First Floor air tight membrane advice

[richo106]

I am just trying to plan ahead in what order to complete things when the roof is on.

 

I am planning on installing an air tight membrane to my first floor ceiling. Firstly could any recommend an air tight membrane that would be suitable for this (with cost in mind)? Also do I just fix this to my joists prior to plasterboarding? What’s the best way to fix this?

 

I am thinking of running it down the block wall slightly and taping it to the wall? Is that the best way also?

 

How early after you are weather tight would you do this? As I thinking of doing this near the start along with the ceiling plasterboard so I can concentrate on getting my MVHR ducting through and sealed well.

 

Any advice greatly welcomed like usual

 

Many Thanks 

[Miek]

Some thought should be given to how the light fittings will work too. Lots of folk on here add a service void which helps maintain the airtight layer, but this also adds cost. 

[richo106]

5 minutes ago, Miek said:

Some thought should be given to how the light fittings will work too. Lots of folk on here add a service void which helps maintain the airtight layer, but this also adds cost. 

I’ve seen some airtight cable grommets for standard pendants, and then for downlights I’ve seen air tight covers that  sit over the top that I’ll tape down etc

[jayc89]

I have a cold roof. Planning on running Intello Plus across the underneath of the floor joists, lapping over the VCL on the walls. You could use a plaster-able tape to secure it to the blockwork prior to wet plastering. 

[Iceverge]

You could just use a polythene sheet. Lightly to tear if not treated well.  

 

I would spring for something with some reinforcement involved.  Not terrible dear either. 

 

 

 

Have you abandoned the warm loft idea? 

 

 

 

 

[richo106]

11 minutes ago, Iceverge said:

You could just use a polythene sheet. Lightly to tear if not treated well.  

 

I would spring for something with some reinforcement involved.  Not terrible dear either. 

 

 

 

Have you abandoned the warm loft idea? 

 

 

 

 

Yes unfortunately I have abandoned the warm loft idea, my roofer said it could do it that way but we could justify the increase in cost. I would of liked to of had a warm loft 

[Iceverge]

Not to worry you can still make it work. We have a cold loft and it works fine. 

 

1. Lay a wide strip of VCL on top of all the internal wall plates (if you have any) pre roof commencing. You don't want to end up trying to seal around roof timbers where they rest on internal walls.

 

2. Make sure that any of the internal walls without wall plates have enough space to pull a VCL through. say 25mm, also it'll make the sparkys life easy. ( easy job = non damaged VCL) 

 

3. Apply VCL to the bottom of the ceiling joists. and return to the external walls. 

 

4. Put 22*70mm battens below it to create a service void. Run them at 90deg to the ceiling joists so you can run wires easily. 

 

5. Put 2 layers of standard plasterboard for noise, staggered and sealed to external walls. 

 

6.  Install MVHR plenums after the VCL and tape as appropriate. 

 

7. Forget the airtight box in the loft.  Make something like a base of timber suspended on wires or a platform on isolating rubber feet for the MVHR to sit on. put the upstairs MVHR manifolds on the floor of the loft and run your ducts from there. Cover the whole lot of the ducting in insulation.  Wrap the MVHR unit if it isn't insulated already (many are) and insulate all exposed ducting with flexible foil backed mineral wool and tape. 

 

8. Run 2 large ducts through your airtight layer to the downstairs and host your manifolds somewhere along the line. Sealing around ten to twenty ducts would be a total pain and they will need to be spaced out lots to do it properly. 2 large ones would be easy. 

 

9. Substitute a sheet of OSB for the VCL above a suitable cupboard for all the wires that have to go in the attic We had 5. In hindsight just the TV aerial would have been needed.  Get the electrician to drill ONE small hole for ONE wire and you can seal afterwards with sealant/tape. 

 

10. If you can afford a blown cellulose insulation or mineral wool it'll cover the ducting much easier. Cellulose is my fave, helps airtightness, good for heat protection, noise, non itchy etc etc etc. 

 

11. Otherwise consider something denser like Rockwool which will help with sound proofing too. 

 

 

Any questions just ask. 

 

 

 

 

[richo106]

On 20/02/2023 at 22:57, Iceverge said:

Not to worry you can still make it work. We have a cold loft and it works fine. 

 

1. Lay a wide strip of VCL on top of all the internal wall plates (if you have any) pre roof commencing. You don't want to end up trying to seal around roof timbers where they rest on internal walls.

 

2. Make sure that any of the internal walls without wall plates have enough space to pull a VCL through. say 25mm, also it'll make the sparkys life easy. ( easy job = non damaged VCL) 

 

3. Apply VCL to the bottom of the ceiling joists. and return to the external walls. 

 

4. Put 22*70mm battens below it to create a service void. Run them at 90deg to the ceiling joists so you can run wires easily. 

 

5. Put 2 layers of standard plasterboard for noise, staggered and sealed to external walls. 

 

6.  Install MVHR plenums after the VCL and tape as appropriate. 

 

7. Forget the airtight box in the loft.  Make something like a base of timber suspended on wires or a platform on isolating rubber feet for the MVHR to sit on. put the upstairs MVHR manifolds on the floor of the loft and run your ducts from there. Cover the whole lot of the ducting in insulation.  Wrap the MVHR unit if it isn't insulated already (many are) and insulate all exposed ducting with flexible foil backed mineral wool and tape. 

 

8. Run 2 large ducts through your airtight layer to the downstairs and host your manifolds somewhere along the line. Sealing around ten to twenty ducts would be a total pain and they will need to be spaced out lots to do it properly. 2 large ones would be easy. 

 

9. Substitute a sheet of OSB for the VCL above a suitable cupboard for all the wires that have to go in the attic We had 5. In hindsight just the TV aerial would have been needed.  Get the electrician to drill ONE small hole for ONE wire and you can seal afterwards with sealant/tape. 

 

10. If you can afford a blown cellulose insulation or mineral wool it'll cover the ducting much easier. Cellulose is my fave, helps airtightness, good for heat protection, noise, non itchy etc etc etc. 

 

11. Otherwise consider something denser like Rockwool which will help with sound proofing too. 

 

 

Any questions just ask. 

 

 

 

 

Hi @Iceverge

 

I am just referring back to this post as I am getting to the point now.

 

What is the best way to fix it to the underside of the joists? Staple and then tape over them?

 

Do you tape each joint air tight tape? Any recommendations?

 

Do I also tape the the end to the block wall also?

 

Just looking at air tight tape…not cheap is it

 

Any advice/information greatly appreciated 

 

[Iceverge]

4 hours ago, richo106 said:

What is the best way to fix it to the underside of the joists? Staple and then tape over them?

 

If it's the membrane you're speaking of then yes. 

 

Regarding the tape so long as its one designed for purpose it'll be ok. Maybe buy one roll each of a couple of types and see what you think. Its generally only as good as it is easy to use. 

 

 

 

 

Regarding sticking the last sheet of the PIR to the blockwork. 

 

 

What is your wall finish going to be? Boarded or wet plaster? 

 

 

 

[Iceverge]

 

This is the stuff we used by another name but it was a bit cheaper. It was excellent. 

 

It was cheaper than the Soudal Equivalent and tougher. We used a little bit of tyvek too but it wasn't even close to being straight. 

Edited April 24, 2023 by Iceverge

[richo106]

36 minutes ago, Iceverge said:

 

If it's the membrane you're speaking of then yes. 

 

Regarding the tape so long as its one designed for purpose it'll be ok. Maybe buy one roll each of a couple of types and see what you think. Its generally only as good as it is easy to use. 

 

 

 

 

Regarding sticking the last sheet of the PIR to the blockwork. 

 

 

What is your wall finish going to be? Boarded or wet plaster? 

 

 

 

The finish is going to be boarded 

[richo106]

33 minutes ago, Iceverge said:

 

This is the stuff we used by another name but it was a bit cheaper. It was excellent. 

 

It was cheaper than the Soudal Equivalent and tougher. We used a little bit of tyvek too but it wasn't even close to being straight. 

Does this stick to block work or do I need primer?

[Iceverge]

No, it won't stick to blockwork. Membranes, OSB, Foil faced boards, metal all ok. If you tried to stick it to block it'd just pull off a thin layer of dust. 

 

With boarding you'll be depending on the board adhesive to seal around the edges of the boards and any penetrations like sockets etc for airtightness. 

 

I would tape a strip of membrane to the bottom PIR board and stick it to the blocks with some airtight sealant like this. 

 

 

Then make sure the boarders cover the joint between the blocks and membrane with a continuous bead of boarding adhesive, it's as important to prevent any damp air escaping from the house to your roof and rotting it, as it is for energy saving and comfort. 

 

It can be easier to leave the membrane too wide, then seal and then trim back to 50-100mm once done. 

 

This is the stuff. Wear gloves as it's like permanent chewing gum. 

 

 

 

 

 

 

[richo106]

Thank you