FAQ · Ceiling mould & condensation · SE Queensland
Why does mould keep growing on my ceiling, and how do I stop it?
Four things wet a Queensland ceiling: a roof leak, wet-season humidity, steam with nowhere to go, and air-conditioning. Gaps in the insulation add a fifth, in dead-straight lines. Work out which one you have before you fix anything.
I get this one a lot. Someone’s scrubbed the same patch of ceiling three times and it keeps coming back. Before you buy anything, read the pattern. A hard-edged stain, a whole corner, a strip over the bathroom, drips along a duct run and a neat grid of lines are five different problems with five different fixes. Here’s how to tell them apart, which one insulation fixes, and what to do about the rest.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
Real reviews, real jobs
What our customers say
Genuine Google & hipages reviews from Comfort Zone customers across SE Queensland.
- J
Jessa B.
Brisbane
“It dropped about 4 degrees straight away, and we added another 3 with the second job. I appreciate Peter's honesty, and the team showed pictures before and after.”
- Ahipages
Angela M.
SE Queensland
“The fact that I can't even tell it's 6 degrees outside when I wake up in the morning speaks for itself. Have wasted so much money attempting to heat and cool an uninsulated home. Worth every $.”
- I
Iain V-B.
Brisbane
“Quick and polite service. Great follow-up advice and photos sent for our records. Above and beyond what we expected. Would highly recommend.”
- Ghipages
Gerry S
Fitzgibbon, 2023
“I used Comfort Zone and they have a done an excellent job.”
- Jhipages
Jung K
Riverhills, 2023
“An experienced family operation. Highly recommend. Thank you for the great job!”
- Dhipages
David H
Sunshine Coast, 2021
“Completed the job as quoted and to a high standard. Great personal service. Would highly recommend Comfort Zone for ceiling installation work.”
- Bhipages
Benjamin H
Carseldine, 2019
“Very good explanation about their works. Advice of existing problems with the roof. Clean work. Very professional.”
- Ihipages
Ian G
Burnside, 2019
“Good information, communication and professionalism.”
- Dhipages
Danny D
Boondall, 2018
“He explained everything he was going to do and the different types of insulation they used. He talked through the different options but made a recommendation for the one most people use, which is the one I chose. He was very understanding towards what I needed and not about himself.”
- Ghipages
Graham R
Riverhills, 2018
“Comfort Zone. Turned up ahead of time, completed in about 2 hours, cleaned up. All good. Very motivated installation team.”
- Ahipages
Alex B
West Ipswich, 2018
“Fast, friendly, efficient.”
- Lhipages
Luke D
Mcdowall, 2017
“Peter did a good job. It was a quick and clean service. I'm happy to recommend!”
- Bhipages
Brendon
Brays Creek, 2016
“Peter supplied and installed roof insulation for me. He was very informative and provided good advice.”
- Thipages
Trevor G
Brookside Centre, 2016
“Excellent tradesmen from Comfort Zone Insulation. They were punctual and cleaned up after. Highly recommended.”
- Jhipages
John G
Beaudesert, 2019
“Peter is an honest person who provided me with the information I wanted then performed a good job with great results for the benefit of myself and my family.”
- Phipages
P Peter
Alstonvale, 2024
“Connected with Comfort Zone Insulation and would recommend them”
- Nhipages
Nola M
Birtinya, 2024
“They were courteous and competent.”
- Jhipages
Jennifer's E
Upper Caboolture, 2024
“Excellent customer service. Highly recommended. Has a profound knowledge of insulation products and has the best interest of his customer.”
- Jhipages
Jennifer
Upper Caboolture, 2024
“Excellent customer service. Highly recommend. Has a profound knowledge of insulation products and has the customer best interest.”
- Dhipages
Diane A
Ormeau, 2024
“Peter and crew did a great job I would definitely recommend them”
- Thipages
Timea
Highland Park, 2023
“I was extremely satisfied with the service they provided. They gave a very thourough explanation of the materials used, the way the work will be carried out and the price I had to pay was the exact amount quoted, no hidden costs included. They arrived on time, well prepared and workwas carried out exactly how they said it would be, they were super efficient, well prepared and were kind enough to even clean up after themselves. The services they provided was second to none! I don't hesitate to recommend them for any insulation job!”
- Mhipages
Mark
Pottsville, 2017
“Michelle, we are done - Peter from comfort zone insulation was very helpful. very honest with his recommendations - in fact he told me that the product my daughter had if installed correctly was superb. Thanks Peter you are a champion and i would recommend you to any person that was wanting professional advice and old school service.”
- Jhipages
Jessica
Pottsville, 2016
“This business offers a fantastic product that other businesses did not. Pump in ceiling insulation. Knowledge of the industry second to none.”
- Jhipages
Jack
Pottsville, 2023
“Excellent communication and informative. Professional.”
- Thipages
Tony P
Redland Bay, 2023
“Very knowledgeable about insulation”
- Shipages
Steve
Redland Bay, 2017
“Excellent job and reasonable price.”
- Bhipages
Bruce H
Kuluin, 2023
“Prompt and efficient quoting.”
- Ghipages
Gerry S
Fitzgibbon, 2023
“I used Comfort Zone and they have a done an excellent job.”
- Thipages
Tamara
Underwood, 2023
“Peter is honest, hard-working and came on time. Knew excally what he was talking about and answered my questions. Would 100% recommend”
- Shipages
Sterling G
Ashgrove, 2023
“Comfort Zone were very knowledge with great communication and follow up”
- Ghipages
Graham R
Riverhills, 2018
“Comfort Zone. Turned up ahead of time completed in +- 2 hours cleaned. All good. Very motivated installation team”
- Jhipages
Jung K
Riverhills, 2023
“An experienced family operation. Highly recommend. Thank you for the great job!”
- Khipages
Kathy A
North Lakes, 2023
“We connected with Peter through HiPages and he was prompt, professional and even came back after the job was complete to assist with a question we had. We would highly recommend Peter for further insulation works.”
- Dhipages
David H
Sunshine Coast, 2021
“Completed the job as quoted and to a high standard. Great personal service. Would highly recommend Comfort Zone for ceiling installationn work.”
- Shipages
Sue H
Sunshine Coast, 2021
“Incredible customer service”
- Ehipages
Eileen C
Cedar Vale, 2021
“Quality work, good customer service, prompt”
- Chipages
Craig M
Woody Point, 2021
“Called within 5 minutes of request. Very knowledgeable and explained job in great detail, provide great advice in prior preparation for works required. Very friendly and helpful.”
- Jhipages
Jenny C
Plainland, 2021
“Although I did not hire Peter I was impressed with the initial contact and the knowledge he was willing to impart. I was treated with respect which I appreciated. I would have hired but I received a lower quote.”
- Qhipages
Quinton
Coomera, 2020
“Professional installation without any short cuts. True to their word with high integrity. Response from Comfort Zone Insulation”
- Ghipages
Gary P
West Kempsey, 2020
“Came & Gave a free quote”
Watch · from up in the roof
The joins between batts are where the lines form, seen from inside the roof.
The photo above is the pattern from underneath. This is where it comes from. Watch how batts have to be cut into every bay and poked under the wires, which leaves a join at every edge. Those joins are the strips that run cold on a winter night. Peter runs the hose and fills the whole roof edge to edge with no gaps, so there is no cold strip for the lines to follow.
Read the transcript
We just put some polyester around the manhole and on top of it, so the cellulose doesn't fall down after the job when customers come up through the roof. This is what it's looking like before — I'll come up here and show you how easy it is to pump in. Now imagine with batts: every one of these little bays has to be cut in, and we have to tear the batts in and poke them underneath all these wires and everything. Whereas with the cellulose, I just grab the hose, turn it on, and pump it in. So now you've got a product that's easy to install like this — it gets installed better and more efficiently, with no gaps, and that's why we know this is a far superior product to the batts. See how easy it is for me to get right out to the edges? I don't have to crawl all the way out there — whereas with batts I'd have to crawl all the way out to cut batts in and push them right to the edge. With the cellulose I just reach out and pump out there, and the wires are still on top of the insulation. We go around and do the whole roof just like that. It's still a difficult job — still very hot, sometimes I've had roofs at 83 degrees Celsius — but it's a whole lot easier this way than with batts. The reason most people don't do it this way is because it costs money and equipment to buy the machines. So I'll turn the machine off — that's one section done. You can see how easy that is, and how there's no gaps in any of it. So why do people put batts in? Because batts are really, really cheap, and someone who installs them only needs a ute and a broomstick handle to call themselves a professional batt installer — whereas I've got thousands of dollars of equipment to install this. Cellulose fibre is a premium product. Rats and mice won't build nests in it because it's loose — a rat just sinks in, so they won't nest in it, whereas with batts they push them up and nest underneath. And it's treated with borax and boric acid — the borax is a natural insect killer; it kills ants, spiders, cockroaches, silverfish and dust mites, so you get rid of the food up here and the lizards, rats and mice that come for it. Fire-wise, cellulose is far more fire-retardant — if there was a fire up here it'd only burn the top of the beam; the fire wouldn't get down past the cellulose on either side, so it actually helps protect a house and prolong the burn time, whereas polyester or fibreglass batts just melt away from the beams. And when fibreglass batts burn there's poisonous gas, whereas this is just recycled paper pulp — good for the environment, a good use of a material nobody else wants. Anyway, that's my information video on cellulose fibre — hope it's helped, and I'll get back to finishing the job.
Mould needs a damp surface. Where the water comes from decides the fix.
Mould feeds on a surface that stays damp, and where the water comes from decides the fix. A leak wets the board from above. Humidity and steam wet it from inside the house. And a ceiling sweats by itself when part of it sits colder than the dew point of the air touching it, the same way a cold glass of water sweats on a hot Queensland day. That last one is where insulation comes in, and only that one.
Insulation changes the temperature of the ceiling, not the amount of water in the air. Where it is missing, thin, or gapped, the board over the gap tracks the roof space instead of the room. On a winter night that strip is colder than the room air, so the room’s moisture settles on it and the mould follows the line. In January the same strip is the hottest, driest part of the ceiling. Summer condensation is a different animal. With the air-conditioning on, wet-season roof air can sweat on the top side of the lining under the insulation, and nobody has shown that gaps make that better or worse. The Australian Government’s yourhome guide calls the framing that lets heat bypass the insulation “thermal bridges” and says plainly that they “reduce the effectiveness of insulation and can also lead to condensation problems.” And the ABCB’s Condensation in Buildings Handbook says that condensation inside the building fabric “can be more damaging than surface condensation because it is often concealed and may go unnoticed” (page 23). Both the winter strip and the summer sweating happen here, and no Australian study I can find counts which does more damage in Queensland.
“You keep saying that the gaps in batts cause a cold strip, but during summer it’s a hot strip. I’m not sure which one of those causes the most mould, whether it’s summer or winter in Queensland.”
The dead giveaway
Mould in dead-straight lines? That is the one pattern insulation explains.
This is the tell I point out to people all the time. Batts are laid in rows. Where two batts don’t quite meet, or one’s been cut short, squashed or skipped around a downlight, there’s a strip with little or no insulation over it. That strip runs colder than the rest of the ceiling on a winter night, so the moisture picks it out first and the mould grows along that exact line. On a January afternoon the same strip runs hotter than the rest, which is why these lines get drawn on cold nights. When you see mould tracing a neat grid or a row of stripes at the usual joist spacing of 450 or 600 millimetres, you’re seeing the gaps between the batts above, lit up like a diagram.
Even small gaps matter more than people think. The same Australian Government yourhome guide notes even a small gap can greatly reduce the insulating value, and a cold strip is exactly where condensation, then mould, sets up shop. A properly pumped Cellulose Fibre Insulation ceiling has no joins, so there is no cold strip left for the moisture to pick out. Patches, a hard-edged stain or a whole mouldy corner are a different story, and no insulation fixes those.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
How it actually happens
Five things wet a Queensland ceiling. Each leaves a different pattern.
Read the pattern before you fix anything. The fix depends on the cause, and insulation is the fix for one of the five.
A roof or flashing leak
Water comes from above. The stain has a hard edge, spreads from one point and gets worse after rain. Look for a cracked tile, a blocked valley, rusted flashing or a dripping pipe, not for an insulation problem.
Wet-season humidity
From November to March the air inside a closed-up Queensland house is loaded with moisture. It settles on the coolest surfaces in the house, in corners and behind furniture as much as on the ceiling. Moving air and a dehumidifier do more than any ceiling product.
Steam with nowhere to go
A bathroom, kitchen or laundry fan that vents into the roof space, or no fan at all, puts the steam straight back onto the lining above it. The mould sits over the wet rooms and stops where the steam stops.
Air-conditioning
Cold ducts sweat in a roof space that can pass 60 degrees on a summer afternoon, and I have had roofs at 83, so the drips land on the ceiling below. Look along the duct runs and around the vents. An air-conditioned ceiling can also sweat on its top side, where wet-season roof air meets lining chilled from below. That one shows up in patches, and no Australian source says gaps in the insulation make it better or worse.
Gaps in the insulation
Mould in dead-straight lines at the usual joist spacing of 450 or 600 millimetres is following the joins between the batts above. On a cold night the bare strip over a join is the coldest part of the ceiling, so the room's moisture settles there first. This is the one pattern insulation explains, and the one a full-contact pumped ceiling takes away.
The fix, by cause
Fix the cause you actually have. Insulation fixes one of five.
If the lines map your batts, full-contact pumped Cellulose Fibre Insulation covers the whole ceiling as one continuous blanket. It flows in and fills around every truss, downlight and wall plate, so there are no strips where insulation is missing or batts leave bits uncovered. Remove the cold strip and you remove the surface the moisture was picking out. That takes away the cold strip the lines were following, and nothing else.
Cellulose Fibre Insulation is not a mould killer and it is not a dehumidifier. It does nothing for a leak, for wet-season humidity, for steam, or for a sweating air-conditioning duct. If your mould is a hard-edged stain, a whole corner, or a patch over the bathroom, fix that first and keep your money. Queensland Health’s mould advice after a flood or cyclone is to dry the house out fast with open doors, fans or the air-conditioner on dry mode, repair leaky roofs and plumbing straight away, and not to dry-brush mould off a surface. That order is right for an everyday ceiling too.
People ask me a fair question. In January the strip over a gap is the hottestpart of the ceiling, so why would mould grow there? It doesn’t. Air only gives up its water to a surface colder than its dew point, so in summer that hot strip is the driest part of your ceiling and the lines get drawn on cold nights. Summer ceiling mould comes from the humidity, the steam and the air-conditioning. The Australian Building Codes Board makes the same point, that condensation trouble “is not confined to colder climates or seasons” and turns up in the summer wet season of the humid tropics too.
- Read the pattern first. Lines at batt spacing point at the insulation. Stains, patches and corners point at water from somewhere else.
- Find and fix roof leaks, flashing leaks and any dripping plumbing or air-conditioning line above the ceiling.
- Duct the bathroom, kitchen and laundry fans outside, not into the roof space, and run them.
- Get air moving through a closed-up, humid house over a South East Queensland summer. A dehumidifier beats any ceiling product for this one.
- If the lines are the insulation, pump the ceiling full-contact so there is no cold strip left for the moisture to find.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
When we do your ceiling, every job is photographed and the photos are checked before you’re invoiced, so you can see the ceiling went in full-contact, even the spots you’d never climb up to look at. That’s our system, the same on every job, run by Comfort Zone franchise owner-operators trained to one standard and held to it.
Straight answers
Ceiling mould questions I answer from inside the roof.
Why does mould keep growing on my ceiling?+
Because the thing wetting it is still there. Mould needs a damp surface, and a Queensland ceiling gets wet four ways. A slow roof or flashing leak. Wet-season humidity from November to March. Steam from a bathroom or kitchen fan that is not ducted outside. Air-conditioning, which sweats the ducts in a hot roof and chills the ceiling lining. Gaps in the insulation add a fifth, and that one draws straight lines. Cleaning removes the mould you can see. It grows back until the cause is fixed.
Why is the mould in straight lines across my ceiling?+
Lines at a regular spacing, the usual joist spacing of 450 or 600 millimetres, follow the joins between the batts above. The ceiling over a gap tracks the roof space instead of the room. On a winter night that strip is the cold part of the ceiling, so indoor moisture settles on it first and the mould follows the line. In summer with the air-conditioning on the same strip is the hot, dry part, so summer mould is coming from the humidity, the steam or the air-conditioning itself. Both the winter cold strip and the summer sweating of an air-conditioned ceiling happen in Queensland, and no Australian study I can find counts which does more damage here. Dust also sticks to a damp strip, so make sure it is mould and not soiling before you treat it.
Does insulation actually stop ceiling mould, or do I just clean it off?+
Insulation does not stop mould. If your lines map the gaps in the batts, a seamless pumped ceiling removes those cold strips, and once you have cleaned the mould off the lines have nothing left to follow. It does nothing for a leak, for humidity, for steam, or for a sweating air-conditioning duct. If one of those is your cause, insulation is the wrong thing to buy, and I will say so when I see the photo. Clean the mould once the moisture source is fixed. Before that it comes back.
What else causes ceiling mould besides gaps in the insulation?+
Plenty of ceiling mould in Queensland has nothing to do with insulation. A roof or flashing leak spreads from a hard-edged stain and gets worse after rain. Wet-season humidity grows mould in corners and on cool surfaces across the whole house. A bathroom or kitchen fan that vents into the roof space, or no fan at all, puts the steam straight back onto the ceiling. Air-conditioning ducts sweat in a hot roof space and drip, and the chilled lining pulls moisture out of the roof air. Queensland Health names excess moisture, long periods of heat and humidity and pooled water as what grows mould, and says to repair leaky plumbing and roofs as soon as possible. Find the pattern that matches yours before you spend money on any of it.
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Three scrubs, same mould patch. Find the cause first.
If the lines map your batt gaps, a full-contact pumped Cellulose Fibre Insulation ceiling takes away the gaps they were following, and the quote for it is fixed and in writing. If your problem is a leak, humidity or the air-conditioning, I’ll tell you that, and you keep your money.
Peter Johnson
Owner / installer · Comfort Zone Insulation Team® · Since 1986
Want the bigger picture? Read whether cellulose settles or grows mould over time, see why cellulose is the only product I’d use in my own home, or just get your quote started.