crossings are the job. The flat areas in between almost look after themselves.

Thermal bypass is defined in the 2021 retrofit guidance as the unintended penetration or circulation of external air on the warm side of the insulation layer, rendering the insulation ineffective. The same guidance reports that allowing gaps within and through the insulation layer can increase heat loss through an insulated wall by up to 300 per cent. That figure is the reason air sealing belongs to the insulation job rather than sitting next to it: a wall can be fully insulated to specification and still lose heat as if it were not.

The workmanship remedies are mundane and specific. Rigid boards should be tightly butted at corners, around openings and to each other, with vertical joints staggered, the installation inspected before it is covered, and gaps greater than 10 millimetres at board joints and at the perimeter packed out; some rigid boards should also be taped, particularly where their face forms the vapour control layer or the air barrier. Flexible insulation in studwork should be friction-fitted, cut slightly wider than the space it fills so that it grips rather than sags.

The moisture consequence sits alongside the heat one. Air leakage into an insulation system carries a large amount of moisture with it and significantly increases the risk of interstitial condensation, which is condensation forming inside the construction where there is an air gap. Sealing the warm side is therefore not only about heat loss: it is the measure that keeps water vapour out of the places where it would condense unseen.

The hard rule first. Any open-flued appliance in the house takes its combustion air from the room, and the permanent ventilation serving it is not a leak to be sealed. Purpose-provided ventilators, air bricks and sub-floor vents serving combustion appliances are covered in the draught-proofing guidance and are a stop item on any air sealing scope of work. If there is a gas fire, a back boiler, a stove or an open fire, its air supply is designed in and has to stay.

Radon is the second case where sealing does something different from what people assume. The UK Health Security Agency's radon service says that simple actions such as sealing around loft hatches, sealing large openings in floors and adding extra ventilation do not reduce radon levels on their own, although combined with other effective measures they can improve the reduction achieved. In other words, a sealing programme is not a radon remedy, and treating it as one in an affected area would be a mistake.

Where radon needs reducing, UKHSA describes a radon sump as the most effective way to reduce indoor levels, particularly under solid floors, with positive ventilation blowing fresh air into the building, usually from the roof space, and natural or active under-floor ventilation as the other standard approaches. If the property is in an affected area, test first, then design the sealing work around whatever radon measure is needed rather than on top of it.

This is a labour and access job, so the quotation tracks how difficult the house is to get at rather than its floor area. What moves it is the number of penetrations to be sealed and how buried they are, whether floors have to be lifted or ceilings opened, the construction of the ground floor, whether a survey with a smoke pen or thermal camera is included, whether the ventilation is being assessed and upgraded as part of the same contract, and how much making good and redecoration follows. Work carried out alongside insulation is usually cheaper per point sealed than the same work done later on its own.

LokalMatch exists to connect you with the trades, not to perform the work. Write down what the house does, what you want fixed and your postcode, and firms covering your area will get in touch. There is nothing to pay as a homeowner; the businesses pay for the enquiries they receive. LokalMatch takes no part in the work itself, sets no prices, and does not vet, rank or recommend anyone who replies, so asking how the ventilation side is being handled is a question to put to each of them yourself.