How to stop condensation on Colorbond roof with no sarking?

Question

Renovating house in regional victoria - so extreme variances to temperature…

I have noticed as I renovate, various things in this old 50 year old structure - how building techniques / products have changed.

So it has corrugated roofing - as far as I can tell - no leaks. I know this was replaced around 20 years ago and it still looks good. Its flat roof at around 10 degrees. Skillon type roof - and there is no cavity….

In the back room, I have taken down the old masonite ceillings and walls, found insulation in the ceiling - old / musty… Always intended to replace this and to actually plaster it.

My concern is condensation. As I have no ceiling / no insulation currently - and there is no sarking I can see easily a lot of condensation. Concerned if I just plaster up things with some insulation - I am just hiding an issue.

I dont have the option to remove the sheets of roofing material - they are some 30m long and will be too big of a job.

So what can I do ? Retrofitting….  ChatGPT contradicts Claude - great AI war in my mind…

Because I cannot guarantee airflow - the picture shows the battens do not sit on top of the eaves - they are cut into the eaves… Meaning the eaves and battens are actually the same height (damn it!) This further restricts the airflow…

ChatGPT is suggesting I cannot let any insulation material touch the roofing… as condesnation will still occur and rot / mould that insulation.  Claude suggests cutting to fit Anticon-style blanket insulation pushing hard up against the roofing material and then to add as much insulation batts as possible to fill the space.

Quite contradictery really? I know I cant make it perfect - just as long as I am not introducing mould on to the plasterboard is probably the best I can achieve…

Any advice ?

MitchellMc · Answer

Hi @kchambers,

You've certainly uncovered one of the downsides of older roofing systems. Without sarking, it's not unusual for condensation to form on the underside of the roof sheets, particularly where you can have very cold nights followed by warmer days.

Condensation occurs when warm, moisture-laden air inside the home comes into contact with the much colder underside of the roof sheeting. As the air cools below its dew point, the moisture condenses into water droplets. The aim is to reduce that temperature difference where possible and allow any moisture that does form to dissipate.

Unfortunately, sarking isn't really an option to retrofit as it sits between the roof sheeting and the roof battens. Installing it correctly would require removing the roof sheets, which you've explained isn't practical in your situation.

I'd also be cautious about pushing insulation directly against the underside of the roof sheeting unless it's part of a roofing system specifically designed for that application. In most cases, insulation should sit at ceiling level rather than directly against the roof, allowing some separation between the insulation and the roof sheets.

One of the best things you can do is improve ventilation within the roof space. Passive eave or gable vents, where practical, or roof ventilators can help equalise the temperature and moisture levels between the roof cavity and the outside air. This reduces the conditions that allow condensation to form in the first place. From your description, it sounds like the framing limits airflow from the eaves, but a builder may be able to suggest suitable ventilation options for your particular roof design.

Given the limited cavity, I'd recommend consulting an experienced builder before insulating and lining the ceiling. They'll be able to assess how best to ventilate the space and recommend an insulation system that won't trap moisture against the roof or behind the new plasterboard.

Let me tag @Nailbag to see if he has any thoughts.

You might also like to check out some of these previous discussions: How to add insulation under tin roof? by @karapoti,  How to add insulation to a tin roof home? by @smiley13 and How to install insualtion under tin roof? @javajive.

Here are some handy step-by-step guides:

* How To Insulate Ceilings

* How To Insulate Walls

* How To Insulate Your Home For Extreme Weather

Please let me know if you have any questions.

Mitchell