Insulation in roof space

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WWHB

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#319200 1-Apr-2025 19:18

Does anybody know why we insulate above the ceiling gib in a roof space and not on the underside of the roof. I do a lot of work in roof spaces and it would be a lot easier to be able to do stuff it was insulated on the underside of the roof. Both the plumbing and electrical would be accessible as well as being easier to move around without damaging the ceiling.

JemS

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#3359569 1-Apr-2025 19:25

So the heat stays in the living areas and is not lost into the roof cavity.

tweake

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#3359575 1-Apr-2025 19:56

its simply cost. it costs a tad more to so under the roof and you need to do gable ends like a wall as well.

personally i think its a good to fit the insulation under the roof, even better if the roof is sheathed so its an air barrier as well. that way you turn the ceiling space into a useable space. a good place for storage and for ducted heat pumps and ventilation systems.

but average kiwi is still stuck in heating one room (or just the person) and not really embraced whole home heating.

Batman

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#3359582 1-Apr-2025 21:52

Not that simple. The roof needs to breathe as wind can blow water under the tiles, silicon around nails imperfect etc. So you will need to create a cavity between your insulation and the roof itself. Not impossible I guess, but you will need a very high roof.

Handle9

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#3359583 1-Apr-2025 21:59

It’s cheaper and much easier to do while using less material.

It also makes it much easier to repair the roof if need be.

tweake

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#3359694 2-Apr-2025 10:26

Batman:

Not that simple. The roof needs to breathe as wind can blow water under the tiles, silicon around nails imperfect etc. So you will need to create a cavity between your insulation and the roof itself. Not impossible I guess, but you will need a very high roof.

not really.

hot roofs (insulation under the roof) is commonly done. many thousands of houses in nz have this feature today and it has been around a long time (all cathedral/open/exposed ceilings). yes, there is a cavity between the insulation and the roof itself typically created by the purloins (or double space if using metal roofing on a sheathed roof).

its not new, its commonly done. like most thing in nz housing, its done for the look rather than the performance or practicalities.

Jaxson

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#3359703 2-Apr-2025 11:14

We live in a fairly moist climate and you run into issues with moisture condensing on cold surfaces, so there are benefits in boxing off the rooms and leaving the cavity space free.

It can be done, a lot of 70s houses just have a thin sloped ceiling and not cavity above. Depending on the slope the cavity can be quite small so agree it's not very nice working up there a lot of the time.

Kickinbac

427 posts

Ultimate Geek

#3361251 6-Apr-2025 10:44

This article by BRANZ has some good information on the topic.

https://www.buildmagazine.org.nz/articles/show/cold-roofs-warm-roofs

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tweake

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#3361256 6-Apr-2025 11:23

i think the terminology gets a bit mixed and i've used it a bit incorrectly.

a true hot roof is something like a sheathed roof with spray foam sprayed underneath it and typically has asphalt tiles direct on the sheathing. often skillion roofs (as in the branz article) is incorrectly (including by me) as a hot roof to differentiate it from a normal open ceiling.

what op is referring to is simply a skillion roof but it has normal flat ceiling instead of the gib up under the rafters.

skillion roof

Goosey

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#3361294 6-Apr-2025 12:58

It’s also for noise insulation too.