TGD L ACD - AT line outside single leaf?

Lumen

This is the latest doc, I think, from 2011.

https://www.housing.gov.ie/housing/building-standards/tgd-part-l-conservation-fuel-and-energy/acceptable-construction-details

The air barrier line – The air barrier is a layer within the building envelope which will adequately restrict the passage of air between the internal and external environments. The barrier should closely follow the line of the inside face of the insulation in the exposed elements of the fabric of the building

My emphasis.

And then under the ACD itself...

https://www.housing.gov.ie/sites/default/files/migrated-files/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad%2C18752%2Cen.pdf

...the blue AT layer is shown on the inner face, even for a solid masonry wall.

Why?

Doesn't an AT layer on the external face solve the same problem but with fewer complications from wall and floor junctions?

Calahonda52

1: can you reference the exact detail to which you ref in the ACD from the list on the second page.

What role(s) do you see the A/T layer fulfilling?

My view it that it acts as a moisture penetration barrier, which reduces the amount of moisture that can reach the dew-point in the wall?

perhaps I am wrong

Lumen

Calahonda52 wrote: »

1: can you reference the exact detail to which you ref in the ACD from the list on the second page.

The blue line in the diagram below.

edit: wait, I've realised that there is no blue line on the wall, only the floor. Does this mean there is no approved detail for wall air tightness in this solid+EWI construction?

Calahonda52 wrote: »

What role(s) do you see the A/T layer fulfilling?

My view it that it acts as a moisture penetration barrier, which reduces the amount of moisture that can reach the dew-point in the wall?

I agree. But for this build up (all insulation external) the dew point is surely going to be in the EWI layer.

So an AT layer on the outside of the block would fulfill that requirement, and seems easier w.r.t junctions and penetrations than doing it on the inside.

Calahonda52

So how will u have continuity with floor and ceiling?
My test is to draw a pencil along the a/t layer on the section drawings...
You need to do wufi calcs to see where the DP will be

Lumen

Calahonda52 wrote: »

So how will u have continuity with floor and ceiling?
My test is to draw a pencil along the a/t layer on the section drawings...
You need to do wufi calcs to see where the DP will be

Ah, that's a good point. I guess you could apply an air tight paint to the top face of a course of blocks e.g. at DPC level or even below, and join up with the floor AT layer, and then do similar at the top of the wall.

The obvious advantage seems to be that you can then chase whatever services you like into the internal face of the wall without worrying about compromising the AT layer.

Calahonda52

Lumen wrote: »

Ah, that's a good point. I guess you could apply an air tight paint to the top face of a course of blocks e.g. at DPC level or even below, and join up with the floor AT layer, and then do similar at the top of the wall.

The obvious advantage seems to be that you can then chase whatever services you like into the internal face of the wall without worrying about compromising the AT layer.

I spec a service cavity where possible.
SAS here did the chasing option on a near PH and the smoke test showed where the back of a few sockets were not made airtight, something that would have bee hard to find without the smoke

Wartburg

I´d say there´s an important detail pointed out wrong in the drawing. Normally you have the radon barrier either underneath your subfloor or between your subfloor and finishing floor. This barrier acts as air tight layer for the ground. If you do not have the radon barrier, there should be at least a heavy gauge builders film, which does the same.
To continue the air tightness layer at the floor to wall junction on ground level, you just need to connect the radon/ builders film with your air tightness layer at the walls (mostly plaster). The most buildings nowadays have the radon membrane raised above the first block on the building envelope and I advise in this case to seal the envelope area from the (raised) radon barrier approx. 50mm further up the finishing floor level. I recommend air tight paint for this detail, because it can be easily applied by almost anyone.


As a side note: WUFI for a basic detail in a block built structure = taking a sledge hammer for cracking a nut (my personal oppinion).

Calahonda52

Wartburg wrote: »

1:
I´d say there´s an important detail pointed out wrong in the drawing. Normally you have the radon barrier either underneath your subfloor or between your subfloor and finishing floor. This barrier acts as air tight layer for the ground. If you do not have the radon barrier, there should be at least a heavy gauge builders film, which does the same.
To continue the air tightness layer at the floor to wall junction on ground level, you just need to connect the radon/ builders film with your air tightness layer at the walls (mostly plaster). The most buildings nowadays have the radon membrane raised above the first block on the building envelope and I advise in this case to seal the envelope area from the (raised) radon barrier approx. 50mm further up the finishing floor level. I recommend air tight paint for this detail, because it can be easily applied by almost anyone.


2: As a side note: WUFI for a basic detail in a block built structure = taking a sledge hammer for cracking a nut (my personal oppinion).

1: That won't be an ACD then
2: Fools and horses etc

Wartburg

Calahonda52 wrote: »

1: That won't be an ACD then

It´s pointless to discuss about ACD as long as the most common building strategies here in Ireland consist of components like concrete blocks, cavity wall and loose DPC layers (as a pretended strategy against dampness).

Lumen

Wartburg wrote: »

It´s pointless to discuss about ACD as long as the most common building strategies here in Ireland consist of components like concrete blocks, cavity wall and loose DPC layers (as a pretended strategy against dampness).

Well, except this thread specifically is trying to make sense of the ACD in the TGD, so it's a reasonable point. Although the ACD doesn't seem to make sense even on its own terms because they seem to have omiitted the air tightness detail from the wall.

Calahonda52

Lumen wrote: »

Well, except this thread specifically is trying to make sense of the ACD in the TGD, so it's a reasonable point. Although the ACD doesn't seem to make sense even on its own terms because they seem to have omitted the air tightness detail from the wall.

I have put him on my ignore list.