Gap between insulation, DPM querys

[ninja432]

I am doing a garage conversion where I'll be fixing in a wooden frame, batons, joists, insulating and then boarding up.

 

I can't find Dam Proof Membrane wider than 4 metres anywhere. My floor is over 4 metres wide and long so I guess I have to overlap it. ~ How big should the overlaps be? ~

 

Also I was going to screw the joists to the concrete floor but then I would have to drill through the membrane. Does this matter?

 

And finally should I leave some sort of gap, somewhere between the outer wall, DPM, insulation and inner wall board, for ventilation?

[Jeremy Harris]

The normal overlap for a DPM joint is at least 150mm, with DPM sealing tape used at the joint.

 

Bolting down through the membrane is fine, all timber frames end up being bolted down through the DPM.

[PeterW]

Overlap of 30cm is fine - just use duct tape to secure the join. 

 

Not  sure on your other comment though - can you put a drawing up as can’t envisage the wall build up. 

[Russell griffiths]

Are you saying you have a concrete floor that you are putting joists onto ?

if this is the case why not use a liquid dpm painted on and screw through that. 

[ninja432]

Ok doing a drawing is not doable right now. Maybe I can explain:

 

I want to put a DPM against the wall. Then bolt the batons to the wall. Then put insulation against the wall. Then board the wall.

 

The wall is made of breeze blocks. 

 

Does that make sense?

[Jeremy Harris]

1 minute ago, ninja432 said:

Ok doing a drawing is not doable right now. Maybe I can explain:

 

I want to put a DPM against the wall. Then bolt the batons to the wall. Then put insulation against the wall. Then board the wall.

 

The wall is made of breeze blocks. 

 

Does that make sense?

 

 

So, if I've understood correctly, you're trying to use DPM as a vapour barrier, but want to put it the wrong side of the insulation layer, is that right? 

 

Doesn't sound sensible to me, TBH, as it's critical that any vapour tight layer be on the warm side of the insulation layer.  Failure to do this may result in interstitial condensation on the outer face of the timbers, where they are fixed to the cold wall, which will cause them to rot.

 

The normal build up for something like this would be to fix the battens to the wall, fit the insulation, then fit a vapour tight layer inside that, and behind the final wall finish on the inside.

[ninja432]

Ok JS Harris, thanks, I didn't realise I had the DPM placement incorrect. And what about leaving a gap somewhere between one of these layers, is that necessary?

 

 

Russel Griffiths,  yes, a concrete floor that I am putting joists onto. I didn't even know you could get liquid DPM, thanks, I will look into that.

[Jeremy Harris]

29 minutes ago, ninja432 said:

Ok JS Harris, thanks, I didn't realise I had the DPM placement incorrect. And what about leaving a gap somewhere between one of these layers, is that necessary?

 

 

Russel Griffiths,  yes, a concrete floor that I am putting joists onto. I didn't even know you could get liquid DPM, thanks, I will look into that.

 

Is the block wall single skin or a cavity wall?

 

If it's single skin, then it might be a good idea to include a cavity between the timber inner frame and the outer wall.  If it's a cavity wall then there's no need for an additional ventilated cavity.

 

With the floor, then I'd be inclined to support the joists above the floor, with a ventilated cavity beneath.  No need for a DPM on the concrete side, just a vapour membrane on the warm side.  Alternatively, you could opt to lay insulation and a membrane on to the concrete floor, then screed over the top for the finished floor.

[ninja432]

13 minutes ago, JSHarris said:

 

Is the block wall single skin or a cavity wall?

 

If it's single skin, then it might be a good idea to include a cavity between the timber inner frame and the outer wall.  If it's a cavity wall then there's no need for an additional ventilated cavity.

 

With the floor, then I'd be inclined to support the joists above the floor, with a ventilated cavity beneath.  No need for a DPM on the concrete side, just a vapour membrane on the warm side.  Alternatively, you could opt to lay insulation and a membrane on to the concrete floor, then screed over the top for the finished floor.

The block wall is a single skin.

 

What is a vapour membrane?

 

Ok so if I support the joists above the floor, would I do this with blocks? And what about the insulation, if the joists are supported above the floor, then the insulation would fall through?

 

What is Screed?

[Jeremy Harris]

Just now, ninja432 said:

The block wall is a single skin.

 

What is a vapour membrane?

 

Ok so if I support the joists above the floor, would I do this with blocks? And what about the insulation, if the joists are supported above the floor, then the insulation would fall through?

 

What is Screed?

 

The vapour membrane is the vapour tight layer on the inside face of the insulation, that's there to prevent moisture in the warm air inside the house from moving out through the structure and condensing in the colder outer layers.  It's really just a plastic sheet that's fixed and taped to the inside surface, or maybe a vapour tight board, or even foil-faced insulation with the joints taped up.

 

The floor joists could be supported on blocks, or on hangars from the walls.  Alternately you could just lay insulation down directly on the existing concrete floor, add a membrane over it then use a screed for the new floor.  A screed is just a layer of concrete or other material that can be poured, levelled and then hardens to form a flat floor.  It can be reinforced to allow a thinner layer to be used if need be.

 

Insulation can be supported in several different ways when fitted between joists.  A permeable membrane or netting can be stretched underneath, or slim battens can be fastened as supports. 

[Russell griffiths]

What is Screed

 

sorry but if you need to ask this, then I think you need to go back to the drawing board and do a design and get it looked at by someone with more knowledge. 

 

Have you submitted building regs application. 

I was under the impression that conversion of a garage to a habitable space would need to comply with building regs. 

[Jeremy Harris]

Conversion of a garage to a habitable room definitely needs Building Regs approval.

[ninja432]

6 minutes ago, Russell griffiths said:

What is Screed

 

sorry but if you need to ask this, then I think you need to go back to the drawing board and do a design and get it looked at by someone with more knowledge. 

 

Have you submitted building regs application. 

I was under the impression that conversion of a garage to a habitable space would need to comply with building regs. 

Ok I have looked up what screed is. Well my drawing board is currently being constructed via this forum, phone calls to a a family member who is a professional builder (albeit one who is difficult to contact at times) and also drawing from experience I have working on a renovation project a few years back.  

 

I am converting the garage to a studio, not living space and in this instance planning permission is not required.

[Jeremy Harris]

Just now, ninja432 said:

Ok I have looked up what screed is. Well my drawing board is currently being constructed via this forum, phone calls to a a family member who is a professional builder (albeit one who is difficult to contact at times) and also drawing from experience I have working on a renovation project a few years back.  

 

I am converting the garage to a studio, not living space and in this instance planning permission is not required.

 

I'm sure that planning permission isn't required, as the floor area is under the limit I suspect, but Building Regulations approval will be required, as a studio will be classified as a habitable room, I'm reasonably sure.  In all probability it won't be too much hassle to gain Building Regs approval, but you will need to make an application and either go for full plans approval before you start work, or opt to do the work via a building notice.  The latter would be probably the best route for a modest conversion project like this.

 

There are details of what's needed here: https://www.gov.uk/building-regulations-approval

[ninja432]

17 minutes ago, JSHarris said:

 

The vapour membrane is the vapour tight layer on the inside face of the insulation, that's there to prevent moisture in the warm air inside the house from moving out through the structure and condensing in the colder outer layers.  It's really just a plastic sheet that's fixed and taped to the inside surface, or maybe a vapour tight board, or even foil-faced insulation with the joints taped up.

 

The floor joists could be supported on blocks, or on hangars from the walls.  Alternately you could just lay insulation down directly on the existing concrete floor, add a membrane over it then use a screed for the new floor.  A screed is just a layer of concrete or other material that can be poured, levelled and then hardens to form a flat floor.  It can be reinforced to allow a thinner layer to be used if need be.

 

Insulation can be supported in several different ways when fitted between joists.  A permeable membrane or netting can be stretched underneath, or slim battens can be fastened as supports. 

Ok so when you say vapour membrane, do you mean this stuff:  https://www.jewson.co.uk/building-materials/thermal-insulation/condensation-control/products/FNPF0510/proctor-breather-membrane-beigec2a050g-100m-x-1000mm/ this is different to DPM right? 

 

Why does the guy in this video put the vapour membrane on the other side of the insulation, to where you say it needs to be? Is he doing it wrong? I am noticing different people have different opinions on where these layers need to be. 

[Jeremy Harris]

1 minute ago, ninja432 said:

Ok so when you say vapour membrane, do you mean this stuff:  https://www.jewson.co.uk/building-materials/thermal-insulation/condensation-control/products/FNPF0510/proctor-breather-membrane-beigec2a050g-100m-x-1000mm/ this is different to DPM right? 

 

Why does the guy in this video put the vapour membrane on the other side of the insulation, to where you say it needs to be? Is he doing it wrong? I am noticing different people have different opinions on where these layers need to be. 

 

 

No, that's breather membrane, and is vapour permeable.  It's intended to go on the outside, in order to allow water vapour to escape through it.

 

It's not a matter of opinion, it's a fact that a vapour tight membrane goes on the inside and a vapour permeable breather membrane goes on the outside.

[ninja432]

1 minute ago, JSHarris said:

 

 

No, that's breather membrane, and is vapour permeable.  It's intended to go on the outside, in order to allow water vapour to escape through it.

What, so are there are 3 types of membrane then?  When you say "vapour membrane", do you mean this then: https://www.jewson.co.uk/building-materials/drainage/foundation-materials/damp-proof-membrane/products/FNPYF003/visqueen-damp-proof-membrane-blue-4m-x-25m/  ??

[PeterW]

No thats just DPM and pretty thick. Find a plastic membrane that is about 300 gauge. 

[ninja432]

29 minutes ago, JSHarris said:

 

 

No, that's breather membrane, and is vapour permeable.  It's intended to go on the outside, in order to allow water vapour to escape through it.

 

It's not a matter of opinion, it's a fact that a vapour tight membrane goes on the inside and a vapour permeable breather membrane goes on the outside.

Ok gotcha, thanks!

[ninja432]

Ok so I have had a good think about all my options and I think I'm going to hang the joists from the wall and allow an air cavity underneath as well as between the outer and inner wall. 

 

So I'm wondering how big the cavity should be, minimum? I don't have a lot of height in this garage so don't want to go too high.

 

Also with the joists, I was going to use 45x95mm's, spaced 600mm apart and they will be about 4.2M long. Will they need any support, aside from the hangers at each end?

Edited May 28, 2019 by ninja432

[Jeremy Harris]

12 minutes ago, ninja432 said:

Ok so I have had a good think about all my options and I think I'm going to hang the joists from the wall and allow an air cavity underneath as well as between the outer and inner wall. 

 

So I'm wondering how big the cavity should be, minimum? I don't have a lot of height in this garage so don't want to go too high.

 

Also with the joists, I was going to use 45x95mm's, spaced 600mm apart and they will be about 4.2M long. Will they need any support, aside from the hangers at each end?

 

The cavity only needs to be enough for ventilation, could be as little as 50mm I think.

 

The joists you're proposing are way too small, I'm afraid.  For a 4.2m span with joists at 400mm centres they would need to be at least 220 x 38 (that's very slightly undersize) and should ideally be either 220 x 47 or  235 x 38. 

 

Joists that were just 95 x 45 spaced on 600mm centres would only span a maximum of about 1.4m.

 

I suspect that laying insulation directly on the existing slab and then laying a membrane and screed would give you the lowest floor build up thickness.

[ProDave]

I am working on an old cabin that is being refurnished.  They have used 95 by 45 floor joists, BUT they are supported to the concrete floor about every metre with packers.  So you can do that if there is a reason you must limit the thickness of the floor make up but you must support them at several places not just the ends.

[ninja432]

Quick question about the joists, if I need to use packers underneath, will the plastic type do the job, or do I need something like steel or roofing slate?

[Jeremy Harris]

8 hours ago, ninja432 said:

Quick question about the joists, if I need to use packers underneath, will the plastic type do the job, or do I need something like steel or roofing slate?

 

 

Slate is best for floor packers, as it doesn't compress with time.  A bit more fiddly to get the levels spot on, but it's what I'd use.

[Russell griffiths]

Ok so I’m going to mess with your head a bit. 

@ninja432

how about a floating floor, it used to be popular, and if this is just an office ,gym type room would be more than adequate. 

 

Eps insulation, laid on concrete floor, moisture resistant tongue and groove chipboard on top all glued together to make one big sheet, edges held down by skirting. 

No joists, no hangers, no screwing or packing, 

It would depend on the quality of the floor underneath