Insulation question.

[skipdiver]

I'm ordering the stuff today to start making my workshop a bit more pleasant. I'm not looking for eco-house levels, just to take the edge off as i don't like being too warm when i'm working. It's more to make it a bit more bright and pleasant, whilst keeping my tools and machinery in better order. Doing the walls with 25mm Celotex and 11mm OSB painted white. The floor will be 25mm jablite with 18mm chipboard but i'm undecided on the roof. Got 4" rafters with breathable membrane, tiles and battens on top. My initial plan was 100mm wool of some type but reading the Knauf site it recommends counter battens for air flow above the membrane. I thought i'd be ok with just breathable membrane but i don't want to build in problems for later. Second thought is 75mm Celotex with 25 mm air gap above and air vents in the soffits. Ceiling will also be covered in OSB. Big difference in price between the two and i would favour wool for the price, ease of fit and the fact that i'm not looking for absolute total warmth here. Would wool pushed up against breathable membrane cause problems, or has anyone done this with no adverse effects? I could get 90mm batts and leave a bit of a gap to the membrane.

Any thoughts appreciated.

 

[Paul200]

I just typed a long reply to this and was then told I wasn't logged in and lost the lot! Basically what I said was you have an air gap between the tiles and the membrane courtesy of the counter battens - this should be sufficient to dry the membrane if it needs it. My summer house roof is built in the same way as you propose but with wrinkly tin instead of tiles. It's been up 18 months now (in sunny south west Scotland!) and no problems so far. I would check that you don't have any obvious small leaks first though. One thing I noticed before my tin went on was the membrane would let water where it was stapled, so I went round sealing all the staple holes! Probably unnecessary once the roof covering went on but it concerned me at the time.

 

[skipdiver]

Thanks for the reply but i don't have counter battens and that's the thing that is giving me pause for thought. Breathable membrane on top of 4"x2" rafters, then 2"x1" batten with tiles on top. Can i just stuff in 100mm wool without causing problems is my dilemma. Reading the internet suggests not but this is a workshop, not a house......

 

[Fitzroy]

I think you know the answer to your question, it's not certain. Ideally you would have a vapour barrier, insulation, vented air gap then the roof, but this is the expensive option. The alternate risks getting condensation or penetrating moisture trapped in the insulated but un-vented space, with the potential to cause damp related issues. However as you said the space is not always occupied so moisture from people being in the space is reduced, its a workshop not papered and painted room, and roof moisture issues take years to be realised even in a permanently occupied house.

I've had a number of similar arguments with myself during my workshop build, what normally won out was price. if I could do the right thing for less than a few hundred quid i did, if not i took the cheaper option. The one I am most concerned about was putting mineral wool under the floor rather than kingspan/celotex, and the thought that I have created a perfect rat hotel!

F.

 

[skipdiver]

Just been on the Knauf site and you can fit earthwool rafter roll or flexible slab to this kind of roof as long as you leave a 50mm air gap. They then suggest insulated plasterboard under that to achieve u values but i don't need that. I'm thinking thicken out the rafters with 2x2, then 100mm of rafter roll to maintain the 50mm gap.

 

[slate1234]

if the ceiling is having osb on it can you lay the insulation on top of that in between the ceiling joists, you can also buy kingspan seconds online at diffrent thicknesses ie 75mm might be 60mm/65mm, like others have said it is only a workshop and should be plenty of fresh air, but still feel you shouldn't just push the wool up under the tiles.

 

[NickN]

When I get round to doing mine, I had thought about putting a layer of vapour barrier plastic under the OSB ceiling, so that no (or very little) room moisture could get into the space above, and the insulation. Same principle as I will do with the walls. Or in other words...
OSB -> vapour barrier stapled to -> 2 x 2 studwork with insulation wool stuffed in -> breathable membrane stapled to -> 4 x 2 exterior studwork -> cladding (or roofing). So my air gap is on the outside of the breathable membrane, no other choice, as it was not put on before construction of the shed, unfortunately.

In your case with the breathable on the outside of the 4 x 2 (or whatever it is) and you're looking to fill that with insulation, then I'd just go for 50mm insulation (the Rockwool Flexi Slabs come in that thickness) and leave that air gap. Should still be a good difference compared to nothing.

 

[skipdiver]

if the ceiling is having osb on it can you lay the insulation on top of that in between the ceiling joists, you can also buy kingspan seconds online at diffrent thicknesses ie 75mm might be 60mm/65mm, like others have said it is only a workshop and should be plenty of fresh air, but still feel you shouldn't just push the wool up under the tiles.

I'm not doing the ceiling. I'm insulating between the rafters and boarding over them. Think i'm going to do what NickN suggests and just use 50mm flexi slabs. I'm not looking for ultimate warmth, just to brighten up the workspace and keep the chill off. I don't even have soffits on at the moment and the doors need draught proofing too, so along with the insulation and boarding to roof, walls and floor, i think that will be enough for me.

Thanks for all the input.