Retrofit, Windows & Extensions Hub
Insulation, new windows and extensions can leave a home damper, hotter or no warmer when done without diagnosis. These guides explain why common upgrades fail and how to get them right.
Book a Diagnostic SurveyEvery upgrade changes how a building handles heat, air and moisture. Sealing without ventilating traps damp; insulating without detailing junctions creates cold bridges. Measuring first — and detailing properly — is what makes the spend pay off.
Recommended diagnosis: Building Physics Assessment
Retrofit Mistakes
All 10 guidesWhy did my retrofit make things worse?
It is a dispiriting and surprisingly common story: money is spent on insulation, new windows or draught-proofing, and the home ends up damper, stuffier or no warmer than before. The work was not necessarily done badly — but it was almost always done without first measuring how the building actually behaves. A retrofit that ignores moisture, airflow and the order of operations does not just underperform; it can move problems around and create new ones.
Read the guideDid insulating my walls cause damp or mould?
Wall insulation should make a home warmer and drier, and done correctly it does. But damp or mould appearing after insulating walls — especially internally, and especially on older solid-wall homes — is a recognised failure mode. It happens when the insulation traps vapour in the wall, leaves cold thermal bridges where moisture condenses, or reduces ventilation. The cause is in the detailing and the building physics, not the idea of insulation itself.
Read the guideDid making my house airtight cause condensation?
Sealing draughts and fitting airtight windows is sound building physics — but only when ventilation is provided to match. Airtightness without a ventilation strategy is one of the most common retrofit mistakes: the home stops losing heat through leaks, but it also stops losing the moisture it produces, so humidity rises and condensation and mould appear. Airtightness and ventilation are two halves of the same measure, and doing one without the other causes the very problems it was meant to prevent.
Read the guideDo damp-proof injections actually work?
Chemical damp-proof injection is one of the most-sold damp treatments in the UK, and one of the most disappointing — not because the chemistry never works, but because the damp it is sold to treat is usually not rising damp at all. When low-level wall damp is actually condensation, penetrating damp or a bridged damp course, an injected chemical course does nothing about the real source, and the damp returns. Whether injections work depends almost entirely on getting the diagnosis right first.
Read the guideWhy do quick fixes like anti-mould paint keep failing?
Anti-mould paint, plug-in dehumidifiers, a bigger boiler, mould sprays, sealant around a window — these are the quick fixes homes reach for, and they share a common fate: they keep failing because they treat the symptom while the cause carries on. A cold, humid surface that grows mould will keep growing it under the paint; a home that loses heat faster than it is made will stay cold with any boiler. The fix that lasts is the one aimed at the underlying building physics, not the visible symptom.
Read the guideWhy is my cavity wall insulation causing damp?
Damp appearing after cavity wall insulation usually means the fill is doing the one thing the cavity was designed to prevent: bridging the gap so water can cross from the wet outer leaf to the dry inner one. This happens when the wall was not suitable for filling in the first place — too exposed to driving rain, a narrow or debris-filled cavity, or an outer leaf already letting water in — or when the insulation was installed badly, leaving slumped or bridged material. The result is damp patches and mould on internal walls that were dry before. It is a building-physics failure, and the fix is to diagnose what is bridging the water and, often, to extract the insulation and put the wall right.
Read the guideWhy didn't my new insulation make my house warmer?
Spending on insulation and feeling no warmer is a common and frustrating outcome — and it almost always means the insulation, though installed, did not address where the home actually loses heat. The biggest losses might have been somewhere you did not insulate; the new insulation might have left gaps, thermal bridges or air-leakage paths that short-circuit it; or the dominant problem might have been draughts rather than poor insulation in the first place. Heat finds the weakest path, so insulating one element while leaving the real losses open changes little. The way to get the improvement you paid for is to find where the heat goes first, then insulate and seal accordingly.
Read the guideWhy is my house damp after insulating the loft?
Finding damp or condensation appear after insulating the loft is a classic retrofit surprise, and it has two common mechanisms. First, insulating and sealing the ceiling reduces the air leakage that used to ventilate the home through the loft, so indoor humidity rises and condenses in the rooms below unless ventilation is added. Second, the loft itself is now colder — the insulation keeps the house's heat out of the loft — so any moist air that still reaches the loft condenses on the cold roof timbers, and if roof ventilation was blocked by the new insulation at the eaves, the loft can become damp and musty. Both are about moisture and ventilation, not a fault in the insulation.
Read the guideCan new windows make condensation worse?
Yes — new windows can make condensation worse, and it is a common and counter-intuitive outcome. Old, draughty windows let a lot of moisture-laden air escape; replacing them with well-sealed units stops that accidental ventilation, so indoor humidity rises. The condensation that used to form on the cold single-glazed windows (where it was visible and wiped away) now forms instead on the next-coldest surfaces — external walls, corners, behind furniture and in unheated rooms — where it is hidden and feeds mould. The windows did not create the moisture; they removed the ventilation that was managing it, so the fix is ventilation, not regret over the windows.
Read the guideWhy did sealing my draughts make the air feel stuffy?
If the air feels stuffy after you sealed up draughts, you have run into the central rule of airtightness: sealing the uncontrolled air leakage that used to ventilate a home, without providing controlled ventilation to replace it, leaves the air stale and humid. Those draughts, though wasteful of heat, were carrying away moisture, CO₂ and stale air. Seal them and the moisture and CO₂ build up — the air feels stuffy, windows mist, and mould can follow — unless you add deliberate ventilation. The answer is not to undo the sealing (which wastes heat again) but to ventilate properly, so the home is both airtight and well-ventilated.
Read the guideWindows & Glazing
All 13 guidesDo I need new windows, or is it something else?
New windows are one of the most-sold home improvements, and often the first thing blamed for cold, draughty or condensation-prone rooms. But the windows are frequently not the main cause — walls, air leakage elsewhere, ventilation and cold surfaces can all produce the same symptoms. Knowing whether you genuinely need new windows, before spending thousands, is what a measured diagnosis provides.
Read the guideWhy are my windows cold and draughty?
Cold, draughty windows have two distinct parts: the glazing itself running cold, and air leaking around the unit, frame and reveal. Single or older glazing has a cold inner surface you feel as radiant chill, while worn seals and gaps let air through. Telling the radiant cold from the air leakage is the key to fixing the right thing.
Read the guideWill new windows stop condensation and mould?
New windows are often bought to cure condensation and mould, and they can reduce condensation on the glass by giving it a warmer surface. But they do not remove the moisture from the home — and by sealing up draughts they can push condensation onto the next-coldest surface, sometimes making wall mould worse. Whether new windows help depends entirely on whether ventilation is addressed too.
Read the guideWhy do my window frames get damp and mouldy?
Damp and mould around window frames, reveals and sills are usually condensation, not a leak. The reveal and frame run cold, humid room air condenses on them, and mould grows in that persistently damp band. Because it looks like water ingress, it is often misdiagnosed — but the cure is warmer surfaces and better ventilation, not sealant.
Read the guideIs triple glazing worth it?
Triple glazing genuinely improves comfort and reduces heat loss at the window, with warmer internal glass that feels more comfortable to sit beside and less prone to condensation. But whether it is worth it depends on the whole building: in a home still losing most of its heat through uninsulated walls and air leakage, triple glazing is rarely the most cost-effective next step. Its real value emerges as part of a well-sequenced, fabric-first approach.
Read the guideWhy are my windows misted up between the panes?
Misting between the two panes of a double-glazed window is the unmistakable sign of a blown (failed) sealed unit — the airtight seal around the edge has broken down, the dry gas or air inside has been lost, and moisture is now condensing inside the cavity where you cannot wipe it away. Unlike condensation on the room-side surface, this is not a ventilation problem; it is a failed component, and the only real fix is to replace the glass unit, though usually not the whole window.
Read the guideWhy is there condensation on the outside of my windows?
Condensation on the outside of your windows — dew or misting on the external pane, usually on clear, still mornings — is almost always a sign that your glazing is working well, not a fault. Because efficient, well-insulated windows let very little heat escape outwards, the outer pane stays cold enough to drop below the dew point of the night air, and dew forms on it just as it does on a car roof or the lawn. It clears as the sun warms the glass, and it generally means the opposite of a problem.
Read the guideIs secondary glazing worth it?
Secondary glazing — a discreet, independent pane fitted on the room side of your existing window — is often very good value, and in some homes it is the better choice than replacement double glazing. It can markedly cut heat loss and draughts and is outstanding for noise, while keeping original windows intact, which makes it ideal for period and listed properties. Whether it is worth it depends on what you are trying to solve and on the condition and significance of your existing windows.
Read the guideWhy are my sash windows so draughty?
Traditional sliding sash windows are draughty largely by design: they work by two sashes sliding past each other in channels, and those moving joints, together with the original counterweight boxes, leave gaps that let air leak in and out. Decades of paint, shrinkage and wear usually make it worse. The good news is that a well-made sash window can almost always be draught-proofed and kept — you rarely need to replace it to stop the draughts.
Read the guideShould I repair or replace my old windows?
Whether to repair or replace old windows is one of the most common — and most over-simplified — decisions homeowners face. The replacement industry presents new windows as the obvious answer to cold, draughts and condensation, but a sound timber window that is draughty and cold can often be repaired, draught-proofed and upgraded for far less, keeping its character and lasting longer than a modern unit. Equally, a window that is rotten, beyond repair or performing very poorly may genuinely warrant replacement. The right call depends on the condition of the existing windows, what is actually making them uncomfortable, and what you want to achieve — which is worth establishing before committing to an expensive replacement.
Read the guideWill new windows make my house warmer?
New windows will usually make a home feel a little warmer — modern glazing has a colder-resistant inner surface and is less draughty — but they are rarely the biggest source of heat loss, so they often deliver less than homeowners expect for the cost. Most homes lose far more heat through their walls, roof and air leakage than through their windows, which are a relatively small area of the total envelope. Replacing the windows while leaving the dominant losses untouched changes little, and can disappoint. Whether new windows are worth it for warmth depends on how much heat your windows actually lose compared with everything else — which is worth measuring before committing to an expensive replacement.
Read the guideHow do I choose the right windows for my house?
Choosing the right windows is not just about picking the lowest U-value or the cheapest quote — it depends on your home's heat loss, the orientation of each window, the home's character and the way the windows are fitted and sealed. The best glazing badly installed, or chosen without regard to solar gain and overheating, can disappoint; while a thoughtful choice matched to each elevation delivers warmth, comfort and light without unwanted summer heat. The right approach is to understand how your windows actually perform in your home before specifying, so the considerable spend matches the building rather than a generic recommendation.
Read the guideWhy do my new windows have condensation?
Condensation on new windows is one of the most common surprises after a window replacement, and where it forms tells you what is happening. Condensation on the inside of the glass usually means the room's humidity is high and ventilation is now too low — because new, well-sealed windows removed the draughts that used to let moisture escape. Condensation on the outside of the glass, by contrast, is a sign the glazing is performing well. Condensation between the panes means a sealed unit has failed. So new-window condensation is rarely a fault with the glass; it is usually a ventilation message, and sometimes good news.
Read the guideExtensions & Conservatories
All 8 guidesWhy is my extension cold and hard to heat?
Extensions are often colder and harder to heat than the original house because they expose more surface to the outside per square metre, usually contain a lot of glazing and roof area, and frequently have thermal bridges where the new structure meets the old. Add heating that was sized for the original home rather than the extension, and the room struggles to stay warm. The causes are specific and measurable — and fixable once identified.
Read the guideWhy is my conservatory too cold in winter and too hot in summer?
A conservatory is too cold in winter and too hot in summer for one underlying reason: it is almost entirely glass and roof with very little insulation or thermal mass. Glass lets heat out rapidly in winter and lets solar heat flood in during summer, with nothing to buffer either, so the room swings to extremes. Making a conservatory usable year-round means reducing those swings — improving the roof and glazing, controlling solar gain and ventilation — or accepting its limits.
Read the guideShould I replace my conservatory roof with a solid roof?
Replacing a conservatory's glass or polycarbonate roof with a solid, insulated roof is often the single most effective way to turn an unusable, baking-then-freezing conservatory into a comfortable year-round room — because the roof is usually where most of the heat is lost in winter and gained in summer. But it only delivers if the rest of the structure can keep up: the glazed walls, the floor and any dwarf walls also need to perform, and the existing base and frames must be able to carry and suit a heavier roof. So it is usually worth it, provided it is done as part of the whole picture.
Read the guideWhy is my single-storey extension so hot in summer?
Single-storey extensions — especially open-plan kitchen-diners — often overheat in summer because they combine the very features that admit and trap heat: large rooflights and glazed doors that pour solar gain in, a big flat or shallow roof fully exposed to the sun, and frequently limited ventilation with glazing on only one side. Heat gets in fast through the glass and roof and cannot easily be purged. It is a design-and-ventilation problem, fixable with shading, roof improvements and better cross- and night-ventilation rather than air conditioning.
Read the guideWhy is there damp or cold where my extension joins the house?
Damp, cold or mould at the line where an extension meets the original house is almost always a junction problem: the insulation and airtightness of the new build do not connect continuously with the old, leaving a thermal bridge and often an air-leakage path right at the join. That cold, leaky line then runs colder than the surrounding surfaces, so it attracts condensation and mould and feels cold. It is a detailing issue at the interface between old and new, not a coincidence, and it is fixed by making the junction continuous.
Read the guideWhy is my orangery or glass extension uncomfortable?
Orangeries and glass extensions are uncomfortable for the same reason they are appealing: they have a large area of glazing, which loses heat readily in winter and admits strong solar gain in summer, so the space swings between cold and hot far more than a conventional room. Glass has a much higher heat loss and far higher solar gain than an insulated wall or roof, so a room dominated by it is hard to keep at a steady, comfortable temperature. The discomfort is a predictable consequence of the construction, and it is addressed by reducing the heat loss and the solar gain — better glazing, shading, a more solid roof, and adequate heating and ventilation — once the dominant problem is identified.
Read the guideWhy is my extension getting condensation?
Condensation in an extension usually comes down to cold surfaces and inadequate ventilation, often concentrated where the extension meets the original house. Extensions present a lot of glazing and a large roof and wall area for their size, so they run cold if the insulation or airtightness is sub-standard; thermal bridges at the junction with the existing building create cold lines where condensation and mould form; and a new, well-sealed extension can trap moisture if it was not given proper ventilation. The fix is to find which of these dominates — cold fabric, bridging at the join, or a ventilation deficit — and address it, rather than just wiping the symptom.
Read the guideHow do I make my extension warm and comfortable?
Making an extension warm and comfortable year-round means getting four things right together: good, continuous insulation; a well-detailed, airtight junction with the original house; the right glazing for the amount and orientation of glass; and heating sized to the extension's real heat loss. Extensions are uncomfortable when one of these is weak — thin insulation or a cold junction makes them cold and condensation-prone in winter, while too much unshaded glazing makes them overheat in summer. The route to comfort is to assess the extension's heat loss and gains and fix the weak links, rather than simply turning up the heating in winter and suffering the heat in summer.
Read the guideOverheating & Summer Heat
All 8 guidesWhy does my house overheat in summer?
A house overheats in summer when more solar heat enters during the day than can be removed at night. Sun through unshaded glazing and a hot roof pours energy in; if the home is then unable to purge that heat overnight — because windows stay shut, ventilation is limited or the structure holds onto warmth — temperatures climb day after day. Overheating is a building-physics balance of solar gain in versus heat out, and it is increasingly common in modern, well-insulated and glazed homes.
Read the guideWhy is my bedroom too hot at night?
A bedroom that is too hot to sleep in usually combines three things: it gains a lot of heat during the day, it is high in the house where heat collects, and it cannot purge that heat at night. Being upstairs, often under a sun-heated roof, with windows kept shut for security or noise, the room stores the day's warmth and holds it through the night. Cooling it means cutting the daytime gain and enabling secure overnight ventilation.
Read the guideHow do I keep my house cool without air conditioning?
You can keep a home comfortable in summer without air conditioning by working with building physics rather than against it: stop the heat getting in during the day, and purge the heat that does build up at night. In practice that means shading the windows that gain the most sun, keeping the fabric (especially the roof) insulated, closing up against the hot daytime air, and ventilating hard once the outside cools in the evening. Done in the right order, these passive measures keep most UK homes comfortable through all but the most extreme heat.
Read the guideDo I need external shading, or will internal blinds do?
For controlling overheating, external shading is far more effective than internal blinds, because it stops the sun before it passes through the glass, whereas an internal blind only intercepts the heat once it is already inside the room. Internal blinds help with glare and privacy and offer some benefit, but they cannot match external shutters, blinds or overhangs for keeping a room cool. So if your aim is to stop a room overheating in summer, the honest answer is that external shading is what really works.
Read the guideWhy is my top-floor flat or loft room so hot in summer?
Top-floor flats and loft rooms overheat in summer because they sit directly beneath the roof, which is the surface of the building exposed to the most intense solar heat — and they usually have the least insulation and the poorest night-time cooling above them. The roof absorbs a great deal of heat through the day and radiates it down into the room; rooflights and dormer windows add direct solar gain; and the heat that builds up has nowhere to escape because hot air collects at the top of the building and there is no purge route. It is a combination of solar gain through the roof and glazing, weak insulation, and a lack of night ventilation — all of which can be addressed once the dominant cause is identified.
Read the guideWill better insulation make my house hotter in summer?
A common worry is that insulating a home will make it hotter in summer by 'trapping heat' — but this misunderstands how insulation and overheating work. Summer overheating is driven by solar gain (sunlight entering through windows and heating the roof) and by an inability to get rid of that heat through ventilation, not by insulation. Insulation actually slows heat from passing through the fabric in both directions, so a well-insulated home gains heat from the hot outdoors more slowly and, once cooled, stays cool longer. The real causes of a hot house are unshaded glazing, a sun-baked roof and a lack of night ventilation — and insulation, far from causing the problem, is part of the solution.
Read the guideWhy does my bedroom stay hot all night?
A bedroom that stays hot all night has absorbed more heat during the day than it can release after dark, and cannot purge it because night ventilation is inadequate. Bedrooms — especially upstairs and under the roof — gain heat through sunlit windows and a hot roof during the day, and store it in the walls, ceiling and contents; if the room is closed up and there is little cool night air flushing through, that stored heat keeps radiating into the room overnight, so it never cools enough to sleep comfortably. The fix is to keep the heat out by day and purge it by night, not to add air conditioning.
Read the guideHow do I stop my loft conversion overheating?
Loft conversions overheat in summer because they combine the worst gains in the house: a large roof surface baking in the sun above thin insulation, roof windows (rooflights) that admit intense overhead sunlight, and a position at the top of the house where heat collects. The result is a room that becomes uncomfortably hot and stays hot. The way to cool it without air conditioning is to cut those gains — improve the roof insulation, shade the rooflights externally — and to purge the stored heat with night ventilation, so the conversion becomes comfortable in summer as well as warm in winter.
Read the guide