On a new build every plane is open and you choose freely. On an existing building the walls are lined, the roof is sheeted and the floor has a house sitting on it, so the question stops being what performs best and becomes what you can reach. That single constraint reorders everything: which element you do first, which product suits, and what the money buys.
In short
- Access decides the retrofit, not R-value.
- The ceiling is nearly always the first job, because you can get to it.
- Anything under roof sheeting is a re-roof decision, not a retrofit one.
- Walls are the hard one. Doing them properly usually means the cladding or lining is coming off anyway.
- On an existing building, fix moisture and ventilation before adding insulation.
Most of the guidance you will read is written for new construction, because that is where the code requirements bite. It is not much help when you are standing in a 1980s house wondering where to start.
The constraint changes
| New build | Retrofit | |
|---|---|---|
| What limits you | The code target and the budget | What you can physically reach |
| Choice of product | Anything the specification allows | Whatever suits the access you have |
| Under the roof sheeting | Open, and the natural place for the blanket | Closed. Not available without lifting sheets |
| Wall cavities | Open before lining | Behind plasterboard or cladding |
| Sequencing | Fits the build program | Fits around people living there |
| Proof required | Energy report, Total R-value calculations, certifier | Usually none, unless a program or approval is involved |
The last row matters more than it looks. On a new build the number is the point, because someone has to sign it off. On a retrofit nobody is checking, which means the honest measure of success is whether the building is more comfortable and drier, not whether the packet said R6.
Element by element
| Element | New build | Retrofit |
|---|---|---|
| Ceiling | Batts before the ceiling lining goes up | Straightforward through the manhole if the roof space is accessible. Usually the best value job on the house |
| Under roof sheeting | Blanket, spacers, mesh and thermal break | Only during a re-roof |
| External wall | Batts in the open cavity, plus wrap | Hard. Practical only when the lining or the cladding is off |
| Underfloor | Easy from below before the floor is loaded | Feasible where the sub-floor has access, and one of the better retrofit jobs |
| Internal walls for acoustics | Cheap while the frame is open | Only during a renovation that opens the wall |
| Slab edge | A design stage decision | Not retrofittable in any practical sense |
Read down the retrofit column and a clear order falls out. Ceiling first, underfloor second where there is access, everything else when the building is already open for another reason.
When a re-roof is the opportunity
This is the point most homeowners miss, and it is worth more than anything else in this article.
Everything that sits under metal roof sheeting, the blanket, the spacers, the safety mesh and the thermal break, has exactly one install window and it closes when the sheets go down. On an existing house that window is shut. It reopens only when the roof is being replaced.
So if a re-roof is on the horizon, that is the moment to decide the whole roof build up rather than treating it as a like for like replacement. Our guides to metal roof insulation and installing a roofing blanket cover what goes in there.
Miss it and the next opportunity is another twenty or thirty years away.
Walls, the hard one
Walls are where retrofit ambitions usually stop, and it is worth being straight about why.
The insulation goes inside the stud cavity, and that cavity is closed on both faces. Reaching it means removing the internal lining or the external cladding, and both of those are renovation jobs rather than insulation jobs. There are injected products on the market that avoid opening the wall, and they are a genuine option, but they are a different trade with their own considerations rather than something we supply.
What that means practically:
- If a room is being re-lined anyway, insulate the wall while it is open. The material cost is small next to the labour already being spent.
- If the cladding is being replaced, the same logic applies, and you also get the chance to add a wall wrap.
- If neither is happening, spend the money on the ceiling and the sub-floor instead, where it will do more for less disruption.
Remember too that the cavity depth caps what fits. A 90 mm stud is roughly an R1.5 to R2.7 proposition, as our guide to what R-value goes where sets out, so opening a wall does not deliver a ceiling level number.
What order to do it in
If you are working through an existing house with a finite budget, this order gets the most out of it.
- Check the exhausts. Bathroom, kitchen and laundry fans and the dryer should discharge to outdoor air, not the roof space. Cheapest job on the list and it prevents the most damage.
- Deal with any existing moisture. Wet insulation, damp sub-floor, rusting fixings. Adding insulation over a moisture problem makes it worse.
- Ceiling insulation. Best return per dollar, easiest access, biggest temperature swing above it.
- Roof space ventilation. Intakes at the eaves and an extractor high, sized to the space. Do it with the ceiling job so the eave clearance gets respected.
- Underfloor, where the sub-floor can be worked in.
- Walls and the roof build up, when the building is open for another reason.
Do the moisture work first
This is the difference between a retrofit that improves a house and one that damages it.
Adding insulation at ceiling level makes the roof space colder, because less heat is leaking up into it. A colder surface condenses more readily. If the building already has moisture finding its way into the roof space, more insulation can make the condensation worse rather than better.
The same applies under the floor. A damp sub-floor with new batts in it is a damp sub-floor with wet batts in it, and wet insulation does not recover.
So on any existing building, look before you insulate. Our guide to diagnosing a sweating roof covers what to look for, and roof space ventilation and insulation covers how the two interact.
Topping up existing insulation
Plenty of houses have something in the ceiling already, and the question is whether to add to it or replace it.
| What is up there | What to do |
|---|---|
| Thin, dry, evenly laid batts | Top up over the top, laid across the joists to cover the framing |
| Batts that are wet, slumped or mouldy | Remove them and find out why before replacing |
| Patchy coverage with gaps at the edges | Fix the gaps first. Gaps cost more than a low R-value |
| Only foil laid under the roof, nothing at ceiling level | Treat the ceiling as uninsulated |
| Anything compressed by stored boxes or a walkway | Compressed insulation is not delivering its rating there |
The gap point is worth repeating. A small uninsulated area costs more than the difference between two adjacent R-values, so on a retrofit the coverage and the fit matter more than the number on the pack.
What goes wrong
- Chasing a new build R-value in an existing house. The constraint is access, not ambition.
- Missing the re-roof window. Everything under the sheeting has one chance, decades apart.
- Insulating over a moisture problem. It gets worse, not better.
- Blocking the eaves with the new batts. The ventilation stops working the day the insulation goes in.
- Topping up over wet or slumped batts. Remove them and find the cause.
- Spending on walls before the ceiling. More disruption, less return.
- Ignoring downlights. Fittings that cannot be covered leave holes in the layer you just paid for.
We supply it and we install it
AWBM supplies ceiling, wall, underfloor and roof insulation plus ventilation across Victoria and New South Wales, and we install batts, wall wrap and sarking. Tell us the age of the building, what is up there now and what access you have, and we will tell you which job to do first and quote it.
Or browse ceiling, wall and roofing blankets.
Frequently asked questions
Can you add insulation to an existing roof?
At ceiling level, yes, and it is usually the best value job on the house. Under the roof sheeting, no, because the blanket, spacers and thermal break sit between the purlins and the sheets. That layer is only available during a re-roof.
What insulation should I do first in an old house?
Check the exhaust fans discharge outdoors, deal with any existing moisture, then insulate the ceiling and sort the roof space ventilation at the same time. Underfloor next where the sub-floor has access.
Can you insulate walls without removing the lining?
Not with batts. Batt insulation goes in the stud cavity, so reaching it means the internal lining or the external cladding comes off. That makes wall insulation a renovation job rather than a standalone retrofit.
Should I top up my existing ceiling insulation or replace it?
Top up over dry, evenly laid batts, running the new layer across the joists to cover the framing. Remove anything wet, slumped or mouldy and find out why before replacing it.
Does more insulation cause condensation?
It can make it more likely. Adding insulation at ceiling level keeps heat in the rooms, so the roof space runs colder, and a colder surface condenses more readily. Deal with moisture and ventilation as part of the same job.
Is underfloor insulation worth retrofitting?
Where the sub-floor has usable access, yes. It is one of the better retrofit jobs because it is reachable without disturbing the house above, and it noticeably reduces cold air movement up through the floor.
What if my house is being re-roofed?
Treat it as the one chance to get the roof build up right. Blanket, spacers, safety mesh and thermal break all go on before the new sheets, and after that the window closes again for decades.
Do I need approval to insulate an existing house?
Usually not for a straightforward retrofit, though incentive programs have their own eligibility rules and any work touching electrical or structural elements has requirements of its own. Check before you start if a program is involved.
Can AWBM advise on a retrofit?
Yes. Tell us the building age, what insulation is there now, whether the roof space and sub-floor are accessible and whether a re-roof is planned, and we will tell you the order to do it in and quote it. Batts, wall wrap and sarking we can install as well as supply.
Sources
- NCC 2022 Housing Provisions, Part 13.2 Building fabric, and Part 10.8 Condensation management
- NCC 2022 Housing Provisions, clause 13.2.3(7) thermal break, for the roof build up
- AS/NZS 4859.1, Thermal insulation materials for buildings, for declared R-values at full recovered thickness
- BlueScope Corrosion Technical Bulletin CTB-11, Roof Condensation and Insulation, October 2022
- Product data from the manufacturers of the products listed above
General information. Retrofit work on an existing building can uncover electrical, structural or moisture issues that need a qualified trade, and code requirements apply to some renovation work, so have anything unexpected assessed rather than covering it with insulation.