Air tightness around pre-cast slabs

doniepony

Hey all, was just talking to my builder about the concrete first floor which will be going in soon. The spec says

'Air tightness barrier to be wrapped around end of pre-cast slabs where the slab enters the wall cavity. Barrier to be turned down inside face of internal leaf of cavity wall, sealed with appropriate sealant and plastered over beneath pre-cast slab. Barrier to be turned up inside face of internal leaf of cavity wall, sealed with appropriate sealant and plastered over above pre-cast slab'

but the builder just said (not very clearly) that the pre-cast concrete supplier has told him that there is no need for the air tightness barrier as they now have a special addition to the end of the slab to close the cavity.

Any opinions on this??

JamBur

Go with spec, even if its just for a belt and braces approach........ absolutely go with spec

BryanF

+1 go with the spec

do tell us more about the builders solution?

sas

doniepony wrote: »

but the builder just said (not very clearly) that the pre-cast concrete supplier has told him that there is no need for the air tightness barrier as they now have a special addition to the end of the slab to close the cavity.

Any opinions on this??

This is most likely the usual crap that keeps getting trotted out. The cores aren't the only problem. Sealing them won't make an ounce of difference if you omit the wrap.

The builder doesn't want to do it because it's tricky.

See here
http://www.boards.ie/vbulletin/showthread.php?t=2056249861

doniepony

BryanF wrote: »

+1 go with the spec

do tell us more about the builders solution?

I rang my engineer earlier and he is ringing the supplier tomorrow to ask him about it- I think builder just does not want to do it. He is ringing me tomorrow about it so should know more.

Do-more

Your spec isn't very well written and could be easily mis-interpreted by a builder looking for the easiest option as it only mentions the ends of the slabs.

It's also important (probably more so) to wrap the sides of any slabs that enter the cavity.

Pre-cast slabs are rarely perfectly flat and usually bow up in the middle so you can end up with a gaping hole under the slab as it may not be bearing on the block work along it's side.

Be sure that the wrap the ends and sides of any pre-cast slabs that enter the cavity.

doniepony

Do-more wrote: »

Be sure that the wrap the ends and sides of any pre-cast slabs that enter the cavity.

Thought I had that covered when spec says

'Air tightness barrier to be wrapped around end of pre-cast slabs where the slab enters the wall cavity'.

Is that not clear enough do you think?

sydthebeat

Do-more wrote: »

Your spec isn't very well written and could be easily mis-interpreted by a builder looking for the easiest option as it only mentions the ends of the slabs.

It's also important (probably more so) to wrap the sides of any slabs that enter the cavity.

Pre-cast slabs are rarely perfectly flat and usually bow up in the middle so you can end up with a gaping hole under the slab as it may not be bearing on the block work along it's side.

Be sure that the wrap the ends and sides of any pre-cast slabs that enter the cavity.

i think the specification, as a performance spec, is written clear enough so as not to be ambiguous. It actually outlines the procedure quite well. Wrap ends, seal above and below to inner face of inner leaf.....

It doesnt specify exact materials, but thats quite common when providing a performance specification and not a prescriptive specification.

Do-more

I'm just thinking in terms of real world experiences of how a builder who doesn't want to do a particular task will then follow the instruction's as they are written and argue that he has complied with them.

I'm not sure if you are seeing the point I am making. I will try to illustrate with what I mean with two diagram's.

sydthebeat

in all fairness, if a builder tried to argue the former to me on site as "wrapping the ends" id tear strips off him!!!

the spec says "where the slab enters the wall cavity"....

but i think we're just arguing semantics at this stage.

Do-more

sydthebeat wrote: »

in all fairness, if a builder tried to argue the former to me on site as "wrapping the ends" id tear strips off him!!!

Likewise, but remember the builder doesn't even understand why you are wrapping the slabs and is actively trying to get out of doing the job so my opinion is that it is best to layout what is required in the clearest possible terms so that there can be no possible grounds for ambiguity.

Last such experience I had was with a plumber where the spec. was for all radiators to be piped from the wall. Having completed the task on the ground floor perfectly, I came on site to find him half way through plumbing the upstairs rads out of the floor obviously because it was a lot easier for him. His reply was "Ah I thought you only meant on the ground floor". On that occasion there wasn't even any grounds for ambiguity just an individual who wanted the easy option.

The majority of builders and tradespeople are competent and willing and able to follow instructions but I find it is best to assume you are going to come across the other pigheaded individuals.

sas

Do-more wrote: »

I'm just thinking in terms of real world experiences of how a builder who doesn't want to do a particular task will then follow the instruction's as they are written and argue that he has complied with them.

I'm not sure if you are seeing the point I am making. I will try to illustrate with what I mean with two diagram's.

Not even as simple as that. Hollowcore is supplied with and without a side bearing.

A friend of mine didn't have a side bearing so he only did the detail you've descibed as the "builders version". He got 0.5 so it can be done in certain circumstances.

I did have a side bearing so I did the "correct version".

I absolutely agree though. Having watched an airtightness guy spend 2 full very long days on my site completing the wrap, there isn't a chance in hell that any regular builder (see "I've been doing this for years...") will put the effort in or even be capable of it.

esox28

sas hey,

just been looking back on pics of air tightness membrane,

how did you get the corners to sit so well, can you join the membrane?

about2build

did mine last week, took two airtightness specialists 4-5 hours to complete.....sides of my slabs do not enter the cavity so i plastered block behind where slab was going to sit so that this can be continues up and down from there. Blocklayer, builder and plasterer hadn't heard of the process and my insulation guy hadn't either so i had to go over his head!! Oh and thanks to sas bp et al. on here too:)

sas

esox28 wrote: »

sas hey,

just been looking back on pics of air tightness membrane,

how did you get the corners to sit so well, can you join the membrane?

The guy that did mine basically created a mockup of a corner using timber and created each corner on that. When you intially do it you get a alot of membrane overlapping so he trimmed alot of it out of the way and taped and glued it all. Very slow going.

You can joing the membrane using airtightness tape. In fact you'll have to.

sas

about2build wrote: »

did mine last week, took two airtightness specialists 4-5 hours to complete.....sides of my slabs do not enter the cavity so i plastered block behind where slab was going to sit so that this can be continues up and down from there. Blocklayer, builder and plasterer hadn't heard of the process and my insulation guy hadn't either so i had to go over his head!! Oh and thanks to sas bp et al. on here too:)

Glad to hear you got this sorted. Yes, it's one of those things you just have to take on driving yourself.

You really gotta start posting some pics on the self builds thread.

esox28

Blocklayer, builder and plasterer hadn't heard of the process and my insulation guy hadn't either so i had to go over his head!!

sure its been completed on projects there's pics too prove it right here on boards:) this is my problem too the guys working on my house havent seen an air tight house b4 and such it is becoming difficult to get the right advice...just been talking to my 'wide slab supplier' and he suggests to put a sand cement 2to1 ratio between slab and block to create an air tight seal ! would this method achieve the same result as using an air tight membrane?

Do-more

Not building in concrete myself, but just just thinking about this, I would imagine that a layer of butyl rubber or similar to protect the air tightness membrane from being punctured by the slabs might be a good addition to this detail? Any reason it wouldn't work?

I only think of it because upstairs in my house I have to screw through the airtightness membrane to secure laths to carry the plasterboard on, so I have got some double sided self sealing tape to put on at all the points I have to screw through. Some guys use the double sided rubber tape for joining radon barrier for this detail.

sas

esox28 wrote: »

sure its been completed on projects there's pics too prove it right here on boards:) this is my problem too the guys working on my house havent seen an air tight house b4 and such it is becoming difficult to get the right advice...just been talking to my 'wide slab supplier' and he suggests to put a sand cement 2to1 ratio between slab and block to create an air tight seal ! would this method achieve the same result as using an air tight membrane?

You need to consider air movement under, over and between the hollowcore slabs (in the vertical gap where the slabs meet).

The hollowcore\wideslab suppliers all seem to miss the point here. Some of them seal the cores so they claim no air can pass in. This simply isn't true.

The membrane is the recommended way for a reason.

It would be much harder to put a sand and cement seal at the required locations than to use the membrane in my opinion. The membrane will be flexible too so more tolerant of movement.

We used the membrane and actually layed a bed of mortar along the membrane just as the slabs were being dropped into position. This protected the membrane from damage during slab installation.

esox28

We used the membrane and actually layed a bed of mortar along the membrane just as the slabs were being dropped into position. This protected the membrane from damage during slab installation

yea this method has been put through from the guys at sega, he was saying that by putting the cement mix on top of membrane just b4 the slab goes on it will coussion the slap from damaging the membrane.

he wasent in favour of using the stronger windproof membrane as apposed to the air tight lighter membrane.

sas

esox28 wrote: »

yea this method has been put through from the guys at sega, he was saying that by putting the cement mix on top of membrane just b4 the slab goes on it will coussion the slap from damaging the membrane.
.

Exactly

esox28 wrote: »

he wasent in favour of using the stronger windproof membrane as apposed to the air tight lighter membrane
.

If they are saying their membrane will take the abuse then fine. The other crowd specifcally do recommend it though.

musclesmcginty

I've been looking for a sample drawing or sketch detailing a low thermal loss/no air loss construction detail at the joining of a 250mm insulation wide cavity wall type with a pre cast concrete floor. Any pointers?

Also what additional sturctural support is required on the inner leaf of the ground floor wall? RSJ the only option?

about2build

Thanks sas oh and i placed a role of 4 inch dpc along the top of membrane to take any damage during installation but in fairness to the guys who put it in, i told them about what i was tring to achieve and they were very careful as i watched from a nearby mound of topsoil!!!

Have tried to post photos sas but to no avail.....tips?!?!

sas

about2build wrote: »

Have tried to post photos sas but to no avail.....tips?!?!

PM sent

heavydawson

sas wrote: »

Glad to hear you got this sorted. Yes, it's one of those things you just have to take on driving yourself.

You really gotta start posting some pics on the self builds thread.

Hey sas, would you mind PM-ing me the details of your air-tightness contractors? Did they do the whole house, or just certain parts of it for you?

hexosan

Can you PM these detail also please

muffler

Please send a PM to the person you are looking for the info from. Far too many of these silly "PM me" posts popping up and destroying threads in this forum.