Garage roof insulation?

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Lias

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#311275 1-Jan-2024 16:58

I'm wanting to insulate my 80's era garage because I'm using it for an office and it's been freezing in winter and baking in summer.

The entire thing is steel over a timber frame, the walls are bare metal on the interior, the roof is partially tar papered and partially bare metal.

I'm thinking normal batts for the walls but unsure about the roof. It's a garage so it has no ceiling below the rafters, so I'm assuming I just install insulation directly between the purlins. "Roof" batts are all much too thick for the 100mm depth purlins, so I'm not quite sure whether I should use wall bats or sheets of polystyrene or what.

Anyone have suggestions?

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#3176786 1-Jan-2024 17:10

not sure what you mean by bare metal on the interior. if you mean there is no gib and no house wrap. the metal is the outside cladding. you will need to install house wrap, even if its just pocket install. need to keep insulation dry. batts is fine. i would clad with plywood rather than gib.

roof will need the underlay fixed or installed. which may mean taking the roofing iron off. i would not try to pocket install the underlay. the insulation, and easy one is polyester and simply staple it across the rafters or purlins.

do not use polystyrene, need to use vapor open insulation so it dries.

also what is the heating/cooling system. insulation doesn't heat or cool.

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#3176787 1-Jan-2024 17:12

Having done this previously, I built a ceiling, and then insulated on top of it - could you do the same?

Lias

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#3176811 1-Jan-2024 19:36

tweake:

not sure what you mean by bare metal on the interior. if you mean there is no gib and no house wrap. the metal is the outside cladding. you will need to install house wrap, even if its just pocket install. need to keep insulation dry. batts is fine. i would clad with plywood rather than gib.

No gib, no house wrap, nothing, it's just 2*4 framing and the exterior steel. I'm cladding with cardboard because it's much cheaper than gib or plywood :-)

Lias

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#3176812 1-Jan-2024 19:39

Ge0rge: Having done this previously, I built a ceiling, and then insulated on top of it - could you do the same?

When I build internal walls to make a "proper" office I plan on putting a ceiling in under the rafters, but that's very much a future project. I was rather hoping for a quick (and cheap) interim option.

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#3176816 1-Jan-2024 19:53

The steel purlins are going to act as a pretty big thermal bridge across your insulation. Ideally you want to run battens at 90 degrees to the purlins, then put your ceiling on the underside of that. That'll give you an extra ~40mm to fit thicker insulation (squished slightly under the battens unless you're diligent with the knife).

I'm pretty sure there are issues with insulation against the steel without any building paper, though. You're going to experience pretty serious condensation and dripping once you start increasing the temperature and humidity.

(I assumed that 'purlin' implied steel, and this is wrong. The rest of the comment still applies)

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#3176818 1-Jan-2024 20:09

I have made the assumption from what you have said that you have a skillion roof rather than a pitched roof with conventional trusses? As with a number of the comments above, I can't see a useful solution without packing "downwards" to create a false/lower ceiling. The roofing iron should have roofing "paper" under it - and note, also, that you need a minimum of 25mm air gap between the top of the insulation and the roofing paper/iron for the insulation to work properly, which reduces your insulation thickness available even more. So I'm not sure if there is an easy/simple fix :-)

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larknz

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#3176822 1-Jan-2024 20:16

As mentioned previously you need a moisture barrier under the iron, otherwise you will get water damage to the gib board.

Lias

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#3176830 1-Jan-2024 20:37

edge:

I have made the assumption from what you have said that you have a skillion roof rather than a pitched roof with conventional trusses? As with a number of the comments above, I can't see a useful solution without packing "downwards" to create a false/lower ceiling. The roofing iron should have roofing "paper" under it - and note, also, that you need a minimum of 25mm air gap between the top of the insulation and the roofing paper/iron for the insulation to work properly, which reduces your insulation thickness available even more. So I'm not sure if there is an easy/simple fix :-)

No, it has timber trusses, with timber purlins on top of the trusses, then the roof steel (with tar paper in _some_ areas).

At this stage I'm thinking if I go with 50-70mm polystyrene boards between the purlins (which are 100mm deep so allows for an airgap) as an interim measure, then at a future date when I build an internal wall in the garage to make an office I put a ceiling in with "proper" insulation on top of it.

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#3176831 1-Jan-2024 20:39

What about putting the insulation between the bottom members of the trusses, and the temporary ceiling below that?

Lias

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#3176836 1-Jan-2024 20:43

SomeoneSomewhere:

What about putting the insulation between the bottom members of the trusses, and the temporary ceiling below that?

I don't want to put a ceiling under the trusses in the entire garage because then I lose all that truss storage space (which is currently heavily used). I don't mind doing it just over the office space, but as noted that's not being built yet so I was looking at a "roof" rather than "ceiling" solution.

larknz

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#3176838 1-Jan-2024 20:45

Lias:

edge:

I have made the assumption from what you have said that you have a skillion roof rather than a pitched roof with conventional trusses? As with a number of the comments above, I can't see a useful solution without packing "downwards" to create a false/lower ceiling. The roofing iron should have roofing "paper" under it - and note, also, that you need a minimum of 25mm air gap between the top of the insulation and the roofing paper/iron for the insulation to work properly, which reduces your insulation thickness available even more. So I'm not sure if there is an easy/simple fix :-)

No, it has timber trusses, with timber purlins on top of the trusses, then the roof steel (with tar paper in _some_ areas).

At this stage I'm thinking if I go with 50-70mm polystyrene boards between the purlins (which are 100mm deep so allows for an airgap) as an interim measure, then at a future date when I build an internal wall in the garage to make an office I put a ceiling in with "proper" insulation on top of it.

You need a proper moisture barrier under the iron, otherwise you will have water problems.

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tweake

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#3176891 1-Jan-2024 21:10

No, it has timber trusses, with timber purlins on top of the trusses, then the roof steel (with tar paper in _some_ areas).

At this stage I'm thinking if I go with 50-70mm polystyrene boards between the purlins (

do NOT use polystyrene boards 🤬. they do not let moisture through and you will end up with condensation forming on them.

also do not forget if you insulate up at the purlins you will also need to insulate the gable ends. insulation needs to be continuous for it to work properly. so insulate everything from top to bottom.

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#3176974 2-Jan-2024 07:05

Lias:
No, it has timber trusses, with timber purlins on top of the trusses, then the roof steel (with tar paper in _some_ areas).

At this stage I'm thinking if I go with 50-70mm polystyrene boards between the purlins (which are 100mm deep so allows for an airgap) as an interim measure, then at a future date when I build an internal wall in the garage to make an office I put a ceiling in with "proper" insulation on top of it.

OK - see what you're faced with now. Can't really add anything to what had already been covered in the responses to date. You might have to hold off on the insulation until you're ready for a more major upgrade! :-)

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Lias

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#3177063 2-Jan-2024 11:32

Waaaah? Thanks for the input team, appreciate it even if it's not what I wanted to hear lol.

mattwnz

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#3177115 2-Jan-2024 14:36

Some photos may help to get an idea of what you are working with and condition. But it sounds like one of those cheap steel garages that were never designed to have insulation or internal linings. Ground moisture and clearance around the slab, and from the slab level could also be an issue that may need consideration if you are putting in internal linings..

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