2 airtight compartments

[mvincentd]

A current thread on air tight tests and the comments it's generated has got me thinking....

 

I am due to dot and dab plasterboard, and this 'layer' would constitute my airtight layer.  All my mvhr in/outlets would serve the air within the house compartments bounded by this airtight layer.

 

I have a warm flat roof, so if prior to dot and dab I also make the 'shell' airtight (as recommended) by using blower proof on the internal block walls, air tape on all the wall/roof junctions, etc......I will end up with an inner airtight box inside an outer airtight box.

 

One of those airtight boxes will be above ceiling and below roof deck, will contain joists, mvhr ducting, services etc.......but will not have any mvhr actively serving that area.

 

Is this fine and correct or should I be looking to achieve a not-brilliant level of airtightness in the outer box?  I know i'm not living in the outer box, creating moist air etc.....so guess this is the key point.....but suddenly it feels a bit odd so thought i'd check opinion.

[MikeGrahamT21]

Is dot and dab ever truly airtight?

 

You would definitely need to do the airtighting before the dot and dab if thats the way you are going, parge coat on block work (5mm min) (cheaper than blower proof), and tape all f the junctions. Another method would be airtight membrane and wall battens to create a service void, which is a much nicer solution.

 

If you are wanting to ensure tightness, before you board in either method, get an air test to confirm tightness.

 

Airtightness and moisture diffusion are two very different things also, which aren't particularly related.

[mvincentd]

Appreciated Mike, sorry the main thrust of my question was intended to be "under my current scheme are the contents of my ceiling void gonna be ok?" rather than am i going about airtighting my outer box the best way.

[MikeGrahamT21]

Ahh sorry must have misread. As long as the materials are vapour open on one side, then i don't see a problem with this, normally the membrane nearer the living space would be the airtight vapour membrane, and the outer membrane could be more like a breathable roofing membrane, however you would want it to be airtight to more than 50pa (Tyvek is ok to 50pa) to avoid cold air blowing in the void.

 

I'll let someone else more qualified answer though, one thing you could try is simulating it: https://www.ubakus.de/u-wert-rechner/?

[mvincentd]

ah, thanks for that...hadn't thought about air tight but not vapour tight......i'm now wondering if parge coat falls into that category, otherwise i've precious little of my outer box thats breathable! (monolithic structure of waterproof concrete throughout floor and 2 sides whose waterproof membrane is continuously bonded to the roof membrane, then above the blockwork wall's joist plate i've xps thats about to be air taped to the roof deck and joists).

[MikeGrahamT21]

dont forget though, waterproof doesn't necessarily mean not-breathable, though solid concrete would have a lower vapour permeability figure (i.e. be quite resistant to the passage of water vapour).

 

On a block work house, a parge coat would generally be applied to the inner side of the inner leaf, or it would be wet plastered. What exactly is the construction type of your property?

[mvincentd]

Surely 250mm of waterproof concrete with a fully bonded 0.8mm thick waterproof membrane is going to be more airtight than parged thermalites(or indeed most materials any house is constructed with)?.  All external walls are either poured concrete or they're thermalites/135mm full-fill dritherm/flintblocks.

[MikeGrahamT21]

The poured walls will be airtight for sure, just concentrate on junctions.

 

Any blockwork, 5mm parge coat and wet plaster if you can, or dot and dab with continuous ribbon of adhesive around edges