air tightness and hollowcore

about2build

Hollowcore measured and being manufactured as we type. Looking to wrap the edges of the hollowcore in airtight membrane with the slab sitting onto it and having an amount lapping up to bottom of first floor wall and top of ground floor wall so that it can be integrated in wall plaster at a later stage. My questions are
1) how much in length do i need to leave to integrate into wall
2) do i need to do this on hollowcore that does not abridge an external wall?
3)any other suggestions or tips?!:D

sas

about2build wrote: »

1) how much in length do i need to leave to integrate into wall

You are aware that you don't use airtightness membrane for this job, you use roofting felt? Airtightness membrane isn't robust enough to survive the installation.

The felt comes in 1.5 metres rolls. We used the full 1.5 metres and left ourselves excess above and below to deal with any detailing later. The off cuts wouldn't be useful for anything anyway.

about2build wrote: »

2) do i need to do this on hollowcore that does not abridge an external wall?

You need to ensure a continuous airtightness layer around the perimeter. In my case we had a side bearing on the hollowcore at the gables so the wrap made sense.

https://us.v-cdn.net/6034073/uploads/attachments/31039/114621.JPG

about2build wrote: »

3)any other suggestions or tips?

Fitting the membrane properly will take time. Mine took 1.5 days. Don't assume you can do it properly the morning the hollowcore is coming.

The hollowcore installers play a critical roll here. They can't use the long bars they typically use to pry the slabs into position. Each slab has to be dropped perfectly into position. If they try to slide any of them the membrane will be damaged.

Based on a tip I got here we mixed sand and cement mortar on site and put a layer of it on top of the membrane before dropping in each slab. The mortar won't go anywhere if this is done relatively easily. It protects the membrane too.

If you search the Live Self Builds thread you'll see alot of the detailing around this for my home.

Good luck.

about2build

Thanks sas thats great advice... how did you keep the felt in place while they were dropping in HC?
Ill take a look on the self builds site as well thanks

about2build

Sas you mention a continuous layer all around but will it not be necessary to cut the felt and overlap at the right angles or will it be pliable enough to negotiate all corners??

mickdw

sas wrote: »

You are aware that you don't use airtightness membrane for this job, you use roofting felt? Airtightness membrane isn't robust enough to survive the installation.

I made this point to the guy giving an airtightness day course that I did. He maintained the membrane was perfectly suited to the job. Its not though.

sas

about2build wrote: »

Thanks sas thats great advice... how did you keep the felt in place while they were dropping in HC?
Ill take a look on the self builds site as well thanks

From the time we fitted the felt to the time the hollow core arrived, we put strips of dpc on the membrane (to protect it) and put blocks on top of the dpc. We were concerned the blocks could pierce the membrane.

sas

about2build wrote: »

Sas you mention a continuous layer all around but will it not be necessary to cut the felt and overlap at the right angles or will it be pliable enough to negotiate all corners??

I didn't mean 1 piece, just continuous!

So yes you'll need to tape it at the corners. Not a chance in hell you'll get it around the corners neatly. This is very difficult to do incidently. I had a guy that did it for me.

He preformed corners and we then taped them into the continuous straight parts.

sas

mickdw wrote: »

I made this point to the guy giving an airtightness day course that I did. He maintained the membrane was perfectly suited to the job. Its not though.

ECB is who I dealt with and they recommend the felt.

In theory of course the membrane is fine I assume but it shows a wonderful lack of site experience on behalf of that trainer.

I think this detail is often down played on how difficult it is to execute. The corners need to be bang on so that there isn't too may overlaps causing excess materail on the upstand upstairs and hence a bulge in your corners. The guy that did mine spent alot of time checking and rechecking to ensure this was at least minimized.

qaz123

Got a few questions on wrapping membrane around hollowcore slab.
I'm Looking to get feedback on anyone else who has wrapped membrane around hollowcore. 1) Did ye use airtight membrane or roofing felt.
2)After installation how was membrane attached to block.
3) how did ye ensure hollowcore didn't damage membrane during installation

I planning on installing membrane this weekend. Anyone got any other tips or advise on doing this job

mfceiling

qaz123 wrote: »

Got a few questions on wrapping membrane around hollowcore slab.
I'm Looking to get feedback on anyone else who has wrapped membrane around hollowcore. 1) Did ye use airtight membrane or roofing felt.
2)After installation how was membrane attached to block.
3) how did ye ensure hollowcore didn't damage membrane during installation

I planning on installing membrane this weekend. Anyone got any other tips or advise on doing this job

1. Most people use the airtight membrane.
2. Special tape and glue from airtightness company.
3. Be on site when it's being done or have someone you trust and is knowledgeable there.

hexosan

When they say airtight membrane most are referring to solitex roofing felt which is airtight but much stronger than the airtight membrane like intello or vario.

I wouldn't use anything other than solitex for this application.

qaz123

Going to go with solitex. I've got 150mm inner leaf and 200 mm hollowcore.
Do people cut the solitex 1.5m roll in half to 750mm strips or would that be too tight to wrap around. It would leave around 100mm to bond to block on inner leaf above and below hollowcore

hexosan

Yeah I split the rolls in two on mine. If recommend meshing the solitex to the wall straight away and plastering over it to protect and seal it.

chillit

would using the hollowcore with solid end remove the need for this airtight membrane or am I missing something.

my supplier quoted 10 euro per solid end I think.

hexosan

Solid end makes no difference air will still leak it at each of the joints between the slabs.

BryanF

Radon membrane as heavier gauge would be better than air-tightness membrane perhaps

Metric Tensor

Solid ends are necessary too because unfilled ends will allow leakage from elsewhere in the building along the slabs.

So you need both solid ends and to wrap the slab end.

sydthebeat

BryanF wrote: »

Radon membrane as heavier gauge would be better than air-tightness membrane perhaps

quite difficult to make the proper corner folds with radon membrane, actually pretty impossible without cutting, which is bad m'ok

solitex is fine, you can also place a strip of DPC on a mortar bed over the solitex if your afraid of the slab tearing the membrane

hexosan

Metric Tensor wrote: »

Solid ends are necessary too because unfilled ends will allow leakage from elsewhere in the building along the slabs.

So you need both solid ends and to wrap the slab end.

If your wrapping the slabs them solid ends are completely pointless & a waste of money.

Metric Tensor

I disagree.

I know your point is that the entire slab is inside your envelope and that is true.

But undesigned air flow paths are less than ideal in a modern house. In the worst of cases it's like knocking holes in all the walls between each room.

It's also a partial fail-safe in the event if any tears in the wrapping.

In my opinion for the minimal extra cost it's well worth it.

chillit

I'm a little confused. I realise that wrapping the slab ends appears to be the normal approach. The slab supplier even mentioned this.

However if I do the following I am wondering where the air infiltration will be.

1. mortar bed on top of last block
2. place solid end slab onto mortar
3. grout the cracks between the slabs with recommended mortar/concrete
4. pour structural screed
5. continue with first floor walls
6. I suppose for additional fail safe you could put airtight tape on the inside corner junction between wall and the underside of slab and again between the wall and screed at the top side.

Would this not work and be easier than messing around trying to wrap the end of the slabs.

Any advice much appreciated.

Metric Tensor

Only if you do Number 6 effectively AND seal the joints between the slabs from underneath. Filling the shear key won't be sufficient to stop air leakage along the slab joints from underneath and through your air-tightness envelope.

In practice wrapping the end of the slab is easier and more effective than sealing all the slab joints and sealing the walls to the slabs top and bottom.

BryanF

Item 3, who will be grouting every junction?

chillit

BryanF wrote: »

Item 3, who will be grouting every junction?

most likely myself. With 80-100mm screed on top I am struggling to see how air will get through.

However I am getting the gist that people are firmly in favour of wrapping the slab ends and they are most likely more experienced than me.

Am I correct to assume that you later tape the wrapped membrane to the inside wall and plaster over.

Also I am going with block on flat and external insulation so I suppose another approach might be to plaster a 500-600mm band on the the outside of the slab and blocks before applying external insulation. Obviously this isn't an option for people with cavities.

Thanks for all the advice

Metric Tensor

chillit wrote: »

With 80-100mm screed on top I am struggling to see how air will get through.

From underneath

Wegian

hexosan wrote: »

If your wrapping the slabs them solid ends are completely pointless & a waste of money.

Are solid ends not required if pumping bead into the cavity to prevent seepage of bead into the ID of the hollowcore?

Ps - before the point is made, this is an air tightness thread and my query is not air tightness related..

hexosan

No because once the Hollowcore is wrapped bead won't enter

THE DON FANUCCI

wrapping the hollowcore is over doing it anyway. no need.

sydthebeat

THE DON FANUCCI wrote: »

wrapping the hollowcore is over doing it anyway. no need.

DON, in all seriousness, all your recent posts in this forum show that you are a complete novice when it comes to building so dont you think that your not in a position to be offering such definitive statements like the one above?

the reasons hollowcore should be wrapped are mentioned numerous times in this thread and throughout the forum yet you think its fine just to dismiss these with a flippant statement with absolute no evidential back up??

i can guarantee you that if you tried the wet grout methods that beings suggested above that 1) you would not get a comparable low air tightness result, and 2) it would take you far far longer on site applying this wet sand and cement between the chamfered slabs



imagine this slab in your cavity and you trying to get a complete wet sand and cement seal at the joins.....

THE DON FANUCCI

sydthebeat wrote: »

DON, in all seriousness, all your recent posts in this forum show that you are a complete novice when it comes to building so dont you think that your not in a position to be offering such definitive statements like the one above?

the reasons hollowcore should be wrapped are mentioned numerous times in this thread and throughout the forum yet you think its fine just to dismiss these with a flippant statement with absolute no evidential back up??

i can guarantee you that if you tried the wet grout methods that beings suggested above that 1) you would not get a comparable low air tightness result, and 2) it would take you far far longer on site applying this wet sand and cement between the chamfered slabs



imagine this slab in your cavity and you trying to get a complete wet sand and cement seal at the joins.....

is it possible to get an airtightness value of 1 without the hollowcore wrapped?

sydthebeat

THE DON FANUCCI wrote: »

is it possible to get an airtightness value of 1 without the hollowcore wrapped?

you tell me

893bet

THE DON FANUCCI wrote: »

is it possible to get an airtightness value of 1 without the hollowcore wrapped?

Everything is possible but it would prob be much more effort than actually wrapping the slab! And cost more.

sydthebeat

893bet wrote: »

Everything is possible but it would prob be much more effort than actually wrapping the slab! And cost more.

im even seeing timber joists being completely wrapped now because of the ease of application and better test results.....

chillit

sydthebeat wrote: »

im even seeing timber joists being completely wrapped now because of the ease of application and better test results.....

Hi syd

After they wrap the timber joists what do they do then?

Tape the membrane onto the internal block wall and plaster over the membrane. Is this the way to go? assuming a block construction of course.

893bet

chillit wrote: »

Hi syd

After they wrap the timber joists what do they do then?

Tape the membrane onto the internal block wall and plaster over the membrane. Is this the way to go? assuming a block construction of course.

If going timber first floor I think the easiest course for good Airtightness is not to breach the cavity at all. Use hangers either set in the block work or on timber fixed to the block work.

chillit

893bet wrote: »

If going timber first floor I think the easiest course for good Airtightness is not to breach the cavity at all. Use hangers either set in the block work or on timber fixed to the block work.

what if I am going with block on the flat and external insulation. There will be no cavity at all.

sydthebeat

893bet wrote: »

If going timber first floor I think the easiest course for good Airtightness is not to breach the cavity at all. Use hangers either set in the block work or on timber fixed to the block work.

even when you do that!!





sorry about the size

chillit

thanks for the pics Syd. very informative and helpful

By the way is that just 2 steel nails holding those joist hangers onto the walls. I always get the heebe jeebies when I think of just a nail holding up the joists. I get image of the hanger sliding out if the wall.

BryanF

chillit wrote: »

thanks for the pics Syd. very informative and helpful

By the way is that just 2 steel nails holding those joist hangers onto the walls. I always get the heebe jeebies when I think of just a nail holding up the joists. I get image of the hanger sliding out if the wall.

Where's it going to slide to? Is it not the same shelf at the other end?

sydthebeat

chillit wrote: »

thanks for the pics Syd. very informative and helpful

By the way is that just 2 steel nails holding those joist hangers onto the walls. I always get the heebe jeebies when I think of just a nail holding up the joists. I get image of the hanger sliding out if the wall.

theres a whole inner leaf of blocks constructed over them

chillit

i know its standard practice. I'm just being irrational. I like the idea of the full joist stuck in the wall but I understand the hoist hangers are probably stronger than the actual joist itself.

Does anyone have a strong opinion on ecojoist like in the above pics versus hollowcore. I am at the decision stage and the basic price for the ecojoist versus the hollowcore is coming in the same. By basic price I mean the price of just the joists compared to the price of just the hollowcore slab (no screed costs etc)

To me pros for the ecojoist is the ability to attach the ceiling plasterboard directly onto the joist (no metal ceiling work as need with the hollowcore) and also the ability to run services through the joist. If I use hollowcore I will need to drop the ceiling to run services and it this includes sewage pipes I will need quite a large drop on the ceiling thus creating a much deeper overall first floor thickness.

Pros of hollowcore is that I can have solid walls upstairs and supposedly concrete floors are quieter than wooden.

Hollowcore would have better thermal mass I suppose but i don't know if this is a big consideration.

All thoughts welcome

sydthebeat

youve basically outlined the pros and cons above.

the main one for me is the solid feel under you feet on the first floor with concrete slabs, plus the better fire proofing and better sound proofing.

the lack of requiring a metal rail ceiling is a bit plus for the ecojoist

Calahonda52

chillit wrote: »

Does anyone have a strong opinion on ecojoist like in the above pics versus hollowcore. I am at the decision stage and the basic price for the ecojoist versus the hollowcore is coming in the same. By basic price I mean the price of just the joists compared to the price of just the hollowcore slab (no screed costs etc)

Are you sure about this?
Supply and fit of both are the same per sq meter?

If so then you need to add in the price of the the screed on top and the false ceiling below, compared to say ply or osb fitted upstairs and a skimmed slab below.

IMO, having worked on site with both as opposed to being a keyboard warrior, the space in the ecojoists become pretty crowded soon enough and suffers from the main problem that all services are constricted to essentially one plane so 4 inch sewer pipes and 6 inch MVHR pipes will clash.
The steel bracing looking sexy but is in the way a lot of the time so don't get sucked in by too much sales BS. There may also be a need for bridging between them

The sound proofing alone would make me go slab, plus the clean lines underneath

OP: have sent u pm re the flue question you asked elsewhere

Metric Tensor

I'm with Calahonda - go hollowcore all the way.

The sound difference is amazing both through the floor and the benefits provided by having blockwork walls upstairs.