Insulation roof cabin/shed - air gap?

[kpnuts]

hi,

Sorry if not posted in the correct place. I really need some advice with insulating the roof of my new man cave/ shed. The bespoke building was made by a company and the panels are pieced together. The roof is made up of six 4m panels made of tongue and groove boards and it is a low pitched roof.

The inside rafters (if you can call them that) are only 40-45mm in depth. see photo attached to give an idea.

I have some 40mm PIR board to insulate the roof. My issue is, I have conflicting information. Some people say I should have a decent air gap so any moisture can escape and others say this is not required and just completely fill the 40mm void right up to the cladding? The company I bought the shed from strongly recommend the latter, as then there would be no place for any condensation to build up and they also said a vapour barrier underneath that, then plasterboard/ plywood.

My other option could be to lay horizontal across/under the rafters and fix the boards to the rafters using the large washers and screws. Therefore leaving a 40mm air gap. The building is tall, so not worried about losing head space. However, it’s not like I have eves as in a house for the air flow, so would there be any point?

Lastly, the company added a breathable membrane to all the inside walls against the cladding but nothing to the inside of the roof? So I assume this isn’t required?

I do plan to use the building all year round and have a small heater in there when needed in winter.

Any help would be really appreciated.

Cheers

 

[Notch7]

Neither option is really correct, because you cant create a warm or cold roof.

However since you cant ventilate the void, I would push the insulation up tight, then use 75mm foil tape (I find the diall one from screwfix ok), to tape over each rafter so that you join up with the foil face of each insulation board. The foil acts as a vapour barrier.

If you are plasterboarding use foil backed plasterboard.

The more that you can achieve a complete vapour barrier the less chance of vapour working its way up to the underside of the roof t &g, then condensing.

You can use a vapour control layer, like polythene sheet, but thats a bit awkward where those beams are.

 

[kpnuts]

Hi Notch7,

Thanks for the response. Yes, agreed, it doesn't really fit into either category, considering the company I bought from knew the intended use of the cabin.

Thanks for the advice, I will push all the way in. A lot more cutting than laying over, but I would rather that than risk moisture buildup.

I already have the foil tape , so I will fill any cracks with expanding foam and also the foil tape.

Thanks for the tip regarding foil backed plasterboard, I hadn't though about that. I was going to be using acoustic plasterboard (not sure if you can get a foil backed version of this?).

Alternatively, is there any easy way to ventilate it? Or is it more trouble than its worth? The thing is I also want it to be as sound proof as possible, as I would like to set up a projector/cinema room in there.

many thanks.

 

[Notch7]

You would need fascia vents to the outside in between every pair of rafters -not really practical.

There are 2 ways to cut the insulation: measure very carefully, including any tapering and cut to exact size, but cut with a small angle so the insulation goes in easily bit is snug o the face you can see inside the room.

Or cut about 5mm smaller all round. Cut some little bits of wood say 100mm x 50mm with a 5mm hole in the middle -use the to hold the insulation tight.

Then fill every gap with foam. Buy a metal screw on foam gun and have it set so the foam comes out slowly -set too fast it will blow back and wont go in the hole. You will fail if you try using the plastic tube type foam cans.

Using the foam al, round is the best way -it really locks the insulation in tight. But you need a good 5mm gap for the foam to go in deep enough.

 

[kpnuts]

Thank you Notch7, I really appreciate the time you have taken to give me your advice.
cheers

 

[kpnuts]

Hi Notch7,

Sorry one last question. Could I use this as a Vapour barrier and tape all the joints? As is would also add a little more insulation too and then plasterboard on top? There maybe I wouldn't need the foil back plasterboard either?

https://www.toolstation.com/ybs-general-purpose-thermawrap-insulation/p16779

Cheers

 

[Notch7]

Hi Notch7,

Sorry one last question. Could I use this as a Vapour barrier and tape all the joints? As is would also add a little more insulation too and then plasterboard on top? There maybe I wouldn't need the foil back plasterboard either?

https://www.toolstation.com/ybs-general-purpose-thermawrap-insulation/p16779

Cheers

Taping all joints acts as a full vapour barrier, but I like to fit foil backed plasterboard as an extra belt n braces. Thermawrap used as a vapour barrier would work fine.

Im not sure if it add any useful extra level of insulation without air gsps either side (assuming it works like a superquilt). But its not expensive so is worth doing.

 

[noseall]

Lipstick on a pig comes to mind. What about all the other thermal loss areas and condensation risk elements? Large spans with only 45mm timbers as support are going to sag and rupture or cause to crack the roofing and ceiling materials, after the first four seasons.

Madness.

EDIT: Ive just looked at the piccy. Those 'trusses' look ok and will stiffen.

 

[kpnuts]

Thanks Notch7. I will give it a try. Appreciate your time.

Nosall- thanks for taking a look. Yes, not the best design. But I assumed they knew what they were doing when I bought from a company that specialises in cabins. And is wasn’t cheap!
Yes the trusses are solid. So hopefully ok.
You mention about other places of thermal loss and condensation. Please could you expand on that? I’m obviously not very knowledgable with this type of thing. Hence coming here for advice? Any help you could give would be greatly appreciated.

 

[foxhole]

Given the structure type, insulation would be a complete waste of time and money.

 

[kpnuts]

Given the structure type, insulation would be a complete waste of time and money.

The company actually convinced me this was a better option than a log cabin as it already has the structure to insert insulation in to? And that it is much more difficult with a log cabin as need to add framing for insulation after and the issue with expansion and contraction of a log cabin.
The walls and the rest of the framing in mine have 70mm timbers and 20mm shiplap cladding with breather membrane. Are you saying not to insulate any of it? I feel completely ripped off. This cost me almost £4000! Well that’s ruined my year.

 

[Notch7]

The company actually convinced me this was a better option than a log cabin as it already has the structure to insert insulation in to? And that it is much more difficult with a log cabin as need to add framing for insulation after and the issue with expansion and contraction of a log cabin.
The walls and the rest of the framing in mine have 70mm timbers and 20mm shiplap cladding with breather membrane. Are you saying not to insulate any of it? I feel completely ripped off. This cost me almost £4000! Well that’s ruined my year.

You made the right decision, a stud frame structure is much better than a log cabin. Log cabins have interlocked timbers and corner joints, all places where rain water will lodge -not good for our damp climate.

If you insulate the walls too, you will get a pretty warm structure. It isnt perfect due to the lack of cavity behind the shiplap, but if you leave a gap that will help.

 

[kpnuts]

You made the right decision, a stud frame structure is much better than a log cabin. Log cabins have interlocked timbers and corner joints, all places where rain water will lodge -not good for our damp climate.

If you insulate the walls too, you will get a pretty warm structure. It isnt perfect due to the lack of cavity behind the shiplap, but if you leave a gap that will help.

Thanks. Yes I planned to have an air gap on the walls using small plastic spacers and pushing insulation up against it. Only about 20-25mm gap but better than nothing. Cheers for the reassurance.