A new extension is built to a far higher standard than the house it is attached to. It is better insulated, much more airtight, and usually far more heavily glazed. Those three things are all improvements individually. Together, and undesigned, they produce the three complaints we hear most often about new extensions: it is stuffy, there is condensation somewhere there never used to be, and it is unbearable on a sunny afternoon.
None of these are inevitable. All three are decided at concept stage, months before anyone is on site.
Failure one: sealed without being ventilated
Older houses ventilate themselves by accident. Gaps around floorboards, unsealed loft hatches, open chimneys and leaky windows move a continuous, uncontrolled volume of air through the building. It wastes heat, but it also removes the moisture that cooking, washing, drying and breathing produce.
A new extension does not leak. Sealed joints, taped membranes, insulated floor slab, good windows. The moisture generated inside it — and a kitchen extension is usually where most of a household's moisture is generated — has nowhere to go unless it is deliberately removed.
What follows is predictable: condensation on the coldest surfaces available, mould in corners and behind furniture, and air that feels stale by the evening. The coldest surfaces are frequently in the old part of the house, so the problem appears somewhere other than the new work and gets blamed on the old building.
The fix is that ventilation has to be designed, not assumed. The options run from continuous mechanical extract in the wet rooms with background ventilation elsewhere, up to a whole-house mechanical ventilation with heat recovery system, which supplies filtered fresh air and recovers heat from the air it removes. Which is appropriate depends on how airtight the building actually is, how much of the house is being altered, and what the layout allows.
Ventilation is covered by the Building Regulations, and the requirements that apply to your project depend on the work being done. They should be confirmed for the specific scheme rather than carried over from a previous one.
Failure two: ventilation designed on site
Where a mechanical system is intended, the second failure is trying to install it after the drawings are finished.
Mechanical ventilation with heat recovery needs a route for every duct, a location for the unit that is accessible for filter changes and reachable for condensate drainage, insulated ducting where it passes through cold spaces, and terminals in sensible positions. Those routes compete with joists, steels, drainage, downlights and the ceiling void — the same void everything else wants.
Designed at technical stage, this is a drawing exercise. Improvised on site, it produces flexible ducting squashed round corners, long runs with high resistance, a unit in a cold loft, and a system that is noisy and does not deliver the airflow it was specified for. The homeowner then turns it down, or off, and the moisture problem returns.
Two further points that get skipped:
- Commissioning. A ventilation system has to be balanced and its flow rates measured after installation. An uncommissioned system is a guess.
- Maintenance. Filters need changing. If the unit is somewhere nobody can reach, they will not be.
Failure three: overheating, designed in at concept stage
This is now the most common comfort complaint in new extensions, and it is almost always caused by glazing.
The brief for a rear extension is usually light. That translates into large glazed doors, a wall of glass, a long rooflight or several. Then the extension is oriented — because the garden is where it is — towards south or west, and there is nothing to shade it.
The physics is unforgiving. Solar gain through glass is large; it is larger through horizontal glazing than vertical, because a rooflight faces the sky all day; and a well-insulated, airtight room is very good at keeping the resulting heat exactly where you do not want it. A room that is beautiful in March is intolerable in July.
Overheating is now a Building Regulations consideration for new dwellings, and while extensions and conversions are treated differently, the physics does not care what category the work falls into. The mitigations are the same either way.
In order of effectiveness:
- Shade the glass from outside. Once solar radiation is through the glass, it is in the room. External shading — a brise soleil, a projecting roof or overhang, deep reveals, an external blind, a pergola with planting, a well-placed tree — stops the heat before it arrives. Internal blinds and curtains are far less effective, because they intercept heat that is already inside.
- Size and place the glazing deliberately. Glass on a north elevation gives you even light and very little heat. West-facing glass gives you low afternoon sun that no overhang can shade, and it is the hardest orientation to manage. A rooflight gives you the most heat per square metre of any orientation. None of this means avoiding glass; it means putting it where it earns its place.
- Specify the glass for the orientation. Solar control coatings meaningfully reduce heat gain, at some cost to visible light and, on some products, to the appearance of the glass. Different specifications on different elevations of the same extension is normal and sensible.
- Provide a way to purge heat. A room needs to be able to dump heat in the evening. That means openings high and low, or on two sides, to drive cross ventilation and stack effect. It also means those openings have to be usable — secure enough to leave open overnight, and not compromised by noise or insects. An openable rooflight at the highest point of a room is disproportionately effective.
- Use thermal mass where you can. An exposed concrete floor, a solid screed or masonry left visible absorbs heat during the day and releases it at night. It flattens the peak. This works only if the building can be purge-ventilated at night to dump the stored heat.
Note the order. Shading and glazing decisions are architectural and free to make at concept stage. Cooling equipment is expensive, uses energy, and is what you buy when the first four were not addressed.
Our green house conversion interior in South Norfolk is an instructive case, because a glasshouse is the overheating problem in its purest form — the conversion had to resolve solar gain and ventilation before it could resolve anything else. On the luxury bespoke villa in Norwich, a double-height entrance and stair lobby raises the same question in a different way: tall volumes stratify, and hot air collects at the top unless there is a route out.
The junction between old and new
The specific technical difficulty in an extension, as opposed to a new house, is the join.
The insulation line and the air barrier line both have to be continuous, and both have to terminate somewhere against an existing building that has neither. Where they stop badly you get a cold bridge — a path for heat to escape and a cold surface for moisture to condense on. Typical locations are the junction of new wall and old wall, the point where a new roof meets the existing one, around the perimeter of a new floor slab, and at the head and jambs of any new opening cut through the original wall.
The way to deal with this is to draw it. A continuous line marked on a section, showing where the air barrier runs and how it is sealed to the existing structure, is one drawing and it prevents most of the problems. It also gives the builder something to work to instead of a decision to make with a can of foam.
Where the extension is to a period building with solid walls, this becomes a moisture question as well as a heat one, and it needs handling with more care than a modern house — insulating solid walls carries real risks that a standard specification does not address.
Decide it at Stage 2
All of this belongs at RIBA Stage 2, Concept Design, where we set the sustainability strategy — orientation, glazing, shading, ventilation approach and how the building is expected to behave in summer as well as winter. By Stage 4, Technical Design, it is being detailed and coordinated with the structure and services. On site it is being built, not decided.
An extension designed this way costs very little more than one that is not. An extension that overheats costs whatever the retrofitted shading and cooling costs, and it never works as well as it would have.
Talk to us before the drawings
Our free initial consultation covers site constraints, orientation, what you want the space to do, the planning and technical challenges and indicative timelines.
See our house extensions and architectural design services, or book a free initial consultation. Call +44 (0) 1508 830 310 or email info@archidite.com.