Final check before ordering - garage conversion

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Hi all,

Had loads of really useful help on here, special thanks to Woody on that front. Just wanted to list out everything i'm getting before putting quite an expensive order in (that I can't afford to get wrong). Plus it may help someone seeing this level of detail.

I've spoke to building regs and they're happy with what I've got but wanted to see what the size of cost savings could be. Assuming its latest regulations for sign off and space in the room is a premium so the thinner the better.

So i'm at the stage where all electrics are done. Its all cleared out and I need to do the insulation, flooring and plasterboarding.

Wall 1 - this is a cavity wall, I've got 70mm PIR board in the cavity.

Wall 2 - this is a single skin brick wall with 2 piers. Its going to be battened with 50x100's, have 100mm PIR between piers and then 30mm PIR overlaid over the 100mm and the piers. One thing that was mentioned was that we needed a breathable membrane to go over the brick as the PIR is tight against the wall. Is this right and what's a good value option for this? I've only dealt with the polythene sheets.

Wall 3 and 4 are fully internal so no insulation issues here.

Ceiling - Its a pitched roof being insulated at joist level. There are two vent tiles at the back and a vented soffit at the front. Plan is 125mm between the joists (which is the full depth of the joist) and then 30mm overlaid on the top. Question: How much am I likely to save by just using the rockwool insulation and how much would I need to meet regs?

Floor - It'll be a floating floor based off a concrete slab. Going to have a polythene DPM on top of the slab, running up to the DPC level to then meet the breathable membrane where present. Need to lift the floor up by 220mm to match the connecting room so was going to use 220mm of PIR board (made up of 2 lots of 100mm and a 20mm sheet). Question: Appreciate this is probably an expensive way of doing it. At that level would I see much difference using polystyrene boards instead? What is a cheap way of raising the floor level by 20mm so I don't need a 20mm board or is that going to be the cheapest way to do it? Floor is already level apart from a few little bumps here and there. Assume I need a vapour check layer on floor if using polystyrene boards?

Thanks for reading
 
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Hi all,

Had loads of really useful help on here, special thanks to Woody on that front. Just wanted to list out everything i'm getting before putting quite an expensive order in (that I can't afford to get wrong). Plus it may help someone seeing this level of detail.

I've spoke to building regs and they're happy with what I've got but wanted to see what the size of cost savings could be. Assuming its latest regulations for sign off and space in the room is a premium so the thinner the better.

So i'm at the stage where all electrics are done. Its all cleared out and I need to do the insulation, flooring and plasterboarding.

Wall 1 - this is a cavity wall, I've got 70mm PIR board in the cavity.

Wall 2 - this is a single skin brick wall with 2 piers. Its going to be battened with 50x100's, have 100mm PIR between piers and then 30mm PIR overlaid over the 100mm and the piers. One thing that was mentioned was that we needed a breathable membrane to go over the brick as the PIR is tight against the wall. Is this right and what's a good value option for this? I've only dealt with the polythene sheets.

Wall 3 and 4 are fully internal so no insulation issues here.

Ceiling - Its a pitched roof being insulated at joist level. There are two vent tiles at the back and a vented soffit at the front. Plan is 125mm between the joists (which is the full depth of the joist) and then 30mm overlaid on the top. Question: How much am I likely to save by just using the rockwool insulation and how much would I need to meet regs?

Floor - It'll be a floating floor based off a concrete slab. Going to have a polythene DPM on top of the slab, running up to the DPC level to then meet the breathable membrane where present. Need to lift the floor up by 220mm to match the connecting room so was going to use 220mm of PIR board (made up of 2 lots of 100mm and a 20mm sheet). Question: Appreciate this is probably an expensive way of doing it. At that level would I see much difference using polystyrene boards instead? What is a cheap way of raising the floor level by 20mm so I don't need a 20mm board or is that going to be the cheapest way to do it? Floor is already level apart from a few little bumps here and there. Assume I need a vapour check layer on floor if using polystyrene boards?

Thanks for reading
Vapour check is to prevent (warm) moist internal air migrating into a (cold) timber structure.

Breather membranes allow water vapour to escape a timber structure.

DPM stops water/damp migration.
 
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What's the logic of a breather membrane between brick and insulation if it's tight to the wall?

Can internal moist air make its way through 100mm + 30mm PIR with 4 layers of foil, if properly taped?


Think I would use poly on the floor, 200mm as PIR is going to be costly.
Screed is probably cheapest way to get 20mm, read on here before folk using sand.


If you are having a flat ceiling, then you might as well just use rock wool, think 300mm is recommended.
 
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Vapour check is to prevent (warm) moist internal air migrating into a (cold) timber structure.

Breather membranes allow water vapour to escape a timber structure.

DPM stops water/damp migration.
Thanks. Any recommended ones to use for the membrane? Quite new to it and seem to see a few different specs.

I'm guessing the membrane is one way. So its water vapour can get out but not in?

What's the logic of a breather membrane between brick and insulation if it's tight to the wall?

Can internal moist air make its way through 100mm + 30mm PIR with 4 layers of foil, if properly taped?


Think I would use poly on the floor, 200mm as PIR is going to be costly.
Screed is probably cheapest way to get 20mm, read on here before folk using sand.


If you are having a flat ceiling, then you might as well just use rock wool, think 300mm is recommended.

Its only a small room, so we're talking just north of 8m2 floor area. Think poly floor will save me about £200 but i'm worried about the poly floor dipping as I hear its not got the same strength PIR has. I guess it'll make little difference for the U value given its well over spec and floor is probably the last place i'll be losing heat.

It is a flat ceiling but was thinking of having a bit of storage up there so having the flat boards would help rather than having to get some stilts and board it that way.
 

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