Insulation, ventilation and condensation: a warmer home that stays dry
Warmth, air and moisture are one system. Insulation slows heat loss, draught-sealing stops cold air leaking in, and ventilation removes the moisture a household produces. Start with the ceiling, which is usually the easiest and most effective place to insulate, but have an electrician check older wiring first and keep the required clearances around downlights. Seal draughts around doors, windows and floors, then make sure bathroom, kitchen and laundry exhausts vent outside rather than into the roof space. A tighter home without ventilation can trap moisture, causing condensation and mould, and can be dangerous with an open-flued gas heater.
- 01InsulateCeiling first, after an electrical check
- 02SealDoors, windows, floors and unused chimneys
- 03VentilateBathroom, kitchen and laundry exhausts to outside
A tighter house needs its moisture taken out on purpose.
What you'll notice
Most homes show where heat and moisture are escaping or collecting. These are the common signs.
| Sense | Sign | Where to read next |
|---|---|---|
| Feel | Cold rooms despite heating, or rooms that overheat in summer | Insulation |
| Feel | Cold air around doors, windows, skirting, floorboards or an unused fireplace | Draughts |
| See | Water running down windows on cold mornings, or wet window frames | Condensation |
| See | Black mould spots on ceilings, in corners or behind wardrobes | Condensation |
| See | Bathroom mirrors and walls that stay wet long after a shower | Ventilation |
| See | Damp or stained insulation, or rusty nails, in the roof space | Roof-space condensation |
| Smell | A stuffy or musty smell in a closed-up house | Ventilation |
Heat, air and moisture work as one system
A household adds a lot of moisture to the air every day through showers, cooking, drying clothes and breathing. In a leaky, uninsulated house, cold draughts carry it away, at the cost of comfort and energy. When the same house is insulated and sealed, that moisture has to be removed deliberately, by ventilation, or it condenses on the coldest surfaces. YourHome treats insulation, sealing and ventilation as parts of the same design, and so should any upgrade.
Insulation: where to start
| Location | Ease in an existing home | What to know |
|---|---|---|
| Ceiling | Usually the easiest, from the roof space | Often the biggest gain; check existing depth and condition, and top up rather than compress |
| Underfloor (raised timber floors) | Possible where there is subfloor access | Keep subfloor vents open; see subfloors |
| Walls | Hard without removing linings or cladding | Best done during renovation or recladding; cavity fills need specialist advice on moisture |
| Roof (under the covering) | Usually only when re-roofing | A reflective or insulating layer is easiest to add when the roof is off |
Insulation performance is measured as its R-value: the higher, the better it resists heat flow. Requirements and recommendations vary by climate; the ACT, for example, recommends ceiling insulation of at least R5, and notes that many homes built before 1997 have little or none. Insulation also settles and degrades over time, so an older layer may be thinner than it looks.
Insulation safety: wiring and downlights
Ceiling insulation work has a history of fires and electrical accidents, so the order matters. Sustainability Victoria and the South Australian Government advise:
| Check | Why |
|---|---|
| Have a licensed electrician inspect the wiring first | Old wiring can overheat when covered; SA flags wiring installed before 1986 in particular |
| Keep clearances around downlights | Sustainability Victoria gives at least 200 mm above recessed halogen downlights and 50 mm around transformers, under the wiring rules |
| Consider replacing old downlights | Some modern LED downlights are rated to be covered, which reduces the uninsulated gaps |
| Keep foil-faced insulation away from wiring | Foil can become live; never fix it with metal staples |
| Keep clear of flues, exhaust fans and heaters | Hot fittings need clearance from insulation |
| Use a licensed or certified installer | Required in some states, and safer in all |
See the switchboards and wiring guide for what older wiring looks like. Never enter a roof space with damaged or loose cables, and stop if you suspect asbestos.
Draughts: find them, then seal them
Draughts are often the cheapest comfort improvement. Common leaks are gaps under and around doors, window frames and sashes, gaps between floorboards and at skirting, unused fireplaces and chimneys, wall vents in older homes, and exhaust fans without self-closing dampers. Seal with weather strips, door seals, gap fillers and chimney dampers or balloons.
Two cautions. First, if the home has an open-flued gas heater, sealing can reduce the air it needs and raise the carbon monoxide risk; see heating, cooling and gas safety before sealing that room. Second, some old wall vents provide ventilation the house relies on; ask before blocking them.
Ventilation: remove moisture at its source
The most effective ventilation removes moisture where it is made: bathrooms, kitchens and laundries. Exhaust fans and rangehoods must vent outside, not into the roof space, where the moisture condenses on the underside of the roof. Since NCC 2022, new homes must meet condensation provisions covering exhaust discharge and roof-space ventilation; older homes often do not.
| Check | Good sign |
|---|---|
| Bathroom exhaust fan | Ducted outside, run during and for a while after showers |
| Kitchen rangehood | Ducted outside, not just recirculating |
| Clothes dryer | Vented outside, or a condenser or heat-pump dryer |
| Subfloor vents | Clear, unblocked, cross-ventilating |
| Daily airing | Windows opened briefly, especially after showers and cooking |
Condensation and mould
Condensation forms when moist air meets a surface colder than its dew point: single-glazed or aluminium windows, external corners, poorly insulated ceilings and the underside of metal roofs. Repeated condensation feeds mould. YourHome explains how moisture, temperature and ventilation interact.
Treat the cause before the mould: reduce moisture at its source, ventilate, keep surfaces warmer with insulation and steady heating, and improve glazing where condensation is severe. Wipe and clean small areas of mould; large or recurring mould needs the moisture source found. For roof-space condensation, see metal roofs, and for telling condensation from rising damp or leaks, see damp and moisture.
Common misreads
| What people say | What is closer to right |
|---|---|
| "Seal everything and the house will be warm and healthy" | Sealing without ventilation traps moisture and can cause condensation and mould. |
| "The roof space has insulation, so it is fine" | Old insulation settles and degrades; check its depth, coverage and gaps. |
| "Mould means a leak" | Mould on cold surfaces is often condensation; leaks show as damage beyond the wet area. |
| "The exhaust fan works, so the bathroom is ventilated" | A fan venting into the roof space moves the moisture, it does not remove it. |
| "Insulation can go anywhere in the ceiling" | It needs clearances around downlights, transformers, flues and fans. |
Worked example: mould after new insulation
Illustrative reasoning example, not an inspected property. A 1960s brick-veneer house had new ceiling insulation and draught seals installed last autumn. This winter, black mould has appeared on the bedroom ceilings and the bathroom stays wet after showers.
The house is now warmer and tighter, but the bathroom fan vents into the roof space and the bedrooms are not aired. The fix is ventilation, not removing the insulation: duct the bathroom fan outside, run it longer, air the bedrooms daily and check the roof space for damp insulation near the fan. Clean the existing mould once the moisture source is under control.
Frequently asked questions
Where should I insulate an older house first?
Usually the ceiling: it is the easiest to reach and often gives the biggest improvement. Underfloor insulation suits raised timber floors with subfloor access; walls are best insulated during renovation or recladding.
Is it safe to put insulation over old wiring and downlights?
Only after a licensed electrician has checked the wiring, and with the required clearances: Sustainability Victoria gives at least 200 mm above recessed halogen downlights and 50 mm around transformers. Keep foil insulation away from wiring.
What R-value should ceiling insulation be?
It depends on your climate and local requirements. The ACT recommends at least R5 for ceilings, for example. A certified installer can check what you have and what to add.
Can insulation cause condensation and mould?
Not by itself, but a warmer, tighter house without enough ventilation can trap moisture. Vent bathrooms, kitchens and laundries outside, air the house and keep subfloor vents clear.
Where should a bathroom exhaust fan vent?
Outside, through a duct. A fan that vents into the roof space moves moisture into the roof, where it can condense on the roof and damage insulation and timber.
Why do my windows drip with condensation in winter?
Warm, moist indoor air is meeting cold glass and frames, especially single-glazed or aluminium windows. Reduce moisture, ventilate, keep rooms evenly heated and consider better glazing where it is severe.
Should I block old wall vents to stop draughts?
Ask first. Some older homes rely on wall vents for ventilation, and rooms with open-flued gas heaters need replacement air. Seal draughts elsewhere before closing vents.
How this guide was researched
We treat heat, air and moisture as one system: where to insulate first and how to do it safely, how to find and seal draughts, and how to ventilate so condensation does not follow, with a fictional example.
Limits: R-value requirements and installer licensing vary by climate zone and state. Electrical checks and clearances must follow the wiring rules and a licensed electrician.
Sources
- InsulationYourHome, Australian Government · accessed
- CondensationYourHome, Australian Government · accessed
- Home insulation health and safetySustainability Victoria · accessed
- Installing insulationGovernment of South Australia · accessed
- Ceiling insulationACT Government (Climate Choices) · accessed
- Overview of changes to energy efficiency and condensation in NCC 2022Australian Building Codes Board · accessed