Air-tightness above suspended ceiling

heavydawson

Blockwork wrapped up and I've got an airtightness detailing question.
We've got hollow-core slabs underneath which we'll be running 4" HRV ducts, with suspended ceiling below that. The plastering will come up to the ceiling, which will leave probably 5-6" above the suspended ceiling of exposed blockwork.
From what I've learned about airtightness, that blockwork should be covered in a membrane and folded back onto the underside of the slab (We've already wrapped the hollow-core during installation)

Am I correct in thinking I've got to cover the 5-6" with membrane, or will the plaster meeting the ceiling be enough, assuming any punctures in the ceiling are sealed with air-tightness tape?

BryanF

heavydawson wrote: »

Blockwork wrapped up and I've got an airtightness detailing question.
We've got hollow-core slabs underneath which we'll be running 4" HRV ducts, with suspended ceiling below that. The plastering will come up to the ceiling, which will leave probably 5-6" above the suspended ceiling of exposed blockwork.
From what I've learned about airtightness, that blockwork should be covered in a membrane and folded back onto the underside of the slab (We've already wrapped the hollow-core during installation)

Am I correct in thinking I've got to cover the 5-6" with membrane, or will the plaster meeting the ceiling be enough, assuming any punctures in the ceiling are sealed with air-tightness tape?

Wall plaster is overlapped with Hollow core wrap

heavydawson

BryanF wrote: »

Wall plaster is overlapped with Hollow core wrap

Ceilings are quite high Bryan, so wrap will not come down as far as where plastering will end (at suspended ceiling level). So sounds to me like it'll be:

Wrap overlapped with
Membrane overlapped with
plaster
, from top to bottom.

Or were you suggesting bringing the plaster all the up to the wrap? This would be plastering the service area above the suspended ceiling.

fclauson

yup - plaster all the way up

also ping SAS an email - he has the make up you have

sas

heavydawson wrote: »

Ceilings are quite high Bryan, so wrap will not come down as far as where plastering will end (at suspended ceiling level). So sounds to me like it'll be:

Wrap overlapped with
Membrane overlapped with
plaster
, from top to bottom.

Or were you suggesting bringing the plaster all the up to the wrap? This would be plastering the service area above the suspended ceiling.

I can only guess here but I assume you are getting told that the slab itself "must" be plastered into the wall. You can't do this and get your airtightness detail correct.

The order is as follows:
1. The membrane you wrapped the end of the hollowcore with should be fixed to the wall using expanded metal.
2. Then you scud coat your walls
3. Then you scratch coat your walls bringing it up and over the expanded metal and up against the underside of the hollowcore. This forms your airtight seal.
4. Then you install your suspended ceiling rail system\run services.
5. Slab
6. Skim coat walls\ceiling

heavydawson

sas wrote: »

I can only guess here but I assume you are getting told that the slab itself "must" be plastered into the wall. You can't do this and get your airtightness detail correct.

The order is as follows:
1. The membrane you wrapped the end of the hollowcore with should be fixed to the wall using expanded metal.
2. Then you scud coat your walls
3. Then you scratch coat your walls bringing it up and over the expanded metal and up against the underside of the hollowcore. This forms your airtight seal.
4. Then you install your suspended ceiling rail system\run services.
5. Slab
6. Skim coat walls\ceiling

That all makes sense SAS. To be honest, it was just lack of experience on my part. Thanks all. Appreciate the pointers. Definitely less messy than adding a membrane between the wrap and the ceiling.

Big Dec

Hi,

Just jumping on this post with a further question...

I have pre-stressed slabs with my services (HRV, soil, etc) running underneath.
My issue is that I have a number of internal cavity walls through which the ducting must run. I have a few different roof sections which necessitates cavity walls (almost all ground floor walls are of cavity construction).
Once suggestion has been to remove one block from either side of the cavity and then put a plywood box in place. This would then be covered with membrane and sealed to the walls/ceiling either side of the cavity.
Does this seem like a good way of doing this, or do people have other suggestions?

Thanks,
Declan.

heavydawson

Big Dec wrote: »

Hi,

Just jumping on this post with a further question...

I have pre-stressed slabs with my services (HRV, soil, etc) running underneath.
My issue is that I have a number of internal cavity walls through which the ducting must run. I have a few different roof sections which necessitates cavity walls (almost all ground floor walls are of cavity construction).
Once suggestion has been to remove one block from either side of the cavity and then put a plywood box in place. This would then be covered with membrane and sealed to the walls/ceiling either side of the cavity.
Does this seem like a good way of doing this, or do people have other suggestions?

Thanks,
Declan.

Well Declan, you don't need to make the internal parititions air-tight. Only your external walls and roof need be considered from a membrane perspective. Basically wherever you're puncturing your building envelope.
Maybe I'm mis-understanding, but a floor plan of your house might help

Big Dec

Hi,

I should have been a bit clearer in my earlier post.
See attached picture (bit crude but it gets the idea across).
All walls except dashed red lines are cavity walls.
The house is 1 1/2 storey so the internal cavity walls are all for gables. I have 5 different roof levels. These will be open in the attic as I will only be placing my airtight membrane on the ceiling of the first floor rooms.
This means that I will have possible airflow in the cavity walls. These will all be pumped with insulation but there is still a possibility for air flow.
I need a very high level of air tightness for my heating system to work, so I am taking as many precautions as possible.

Let me know if this clarifies the issue?

Declan.

heavydawson

Big Dec wrote: »

Hi,

I should have been a bit clearer in my earlier post.
See attached picture (bit crude but it gets the idea across).
All walls except dashed red lines are cavity walls.
The house is 1 1/2 storey so the internal cavity walls are all for gables. I have 5 different roof levels. These will be open in the attic as I will only be placing my airtight membrane on the ceiling of the first floor rooms.
This means that I will have possible airflow in the cavity walls. These will all be pumped with insulation but there is still a possibility for air flow.
I need a very high level of air tightness for my heating system to work, so I am taking as many precautions as possible.

Let me know if this clarifies the issue?

Declan.

Definitely helps alright Declan. Thanks for that. I understand why you're concerned about the internal cavities now. It's an interesting one for sure, but can't be that uncommon.
What's in the cavity? Full fill or partial fill insulation?

Big Dec

The cavity will be full fill insulation.
Haven't decided on vendor yet but will be some sort of beads.

heavydawson

Not sure about the plywood box. I'd be drilling out the blocks either side to roughly the pipe size required, then putting a rubber grommet around the duct that has a sleeve which would cover any gabs between the duct and the hole it's being inserted into.
Generally speaking, because you're using a full fill, your thermal looping in the cavity should be minimal, so air leakage should be kept to a minimum, but if you want to be fussy, that rubber seal around any wall penetrations would be the way to go.The rubber may have to be covered with a mesh to allow for plastering over that area, so that needs to be considered too.

Big Dec

Thanks for the response.
I am probably being a bit fussy, but I want to minimize any possibly air infiltration.
I have a load of piping to run. My heating and heat recovery system are combined (house heated with warm air) so have a lot of ducting to run. There will be at lease 8 such piped running through one wall, a soil pipe and then a number of insulated qualpex pipes for my water.
This is a large number of holes to cut & seal. Each run of 75mm diameter ducting is continuous & the pipes are ribbed so would have to pull through the grommet on each side (possibility of these getting damaged?). As you would have to leave a certain spacing between each hole, this amounts to a lot of work. This was my reason for using plywoor to create a channel in the wall and then sealing this with tape & membrane.

heavydawson

Big Dec wrote: »

Thanks for the response.
I am probably being a bit fussy, but I want to minimize any possibly air infiltration.
I have a load of piping to run. My heating and heat recovery system are combined (house heated with warm air) so have a lot of ducting to run. There will be at lease 8 such piped running through one wall, a soil pipe and then a number of insulated qualpex pipes for my water.
This is a large number of holes to cut & seal. Each run of 75mm diameter ducting is continuous & the pipes are ribbed so would have to pull through the grommet on each side (possibility of these getting damaged?). As you would have to leave a certain spacing between each hole, this amounts to a lot of work. This was my reason for using plywoor to create a channel in the wall and then sealing this with tape & membrane.

I understand where you're coming from. Grommets would be nightmare with that amount of pipes so close together. You're general idea about a single fixed opening seems sound, but because there's so many pipes, you're going to have a lot of taping, and I don't know how well the membrane will hold up to having so many holes in it in such a confined area.

One other concern you should look into is how big this opening is going to be. Are you going to compromise the structural integrity of the wall in any way? You might want some sort of a lintel over it if it gets to a certain size. Only your engineer can answer that question.

Big Dec

Sorry, I think you have picked me up wrong (lack of details from me too).
The plywood would just be used to seal the gap in the cavity wall so you would have something like a "hatch" through the wall. Both sides and the top and bottom sealed. Would be open from one room to the next (but hidden within the suspended ceiling).
There are concrete slabs sitting on the wall, but there should be no issues with loading as the slab is about 3m wide and I would just be removing one 450mm block.

BryanF

Big Dec wrote: »

Sorry, I think you have picked me up wrong (lack of details from me too).
The plywood would just be used to seal the gap in the cavity wall so you would have something like a "hatch" through the wall. Both sides and the top and bottom sealed. Would be open from one room to the next (but hidden within the suspended ceiling).
There are concrete slabs sitting on the wall, but there should be no issues with loading as the slab is about 3m wide and I would just be removing one 450mm block.

Dec, What is your architects drawings showing?