Mortar airtight jointing

[WWilts]

How airtight can you make well pointed concave tooled joints that use 1:5 cement:sand mix?
Hopeless? Some hope?

Context: reasonably airtight large Celcon blocks, inner face of inner leaf of traditional cavity wall

(Just a very specific question about the best possible performance of such mortar joints, not an invitation to recommend parge coat/wet plaster etc which are already noted as options)

[Brickie]

I wouldn’t have a clue data-wise about this but I have been on jobs where the inner (cavity) face of the blockwork has been pointed,for this very reason. Years ago the blockwork wouldn’t have been pointed at all,just ‘flushed up’ which is quite haphazard & open to interpretation. 
 

[epsilonGreedy]

My guess is that providing there is a continuous bead of mortar filling 50mm of a block joint width then the mortar joint is airtight. Pointing can be deceptive because it could result in a thin crust of mortar being smeared over a joint with a void.

[Brickie]

7 minutes ago, epsilonGreedy said:

Pointing can be deceptive because it could result in a thin crust of mortar being smeared over a joint with a void.

True,though when you are required to point  up any work you’ll naturally try to keep joints full as it’s easier &  quicker to joint them if they’re full to begin with. 

[tonyshouse]

No point in trying, the blocks crack, pull away from the perps, move! Even if the start airtight it won’t last long ?

[WWilts]

13 minutes ago, tonyshouse said:

the blocks crack, pull away from the perps, move!

So far no cracks, perhaps they will develop in time.
Was considering options including painting the perps with airtight vapour permeable paint.

 

Is it the same with bricks (outer face of outer skin)? Will have to look at brick walls more carefully.

 

Edited December 14, 2021 by WWilts

[Iceverge]

Have you discounted a parge coat? 

 

[WWilts]

8 minutes ago, Iceverge said:

a parge coat? 

Wet plaster going on concrete blocks of internal wall that mistakenly was built into external wall.

Might consider parge coat on (large) Celcon blocks, but painting just the joints with airtight brush paint seems easier.

Edited December 14, 2021 by WWilts

[Iceverge]

How airtight are the blocks themselves? 

 

Out of curiosity I just went out to a medium density concrete block and poured an entire litre of water through it with none spilling over the edges. A wall of these would be plenty drafty. How much better do you expect celcon to be? 

[tonyshouse]

It will be better as it is more like an aero bar, full of bubbles and bit brittle. But the edge surfaces are too smooth offering to little key to mortar.

[Dudda]

You can buy standard blocks and certified airtight blocks which look exactly the same. Here in Ireland the budgets for schools is very small and they want buildings very durable so you'll often find fairfaced blocks are used in schools. The other problem is new buildings, due to regulations, have to achieve a certain level of airtightness. We recently completed a school using airtight blocks that were fairfaced and passed the airtightness requirements.

The only thing to consider is you do have to tape the windows to the blocks so a painted window board was used to hide and protect this tape for the full perimeter of the window not just the bottom of the window where you usually see it.

[WWilts]

11 hours ago, Iceverge said:

How much better do you expect celcon to be? 

Technical advice from Celcon:
0.1 m3/(h·m2) for the blocks themselves
No reason to doubt them.

Perpends recur, 10mm per 640mm
Beds recur, 10mm per 225mm
Large airtight blocks seem attractive as airtightness strategy, minimising area of joints between blocks

Concrete blocks are known to be air-permeable. There is a video on youtube that subjects various blocks to smoke tests. Aircrete (Hebel, Australia) did surprisingly well

Poor man's test for airtightness being applied here: night time torch in cavity, look for light leakage through inner and outer leafs. Perhaps there are better (low cost) tests

Edited December 15, 2021 by WWilts

[Iceverge]

Anything you can do to improve the situation is good, however unless you've a comprehensive strategy to stop all the air leaks you're likely to be disappointed, a bit like stopping some of the water leaks in a roof. 

 

Putting a lot of effort into a halfway house remedy seems like a missed opportunity.

 

Spend a couple of hundred on parging everything, then  tape or airtight paint on all the junctions would be time and money well spent. 

 

Spending ages just pointing the joints is likely to be poor payback for effort Vs outcome. 

[PeterW]

3 hours ago, WWilts said:

Poor man's test for airtightness being applied here: night time torch in cavity, look for light leakage through inner and outer leafs. Perhaps there are better (low cost) tests

But light doesn’t go round corners - air does …

 

blocks are inherently leaky - stick a hoover nozzle on one and see what I mean. 
 

Blower paint on perps is like sticking a padlock on a paper bag .. by the time you’ve done the perps with a paint brush you could have done the whole wall with a broom and a parge coat. 

[WWilts]

3 hours ago, Iceverge said:

pointing the joints

being done routinely as part of good workmanship.

Will cement:sand parge coat improve the airtightness of cement:sand joints? Trying to understand that

[Iceverge]

3 hours ago, WWilts said:

Poor man's test for airtightness being applied here: night time torch in cavity, look for light leakage through inner and outer leafs. Perhaps there are better (low cost) tests

 

A DIY blower door can be rigged for £30.

 

Then get a length of rigid electricians conduit and ram a candle in the end (avoids having to being a step ladder to check all the high up junctions)  The flame is incredibly sensetive to any tiny leaks. The back of your hand is also very sensitive. 

 

We got to 0.31ach using this method.

[WWilts]

8 minutes ago, PeterW said:

blocks are inherently leaky

True.

But Celcon claim very very low leakage for their blocks. No reason to doubt them

[Russell griffiths]

2 hours ago, WWilts said:

True.

But Celcon claim very very low leakage for their blocks. No reason to doubt them

Every reason to doubt them, do not confuse celcon with hebel they are like chalk and cheese. 

Get a blower test done before plastering or parge coat and I bet you will have a heart attack.

A torch won’t show you anything. Any hole big enough to get light through will be a howling gale when you ramp the fan up on the air test.  

[WWilts]

On 15/12/2021 at 14:45, WWilts said:

Will cement:sand parge coat improve the airtightness of cement:sand joints?

 

Context: Inner leaf of cavity wall 630mmx 215mm Celcon blocks with cement:sand (1:6) mortar. Brick or stone outer leaf. Insulation blown in bonded EPS beads.

re Inner leaf airtightness,

still trying to discover whether a cement:sand parge coat will improve the airtightness of cement:sand joints. Workmanship ok (jointing/pointing/concave tooling). 
Decision on parge coat due now-ish

 

 

 

 

PS re airtightness of Celcon aircrete blocks, some were curious:

"Tests undertaken by Building Services Research and Information Association (BSRIA) have shown that H+H aircrete achieved an air permeability of 0.12m³/hr/m² measured at 50 Pascals"

[joe90]

My brick and block build was cement rendered (on the inside including between joists) then wet plastered. No cracks and a fairly good airtight test.

[jfb]

I don’t think a parge coat will make any difference to the air tightness of a decent joint. 

[Brickie]

You’re wet plastering,aren’t you? 
It’s my understanding (though I may be wrong) that a parge coat is used to assist air tightness when dot & dabbing. 

[WWilts]

13 hours ago, Brickie said:

You’re wet plastering,aren’t you? 

Decision to be taken on whether to do anything more for airtightness of mortar joints.

 

Blocks (large format, 630mmx 215mm) shown as airtight in independent tests.
Mortar joints (cement:sand 1:6 mix) if well done might be airtight too.
 

Trying to discover if cement:sand parge will improve the airtightness of cement:sand joints. Perhaps someone here has come across a test or report somewhere.

Edited January 4, 2022 by WWilts