Condensation in the loft, and why it is coming up from below
When you see water dripping off the felt, the first thought is always the roof. It is almost never the roof. It is warm wet air escaping upwards out of your house, meeting the coldest surface in the building. Here are the published ventilation figures you can check your own roof against, and what five specific patterns usually mean.
By The Damp GuyLast updated
The short version
A loft is a cold box with a warm wet house underneath it, so the roof covering is usually the coldest surface in the building. Water on the felt is almost always moisture that came up from your rooms, through the hatch, the downlights and the holes round the pipework, rather than water that came down through the tiles. Which means the fix runs in one order: seal the ceiling, clear the ventilation you already have, then add more only if the roof is short of the figure it was designed around. That figure is published. For a cold loft with traditional felt at a pitch over 15 degrees it is 10,000 mm² of free area per metre of eaves, which is the same area as a continuous 10mm slot. Fans and dehumidifiers come last, because they work on the void while the cause is the leak into it.
What the pattern you are looking at usually means
Five things people describe, and the physical reason behind each. These are what a pattern usually means and what to go and check. Nobody can tell you what is wrong with your roof without standing in it.
| What you are looking at | What that pattern usually means | What to go and check |
|---|---|---|
| Water on one slope, the other bone dry | A cold surface problem rather than a leak. Rain does not care which way a roof faces. Sunshine and a clear night sky do, and the difference between the two slopes in December is large. | Which way the wet slope faces. Then whether the eaves are clear on that side and blocked on the other, because that produces the same picture for a completely different reason. |
| It started after the loft was insulated | The loft got colder, which is exactly what the insulation was fitted to do. The ventilation it already had was sized for a warmer roof space. | Whether the installer pushed the insulation into the eaves. There should be a 25mm clear airway above it, held open by a spacer. |
| Beads of water on the underside of the felt | The underlay is usually the coldest surface in the void, and if it is traditional bitumen felt it is also impermeable, so vapour has nowhere to go but back onto it. | Which underlay you have: bitumen felt, or a modern breathable membrane. The ventilation figure your roof needs is different for each, and so is the fix. |
| It is dripping onto the insulation | Drips that are widespread and follow a cold snap are condensation. A leak is local, tracks one path, and follows the rain instead. | One spot or all over, and which it followed: the last hard frost, or the last downpour. Go up twice and compare. |
| It is a new build in its first winter | Construction moisture. NHBC says up to 8,000 litres of water goes into a house as it is built, and that roof-space condensation may show up during the first heating season because of it. | How old the house is. NHBC expects a new home to reach a moisture-balanced state within 18 months to 2 years, and says most roof-space cases are temporary. |
Every one of those five is a real search. People type them because the thing they are looking at does not match the advice they have been given, and the advice they have been given is almost always to improve the ventilation, with no figure attached.
The reversal is worth stating once, plainly, because it is the opposite of what a drip looks like. Water is not getting in. Water is getting out of your house, upwards, and stopping at the first cold thing it meets. That is the same mechanism as the water on the inside of your windows and the same one behind mould on a wall that never feels wet. The loft version is worse only because nobody goes up there for eight months at a time.
The ventilation figures your roof was designed around
Free area at the eaves, in square millimetres per linear metre. These are the numbers almost nobody gives a homeowner.
| What the roof is | At the eaves | Same as a gap of | At high level |
|---|---|---|---|
| Cold loft, impermeable felt, pitch 10 to 15 degrees | 25,000 mm²/m | 25mm continuous | 5,000 mm²/m, pitch over 35 degrees, span over 10m, or a lean-to |
| Cold loft, impermeable felt, pitch over 15 degrees | 10,000 mm²/m | 10mm continuous | 5,000 mm²/m, pitch over 35 degrees, span over 10m, or a lean-to |
| Cold loft, breathable membrane, ordinary ceiling | 7,000 mm²/m | 7mm continuous | none specified |
| Cold loft, breathable membrane, ceiling sealed to BS 9250 | 3,000 mm²/m | 3mm continuous | 5,000 mm²/m, as an alternative to the eaves figure, not in addition to it |
| Ceiling follows the pitch, impermeable felt | 25,000 mm²/m | 25mm continuous | 5,000 mm²/m, always, plus a 25mm clear pathway and a vapour control layer |
NHBC Standards 2024, chapter 7.2, clause 7.2.15, tables 11 to 13, read 11 September 2026. NHBC states that the roof ventilation guidance in that clause is taken from BS 5250:2021. These are new-build warranty standards, not a retrofit duty on a house that already exists.
A free area in square millimetres per metre is not a gap you can put a tape across, and that is the first thing to get straight. 10,000 mm² spread along one metre of eaves is the same area as a slot 10mm wide running that whole metre. It is the number printed on the packaging of any eaves vent, soffit vent or over-fascia vent you might buy, which is what makes the table above something you can actually shop against.
Two things in that table do most of the work. The first is pitch. Below 15 degrees a felted cold roof wants 25,000 mm²/m, two and a half times the figure for a steeper one, because a shallow roof void has less height for air to move through. The second is the underlay, and that one gets a section of its own further down.
There is a ridge figure too, and it is conditional rather than universal. An additional 5,000 mm²/m at high level is called for where the pitch exceeds 35 degrees, or the span exceeds 10 metres, or the roof is a lean-to or a mono-pitch. An ordinary 30 degree semi of ordinary span needs eaves ventilation and nothing at the ridge.
Now the part that surprises people. If you go looking for those figures in the Building Regulations, they are not there. Approved Document C is the guidance that owns roof condensation in England, and its whole section on roofs runs to four paragraphs with no ventilation figure of any kind in it. What it does instead is point elsewhere: a roof meets the requirement if it is designed and constructed in accordance with clause 8.4 of BS 5250:2002. That is the 2002 edition, and BS 5250 has been revised twice since, in 2011 and again in 2021.
One slope soaking, the other bone dry
This is the single most useful thing on this page, and there is nothing published about it anywhere. Water on one roof slope and not the other points at a cold surface rather than a leak, because driving rain does not respect which way a roof faces and sunshine does.
Start with the thing that is easy to forget: a roof slope spends its whole life pointing at the sky. During the day the south-facing slope is being warmed by the sun and the north-facing one barely is. At night, under a clear sky, both of them radiate heat straight out into space, and a surface doing that can fall below the temperature of the air around it. The Met Office describes the same effect at ground level, where on a clear night with light winds the ground, or the roof of your car, drops a few degrees below air temperature, and cloud acts like a blanket that stops it happening.
That is not a fringe idea, and the best evidence for it is that the relevant standard already builds it in. BS EN ISO 13788, the condensation calculation standard, says that for roofs you should use an equivalent outside temperature that accounts for solar gain and cooling by long wave radiation, and that as a simplified case you can take it by subtracting 2 K from every monthly mean external air temperature. In other words the standard treats a roof as colder than the weather, on a monthly average, before anything else happens. It is a monthly mean and not a clear-night figure, so do not read it as one.
Then there is how much sun each slope actually gets, which is arithmetic over a public dataset rather than anybody's opinion. The European Commission publishes modelled irradiance for any location and any orientation. Here is a location in the East Midlands, in December, which is the month the drips start.
| Roof pitch | South slope, December | North slope, December | North as a share |
|---|---|---|---|
| 30 degrees | 38.15 kWh/m² | 10.86 kWh/m² | 28% |
| 40 degrees | 42.90 kWh/m² | 10.10 kWh/m² | 24% |
| 45 degrees | 44.88 kWh/m² | 9.66 kWh/m² | 22% |
In-plane irradiation from the European Commission Joint Research Centre's PVGIS v5.2, queried 11 September 2026 for 52.6369N, 1.1398W. One modelled location, not a figure for the whole country, and the numbers move with where you are.
Read the bottom row. On a 45 degree roof in December, the north slope receives 22% of the solar energy the south slope gets. Across the whole year the gap is narrower, at 50%, and it opens up in exactly the months condensation appears. The two slopes are the same roof, in the same weather, under the same tiles, and one of them spends far more hours below the dew point than the other. A leak cannot do that. Aspect can.
So one wet slope and one dry one is, more often than not, the roof telling you it is a temperature problem. It is not proof, and here is what else can produce exactly the same picture, so go and rule these out before you accept the neat answer.
- One eaves run blocked and the other clear. Insulation shoved into the eaves on one side only will starve that slope of air. NHBC's requirement is that ventilation pathways remain clear, meaning not blocked by insulation or by the structure, and an installer who ran out of patience halfway round produces a one-sided roof.
- Two slopes at different pitches. The table above shows the north-to-south ratio moving from 62% at a 30 degree pitch to 50% at 45, so an extension or a catslide picks up measurably different energy before aspect is even considered.
- Something below loading one half of the house. A bathroom or a kitchen under one end of the roof, or a duct that terminates over one slope, puts the moisture where the moisture is. I have found nothing published that documents this as a cause of one-sided loft condensation, so treat it as worth checking rather than as an explanation.
Condensation in the loft after insulating, and the two causes that look the same
Adding loft insulation makes the loft colder. That is not a side effect, it is the whole function: you have stopped the heat that used to keep the roof space above the dew point. A retrofit sold as the fix becomes the cause.
Nobody hides this, and that is worth knowing, because it means you were not sold a lemon. The government's own loft insulation factsheet says it in one sentence: insulating your loft will keep your house warmer but make the roof space above colder. The Energy Saving Trust says insulation slows heat escaping from living spaces, making your loft space cooler, and that this makes condensation more likely or can worsen existing damp problems. NHBC puts it in its homeowner guidance: the amount of insulation required to meet modern standards greatly reduces the heat entering the roof space from the rooms below, which can increase the risk of condensation because the surfaces in the roof remain cold.
What none of them then tells you is that two completely different things can have happened, they look identical from the hatch, and they need opposite remedies. This is the bit worth going up to establish.
One: the eaves got stuffed. Somebody rolled the quilt out to the very edge and packed it into the corner where the rafters meet the wall. The ventilation path is physically blocked. The fix is free: pull it back and leave a 25mm clear airway, held open by a spacer so it stays open.
Two: nothing is blocked, the loft is simply colder. The ventilation it has was sized for a roof space that used to be warmed by the house leaking heat into it. That heat has gone. The openings that were adequate are now not, and the fix is more of them, measured against the table above rather than guessed at.
Two more things about the eaves are worth having, because both are wrong on almost every page that mentions them. The first is the number. The published figure is a 25mm clear airway, not 50mm. The 50mm everyone quotes is the nominal gap you build in so that 25mm survives the sag of the underlay. BS 5250:2011 said so in as many words, noting that a nominal 50mm gap is likely to be reduced to 25mm under the drape, and the 2021 edition words it as 25mm plus the maximum allowable drape of 15mm. Quoting the bigger number is not dangerous, it is just not the guidance.
The second is that pulling insulation back is not the same as stopping it short. NHBC's detail asks for a spacer in the eaves so that the insulation can be installed over and beyond the wall plate, to minimise the thermal bridge, without blocking the ventilation path. Stopping the quilt short of the wall plate to make room for air leaves a cold line running the whole way round the perimeter of the ceiling below. The spacer exists so you do not have to choose.
And on depth, since it comes up in the same conversation: the 270mm you keep reading is an Energy Saving Trust recommendation, not a Building Regulations figure. What the regulations actually set is a U-value of 0.16 W/m²K for a roof upgrade in an existing home, with 250mm of mineral fibre given as the typical construction that reaches it.
Felt or membrane, and why it decides everything
What is stapled over your rafters decides how much ventilation your roof needs, and it is the first thing to establish before you buy anything. Traditional bitumen felt is impermeable, so moisture can only leave through openings. A breathable membrane lets vapour through, and the required free area drops accordingly.
The two classes have a definition and a boundary. Type LR is a low resistance underlay, with a vapour resistance of no more than 0.05m expressed as an equivalent air layer, which is 0.25 MN·s/g. Type HR is anything above that, and traditional bitumen felt is firmly in it. Look at the table further up and the difference is the whole argument: 10,000 mm²/m at the eaves for felt, 7,000 for a membrane with an ordinary ceiling.
Now the part the marketing skips. Vapour permeable is not the same thing as air permeable, and most breathable membranes are airtight. BS 5250:2021 makes the distinction directly: most LR underlays are airtight, though some are designed to be air permeable, and it is that second property which allows air movement from the loft space through the underlay into the batten space. A roof may be designed with no loft ventilation at all only where the underlay is both low vapour resistance and air permeable, holds current certification for use in a non-ventilated application from an independent technical approvals body, and has an air permeability of at least 34 m³/m².h at 50 Pa. Most do not meet that. NHBC's position on the other end of it is blunt: a pitched roof with no ventilation provision and an air impermeable outer covering will not be acceptable to it.
So here are the four documented ways a roof with a breathable membrane still runs wet, and the first one catches a lot of houses.
- It is laid over boarding. Where a Type LR underlay is fully supported on plywood, OSB, chipboard or tongue-and-grooved sarking, it should be treated as a Type HR underlay for ventilation purposes. The membrane is breathing into a board that is not. On open-jointed square-edged boards, typically 150mm wide with at least a 2mm gap between them, it may still count as LR.
- It is vapour permeable but airtight. Which most are, and which is why it still needs the 7,000 mm²/m at the eaves rather than nothing.
- The covering above it is air impermeable. Slate, fibre cement, sheet metal and continuous membranes typically are; concrete and clay tiles typically are not. BS 5250:2021 even gives the test. Under an impermeable covering the batten space has to be ventilated separately, and if it is not, the LR underlay is treated as HR again.
- The moisture load below has simply won. A membrane is a route out, not an extraction system. If a bathroom is emptying into the void, no underlay is going to keep up.
This is also why the standard bit of internet advice for this problem, a handful of lap vents, does nothing for a lot of people who try it. A lap vent works by propping open the laps of overlapping impermeable felt so air can pass. On a breathable membrane there are no laps to prop in that sense, because the membrane is already the vapour route. The product is not a fraud, it is just specific to one kind of roof, and nobody says so.
Getting this right matters because of what sits above the underlay and below it. BS 5250:2021 notes that a low resistance underlay lowers the risk of condensation in the loft but can raise it in the batten space, with a corresponding risk of decay to the timber battens, unless there is enough air movement between the battens and through the covering. And the threshold for that decay is knowable: the SPAB puts arresting timber decay at getting the moisture content below 20%, because dry timber is immune to attack. Jagjit Singh, writing in the Building Conservation Directory, puts the minimum for dry rot at about the same figure, with optimum growth well above it. That is the number the whole exercise is aimed at. Above it, what usually grows in a roof is wet rot, and the cut-back distance you get quoted for it is the part of that bill worth reading twice.
Fans, dehumidifiers, and the number that beats both
Both are sold as the answer to a wet loft and both act on the void rather than on the cause. The cause is air leaking up out of the house, and the case for sealing the ceiling instead is not an opinion, it is a factor of ten.
Here is the figure that settles the argument. An ordinary ceiling passes about 300 mm² of air for every square metre of it. A ceiling sealed to BS 9250 passes no more than 30. That is a 10-fold difference in how leaky the lid of your house is, and you can see what it buys in the ventilation table: with a breathable membrane, sealing the ceiling takes the required eaves free area from 7,000 mm²/m down to 3,000. Shut the leak down by a factor of10 and the ventilation the roof needs to cope with it more than halves.
The independent building science says the same thing in different words. Joseph Lstiburek of the Building Science Corporation puts the main strategy for keeping a roof or attic free of moisture problems as the elimination of air movement, particularly air leaking out of the conditioned space, and identifies winter attic moisture as arising from exactly that, often because no ceiling air barrier is present, plus leaky ductwork in the attic. That is written for a cold North American climate rather than for a British semi, and it is the clearest statement of priority I could find from a body with nothing to sell.
So what is actually leaking. The named routes, from NHBC's own ceiling penetration detail and from Approved Document C, are the ones you can go and look at this afternoon: the loft hatch, which should be draught stripped; gaps sealed at services, meaning pipes, cables and the soil vent; and downlighters, which where used should be specified and sealed to limit air leakage. Approved Document C puts it as filling and sealing gaps and penetrations for pipes and electrical wiring, and providing an effective draught seal to loft hatches. There is even a number for the hatch: an access hatch to a cold roof void should leak no more than 1 m³/h at 2 Pa when tested to BS EN 13141-1, or be a push-up cover of at least 5.5 kg compressing a closed-cell seal.
And then the one that is worth checking before anything else on this page, because it is common and it passes every casual inspection: follow your bathroom extractor duct. A fan that runs, sounds right and clears the mirror can still be delivering every shower in the house into the roof void, because the duct was never connected to the tile vent or has fallen off it. That failure is covered properly on the ventilation page, along with the extract rates Part F publishes per room. Where the duct does run through the loft, the Property Care Association's code of practice is that ducting in an unheated void should always be insulated, to reduce the chance of the extracted air condensing inside the duct before it gets outside, and that flexible ducting should be kept to a minimum and not used on runs over a metre. The room underneath usually shows it first, which is why mould on a bathroom ceiling and a wet loft above it are so often one fault rather than two.
The same question decides whether a positive input unit in that loft is worth the money, because it lives up here and draws its air from here.
Against all that, a loft fan or a loft dehumidifier is working on the wrong side of the problem. On dehumidifiers the most useful statement comes from a company that sells them: Meaco says one will not work in an undeveloped loft space because it will be taking the fresh air that comes in under the eaves, which it likens to running one in a living room with all the windows open. In a properly ventilated loft that is precisely right, and it is a firm arguing against its own sale. The general case for and against these machines, including the temperature below which the cheap kind stops doing much, is on the dehumidifier page rather than repeated here.
Positive input ventilation is worth a line because it gets sold for this and it is pointing the other way. A loft-mounted PIV unit takes air from the loft and pushes it into the house. Envirovent, which makes them, says it relies upon air naturally entering the loft space, as well as solar gain, and that such systems need enough ventilation within the loft itself to work. It is a treatment for the house, not for the roof space, and it does nothing about the moisture arriving in the loft from below. The rest of the honest limits are on the ventilation page, taken from the firms that sell the units.
The last lever is the cheapest and the least mentioned: the warmer the roof void, the less of this happens, which is the same reasoning behind warming a cold surface rather than drying the air. You cannot heat a loft, and you would not want to, so up here it comes out as reducing what you send up there rather than raising the temperature of what is already there.
Before you go up there, and when to stop
A loft is a structural space with no floor in most of it. Two of the checks on this page mean crawling about over an unboarded ceiling in the dark, so this part is not filler.
The hazard list that matters is the one written for people who do this for a living. US occupational safety guidance on working in attics names fall hazards at the openings between truss chords, exposed nails, cables, wires, low-hanging rafters and cross-beams, poor lighting, and, most relevant here, truss chords hidden by deep insulation. Its advice is that a piece of plywood or planking to stand on improves footing, and that a fall through the ceiling to the floor below is the injury. It is American and written for employers, and every word of it applies to a bloke with a torch over a bedroom. The Energy Saving Trust says the same thing more gently: if your water tank is a long way from the loft hatch, have secure boards to walk on.
The insulation you added is the thing that hid the joists. That is not an argument against insulating. It is an argument for finding out where you can put your feet before you set off across the void looking for a wet rafter.
One more, which stops the job entirely if it applies. If there is loose-fill or sprayed material in the loft that you cannot identify, and the house was built or refurbished before 2000, do not disturb it. The HSE's position is that asbestos can be present in any building built or refurbished before the year 2000, and that fibres are released when materials containing it are disturbed or damaged. That is a stop, not a caution.
On Building Regulations. Converting a loft into a liveable room needs Building Regulations approval, and that is unambiguous: it is a material alteration under the regulations. Re-roofing is more conditional. The published position is that you will not normally need an application if less than 25% of the total building envelope is affected and less than 50% of the roof is, but that you will need one if you carry out structural alterations, or use a new covering that increases the weight of the roof covering by 15% or more. And where a pitched roof is re-covered, Approved Document L requires the condensation risk in the roof space to be assessed and provision made under Part C. So a re-roof is the natural moment to fix the ventilation, and the regulations expect somebody to have thought about it.
What is not published anywhere is whether simply fitting extra vents to an existing roof needs approval. I looked through the whole of the Planning Portal's roof section and there is nothing on it either way. Any page telling you confidently one way or the other is telling you something it cannot source. Building control at your council will answer it in a phone call, and that is the honest route.
And the end of what a website can do for you. If the timbers are soft, if there is fungal growth on structural members rather than surface spotting, if the roof has been wet for more than one winter, or if there is spray foam insulation up there, this needs somebody who has been in the loft. Ideally somebody with no interest in selling you the treatment afterwards. I cannot tell you what is happening in your roof and nobody can from a description of it.
About this resource
The sourcing limitation on this page is worth stating plainly rather than burying. BS 5250:2021 is paywalled and I have not read it. Every ventilation figure here comes from NHBC Standards 2024, chapter 7.2, which states in terms that its roof ventilation guidance is taken from that edition, and I read it on 11 September 2026 from a third-party copy because NHBC's own site blocks automated access. The Approved Documents, by contrast, were downloaded and read directly. If a number here matters to a decision you are making, go to NHBC or to the standard rather than take it from me.
On the edition, since this is a subject where getting it wrong is easy. BS 5250:2021 is titled "Management of moisture in buildings. Code of practice". It was published in July 2021, with a corrigendum in October of that year, and it supersedes BS 5250:2011+A1:2016. There is no later amendment. The roof ventilation guidance moved out of an annex and into the main body of the standard in that revision, and the pitched-roof figures themselves were left essentially unchanged. One manufacturer's summary of the standard gives the pitch threshold as 10 degrees where NHBC and every other source I read give 15; I have published 15 and have not used that summary for it.
Several things are deliberately absent. There is no figure for how many degrees below air temperature a roof falls on a clear night, because the papers quoting one could not be opened. There is no number for how much loft condensation is normal, because nothing published states one. There is no temperature at which a dehumidifier stops working, since that figure is already sourced on the dehumidifier page and its manufacturer gives two different answers in two places. And there are no costs on this page at all; work costs are handled separately on this site with their own sourcing and their own review dates.
Where a source sells something, it is named as what it is. The underlay material quotes come through a membrane manufacturer's white paper. The dehumidifier statement comes from a dehumidifier maker, and it is quoted precisely because it argues against its own sale. The PIV descriptions come from firms that make PIV units. Nothing on this website earns me a commission, no product is recommended on this page, and I do not survey, treat or quote for work, which is what lets me say when the thing being sold is pointing at the wrong half of the problem. If your damp turns out to be water coming through the fabric rather than condensing on it, none of the advice here applies and you want a different page.
Common questions
Is it normal to get condensation in the attic?
There is no published number for how much is normal, and I am not going to invent one. NHBC's own homeowner guidance says you are unlikely to prevent condensation in a roof space completely, and that most cases are temporary. What changes the answer is whether it dries out between cold snaps or builds up week on week, and whether the roof has the ventilation openings its construction was designed around. A dusting of droplets on the underlay after a still, freezing night is a roof doing what roofs do. Water running down it every cold week in January, into the insulation, is not.
How to tell the difference between a leak and condensation?
Condensation is widespread and follows the cold. A leak is local and follows the rain. Condensation coats a whole slope, or the whole underside of the underlay, and it turns up after still, freezing weather with the heating on below. A leak picks one spot, tracks down one rafter or one nail, and gets worse in driving rain and better in a dry spell. So go up after a hard frost, then again after a wet windy day, and note which one made it worse. If the water follows rain rather than cold, you are looking at penetrating damp and the roof covering, not at condensation.
Is it worth putting a dehumidifier in the loft?
In an ordinary ventilated cold loft, no, and the plainest statement of that comes from a company that sells dehumidifiers. Meaco says one will not work in an undeveloped loft space, because it will be taking the fresh air that comes in under the eaves, and describes it as the same as running one in a living room with all the windows open. The machine is trying to dry an endless supply of outside air. A converted loft room that is sealed and heated is a different question, and so is a cold room generally.
Will condensation clear up on its own?
The water will. The cause will not. Every dry, breezy spell takes moisture back out of a roof space, and then the next still, freezing night puts it back, because the thing making it has not changed. The one honest exception is a new build. NHBC says up to 8,000 litres of water goes into a house during construction, that roof-space condensation may show up in the first heating season because of it, and that most cases are temporary. In an older house there is no construction moisture left to dry, so if it is happening it is happening because of the roof and the ceiling, and it will happen again next winter.
How do you know if your attic has enough ventilation?
There is a published figure, and you can compare your own roof against it. For a cold loft with traditional impermeable felt at a pitch over 15 degrees, the design figure is 10,000 square millimetres of free area for every metre of eaves, which is the same area as a continuous 10mm slot running the length of the eaves. Under 15 degrees it is 25,000, and with a breathable membrane and an ordinary ceiling it is 7,000. Those are NHBC figures, restating BS 5250:2021, and they are new-build standards rather than a legal duty on an existing house. They are still the number your roof was meant to hit.
What happens if an attic is not properly ventilated?
Moisture arriving from below has no way out, so it condenses on the coldest thing up there, which is usually the underside of the roof covering or its underlay. The Energy Saving Trust puts the consequence plainly: without enough airflow, warm air from your home can get trapped in the loft, and that can lead to condensation, which over time can cause damp, mould, or even rot in the roof timber. The figure that matters for the timber is 20%. Below that moisture content, according to the SPAB, dry timber is immune to attack, and the whole point of getting the water out is to stay under it.
How do I get rid of condensation in my loft?
In this order, because the order is what most advice gets wrong. First stop the warm wet air getting up there: draught-seal the hatch, seal round pipes, cables and downlighters, and find out where your bathroom extractor duct actually ends. Second, clear the ventilation you already have, which usually means pulling insulation back out of the eaves to leave a 25mm clear airway. Third, and only third, add ventilation openings if the roof is short of the published free area. A fan or a dehumidifier sits at the end of that list, not the start, because both work on the void while the cause is the leak into it.
Is there a way to check if my attic has moisture in it?
Go and look, and pick your day: after a few days of genuinely cold weather is when it shows. Run a hand along the underside of the underlay between the laps, which should feel dry. Look for water beading on the roof covering, dark staining or black spotting on the rafters, and insulation that is damp rather than dusty. Before you go up, read the safety note above: the joists under deep insulation are not where you think they are, and a foot between them goes through the ceiling. If the timber is soft, if the boards are wet, or if you cannot tell what you are looking at, that is the point to get somebody up there who has no interest in selling you a treatment.