Opinion on airtightness detail

macgabhs

Hi all I'd appreciate any opinions on the attached detail from an airtightness perspective.

My query is at the hollowcore bearing detail at the external wall. I've a 215mm inner leaf which gives enough bearing for the hollowcore and the rising walls upstairs. Therefore my hollowcore slab is not exposed to the cavity. Would you still wrap the ends of the slabs for airtightness or would the fact that the hollowcore is literally blocked at its ends with blocks be enough?

Note the detail I've shown is for gable ends elsewhere a 100mm rising wall is provided again bearing straight off the 215mm downstairs inner leaf.

Calahonda52

You might redo it showing the rest of the A/T layer and then turn it right way around

Block on its own is porous so u will be relying on the bricky to put a layer of mortar on the inner face of the block.
You are also, IMO, creating an unnecessary tolerance detail with the HC and the infill block.
What is minimum bearing for the HC?

MicktheMan

macgabhs wrote: »

Would you still wrap the ends of the slabs for airtightness or would the fact that the hollowcore is literally blocked at its ends with blocks be enough?

Yes, as you cannot rely on the blockwork for airtightness.
What is your AT target?

macgabhs

Minimum bearing for HC is 100mm. Sorry about the photo I'm working from my phone here so didn't realise it would be sideways! The airtighness layer generally is a plasterfinish to the internal walls. What do you mean by unnecessary tolerance detail?

Calahonda52

Horizontally: 100mm bearing plus 100mm block allows you 15 mm tolerance assuming everything square, // and level, given that this is the side support for the HC, may impact on cavity insulation
vertically, if bedding the HC then its 200 plus 15 before it gets out of sync and the brickie wont be impressed with an uneven start to the next row of blocks.

Re The airtighness layer generally
is this a direct labour, Youtube, DIY build?

may help
https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx

Metric Tensor

Is the hollowcore bearing on the wall in question?

In your section you show the cores which would tend to indicate that the side of the slab is being "let in" over the walls.

It's probably just a drawing issue but on the off chance it's the side of the slab then it's a very different case from what's being discussed here.

macgabhs

Have architect and engineer to design and contractor on site but I only really get to visit at the weekends so I'm just trying to get a feel for good practice so when I ask question Monday I'm not totally green!

General comment about AT was directed at block work generally not whole house. Just felt it was the relevent detail in this case. Aiming to achieve m3/(hr.m2).

@Metric the detail occurs all around the perimeter in that sometimes the hollow cores are exposed sometimes they are not.

sydthebeat

You need to wrap the ends where the Hollowcore are bearing.

If the slabs are not bearing, like your drawing, then there's no need for them to actually be built into the wall.

mfceiling

As I always recommend when the "wrapping hollowcore" is mentioned...look up the live self builds thread and find the posts by the poster SAS. This guy had a great eye for detail and everything was done properly (belt and braces if you like). He had photos of how he wrapped his slab and it will give you an idea of the right way of doing this job.

Johnnyhpipe

It doesnt make sense to me:

1. "Non structural screed" is shown bonded directly to the slab - so it becomes a structural screed. Unless you have a debonding membrane in here which isnt shown?
2. Screed bonded to top of slab typically is poured in under 215 leaf and turns down the face of the unit - that's how you achieve the peripheral tie requirements
3. Your slab in section is shown spanning into the page but still has 100 side bearing?

You might want to check out precast slab standard details before this is done on site.

macgabhs

You will have to forgive my drawing skills the hollowcore end bearing is what I am interested in most given that this is where the holes are.

With regard to the screed the original design included a structural screed but the precast supplier has provided their own calcs to show that the structural screed is no longer required. Where the structural screed was returning down the face of the slabs effectively sealing them, if it is omitted in favour for a self levelling screed I'm trying to understand what needs to be done for airtightness so I can ask the right questions as I expect to be presented with options.

Calahonda52

macgabhs wrote: »

Have architect and engineer to design and contractor on site but I only really get to visit at the weekends so I'm just trying to get a feel for good practice so when I ask question Monday I'm not totally green!

General comment about AT was directed at block work generally not whole house. Just felt it was the relevent detail in this case. Aiming to achieve m3/(hr.m2).

@Metric the detail occurs all around the perimeter in that sometimes the hollow cores are exposed sometimes they are not.

is the A/T figure of 3 in the contract?
What interim blower door tests are in the contract to make sure this is met.
based on what you have written "generally" about A/T, unless the A/T game is upped big time, you will be lucky to get 10 and if its not in the contract it will be "tough"
You are paying these guys for this sort of stuff so I am at a loss as to how you are posting sketches like this on a Saturday with a view to some Monday meeting.....
This should all have been agreed before a sod was broken, my guess in the slab guys are more on the ball than the other three amigos? Are they all mates?

What prompted the sketch, which it seems now is about the bearing end and not the side of the slab.