A couple of drylining questions

antiskeptic

I'm planning a concrete cavity block extension, rendered on the outside, battened on the inside with pressure treated battens > then 80mm insulated plasterboard > skim finish.

A couple of questions

On another thread someone advised at least scratch coating the inside of the blocks to air-proof but I imagine the render on the outside could be sufficient? I'm battening horizontally top and bottom and around windows, etc. as well

In terms of vapour barrier: the wall will get some weather so I'm wondering whether it is better or not to have an integral vapour barrier on the insulated plasterboard. I strikes me as keeping things breathable (i.e. no vapour barrier) may be better?

I'm also using the same batten/insulated plasterboard for the original house walls which are solid brick. Would the advice change here vs. the concrete cavity block

DOCARCH

First things first, does your proposed wall construction meet the requirements of the (current) Building Regulations (Part L 2011)?

What u-value are looking to achieve?

antiskeptic

DOCARCH wrote: »

First things first, does your proposed wall construction meet the requirements of the (current) Building Regulations (Part L 2011)?

If the extension is considered a new dwelling the just about (0.22 achieved vs. 0.21 required). If considered an old dwelling the by a country mile.

What u-value are looking to achieve?

Using an 80mm xtratherm board gives 0.22

AnarchistKen

antiskeptic wrote: »

I'm planning a concrete cavity block extension, rendered on the outside, battened on the inside with pressure treated battens > then 80mm insulated plasterboard > skim finish.

A couple of questions

On another thread someone advised at least scratch coating the inside of the blocks to air-proof but I imagine the render on the outside could be sufficient? I'm battening horizontally top and bottom and around windows, etc. as well

In terms of vapour barrier: the wall will get some weather so I'm wondering whether it is better or not to have an integral vapour barrier on the insulated plasterboard. I strikes me as keeping things breathable (i.e. no vapour barrier) may be better?

I'm also using the same batten/insulated plasterboard for the original house walls which are solid brick. Would the advice change here vs. the concrete cavity block

In my opinion vapor barrier is not required as once you render the outer leaf of blockwork the cavity is sealed.

Vapour barrier is largely used when there is a risk of moisture creeping through all surfaces and I can't see this happening here. Blockwork as always needs to be bang on too!

Rest of your details are pretty bog standard stuff. Best of luck with the build.

sinnerboy

AnarchistKen wrote: »

In my opinion vapor barrier is not required as once you render the outer leaf of blockwork the cavity is sealed.

No it's not and the op, is using cavity block not cavity wall

AnarchistKen wrote: »

Vapour barrier is largely used when there is a risk of moisture creeping through all surfaces and I can't see this happening here.

Wrong . Vapour barrier is always used on the warm side of the insulation layer to inhibit moisture laden internal air from entering into the wall fabric resulting in interstitial condensation. Why can't you "see this happening" here ?

AnarchistKen wrote: »

Rest of your details are pretty bog standard stuff. Best of luck with the build.

Yes with that approach one does indeed need luck.

sinnerboy

antiskeptic wrote: »

Using an 80mm xtratherm board gives 0.22

0.21 is the min U Value so 80mm is not enough. . But insulating to this level by dry lining presents a significant risk of interstitial condensation .

antiskeptic

sinnerboy wrote: »

0.21 is the min U Value so 80mm is not enough. But insulating to this level by dry lining presents a significant risk of interstitial condensation .

Would the inbuilt vapour barrier not prevent this (assuming the joints where suitably finished?

And what is it about this level of insulation that presents the significant risk? If it were more or less would the risk lessen?

Thanks..

MicktheMan

antiskeptic wrote: »

On another thread someone advised at least scratch coating the inside of the blocks to air-proof but I imagine the render on the outside could be sufficient?

That's good advice, take it. The external render has nothing to do with airtightness.

DOCARCH

antiskeptic wrote: »

And what is it about this level of insulation that presents the significant risk? If it were more or less would the risk lessen?

More = bigger risk to interstitial condensation.

antiskeptic

MicktheMan wrote: »

That's good advice, take it. The external render has nothing to do with airtightness.

Not doubting, just interested: what is it about a scratch coat on the inside that adds to air tightness that outside render wouldn't?

sinnerboy

If the internal heated air makes it as far as the block hollows it's heat energy is then lost to you as it whirls around uselessly to you in that cold series of voids.

You are playing your defenders too far up the pitch if you try to rely on external render for air tightness.

antiskeptic

sinnerboy wrote: »

You are playing your defenders too far up the pitch if you try to rely on external render for air tightness.

If the internal heated air makes it as far as the block hollows it's heat energy is then lost to you as it whirls around uselessly to you in that cold series of voids.

Assuming the top of the wall is sealed off with a block on the flat (and assume the external render wraps around the top of this block to air-seal) then the air wouldn't be whirling around as such. Is it not the case that air tightness has to do with a transfer of air from outside to inside and once you've an airtight boundary - in this case, external render wrapping around the block on flat up top - there is nothing to induce a transfer of air across the internal face of the cavity block?

sinnerboy

You clearly dont need my help.

antiskeptic

DOCARCH wrote: »

More = bigger risk to interstitial condensation.

With the risk a function of the quality of forming a sound vapour barrier from all those bit's of insulated plasterboard you're trying to stitch together??

Assuming you can't prevent vapour getting through to some degree, and assuming you do get some interstitial condensation does it not evaporate off into the void between back of board and wall?

antiskeptic

sinnerboy wrote: »

You clearly dont need my help.

Quite the contrary.

I'm quite happy having what I say dismantled - as it in the probing and the being shot down that the learning takes place. I'm a mech eng so am naturally interested in the related building field.

It's one thing to specify something that is said to achieve a purpose. It's quite another paying for it! And so I ask the questions..

sinnerboy

antiskeptic wrote: »

I'm a mech eng

Convection currents.

antiskeptic

sinnerboy wrote: »

Convection currents.

..would indeed be set up by whatever heat energy leaks into the cavities from inside. And that would produce losses.

But in terms of air tightness it seems to me you need something to drive the air through the inner cavity block surface and through whatever gaps exist between slab and interior - a pressure differential of some sort.

Convection currents within the wall don't strike me as producing the kind of potential difference necessary to drive anything significant through the wall.

sinnerboy

antiskeptic wrote: »

But in terms of air tightness it seems to me you need

To conduct your own studies and not pose suppositions here.

Start here and here

antiskeptic

Over in the rafter insulation thread, Syd made the point that interstitial condensation in the likes of glasswool between rafters is dealt with by having an air gap above the insulation. The air wafting overhead dries out the condensation like washing on a line.

What would be the case with a rendered cavity wall / battens / insulated plasterboard?

- I assume the concrete aspect is breathable to an extent. If so, would humidity build up between the blocks inner face and the back of the insulation 'sweat' outwards

- If not the above, is it practice to ventilate the cavity slightly to outside, just as one would ventilation under a wooden floor?

sydthebeat

im lost here...

are we talking about a cavity block construction, or cavity wall?
are we discussing air tightness or breathability?

whats the goal of this discussion?

antiskeptic

sinnerboy wrote: »

To conduct your own studies and not pose suppositions here.

Start here and here

I thought it more cut-to-the-chase to start here

-

I wouldn't have thought it supposition to suggest air movement through an element as requiring of a driving force? Newton's Laws of Motion have long established that.

Since it costs money to scratch the inside wall I was merely questioning whether convection currents within a cavity block wall constitute a driving force of note.

I searched for the word "convection" on opening both your links and got no matches. The first link does however mention a driving force of 50Pa when it comes to evaluating permeability - which (indirectly) answers the issue of convection currents and their influence on permeability within a cavity block

antiskeptic

sydthebeat wrote: »

im lost here...

The OP asks a number of questions. One of them has to do with air tightness (and whether that is aided and abetted by scratch coating the inner surface of a cavity block / battened /slabbed wall vs. outside rendering essentially achieving the same thing).

Another enquires into the issue of vapour with a cavity block > batten > insulated slab construction. It appears interstitial condensation could be an issue with 80mm insulation (made worse still, what with 90mm insulation likely required to achieve 0.21). It would seem that halting vapour is a goal and so a vapour barrier is to be preferred over none (answering that part of the question - whatever about avoiding construction of a Swiss cheese- inspired vapour barrier)

The last enquiry replaces the cavity block with solid brick as wonders does that alter things.

Now you're found?






On the former point, I'm not convinced of the need for a scratch coat.

On the latter point, I was wondering what could be done to deal with the risk of interstitial condensation on said construction type.

sinnerboy

antiskeptic wrote: »

I thought it more cut-to-the-chase to start here

Not interested. Educate yourself.

antiskeptic

sinnerboy wrote: »

Not interested. Educate yourself.

Thanks for those links. I'll give them a read..

sydthebeat

antiskeptic wrote: »

The OP asks a number of questions. One of them has to do with air tightness (and whether that is aided and abetted by scratch coating the inner surface of a cavity block / battened /slabbed wall vs. outside rendering essentially achieving the same thing).

Another enquires into the issue of vapour with a cavity block > batten > insulated slab construction. It appears interstitial condensation could be an issue with 80mm insulation (made worse still, what with 90mm insulation likely required to achieve 0.21). It would seem that halting vapour is a goal and so a vapour barrier is to be preferred over none (answering that part of the question - whatever about avoiding construction of a Swiss cheese- inspired vapour barrier)

The last enquiry replaces the cavity block with solid brick as wonders does that alter things.

Now you're found?






On the former point, I'm not convinced of the need for a scratch coat.

On the latter point, I was wondering what could be done to deal with the risk of interstitial condensation on said construction type.

My only comment on this thread is that cavity block wall with internal drylining (on timber battens) is the worst possible construction method to employ. Its highly debatable whether or not you can actually comply with regulations with this method (if you study the issues indepth youd see why).

this is "cheap as chips" building and as such incurs all the problems that have already been mentioned here.

on air tightness, trying to make the external face the air tightness layer is a waste of time.. every opening, every cill, every vent, ever pipe protrusion, wire etc punctures this layer... cills especially destroy its integrity. Exposed concrete is incredibly porous to air, so your hollows in the block will provide air movement from inside to outside.

But anyway, focusing on air tightness on a cavity block extension is a bit of a waste of time. IF you want to increase the tightness, you do as has been suggested by applying a thick layer of fine sand and cement render before battening. Personally i wouldnt bother because what you gain in infiltration losses, you loose many times over in thermal bridges.

antiskeptic

sydthebeat wrote: »

My only comment on this thread is that cavity block wall with internal drylining (on timber battens) is the worst possible construction method to employ. Its highly debatable whether or not you can actually comply with regulations with this method (if you study the issues indepth youd see why).

this is "cheap as chips" building and as such incurs all the problems that have already been mentioned here.

I suppose I'm trying to understand what they are since cost/benefit is an issue.

on air tightness, trying to make the external face the air tightness layer is a waste of time.. every opening, every cill, every vent, ever pipe protrusion, wire etc punctures this layer... cills especially destroy its integrity.

As it happens, there won't be protrusions/sills/etc (bar a c/heating outlet, I suppose I can seal it's interface before rendering). The cavity block is used for long side walls. The end wall, which gets the windows etc is intended to be twin leaf drylined on battens.

Exposed concrete is incredibly porous to air, so your hollows in the block will provide air movement from inside to outside.

Assuming the outside render provides as effective an air tight membrane as internal render, would this issue be circumvented. I made the point to Sinnerboy that I can't see where the driving force comes from to cause an exchange of air between inside and the cavity hollows

Personally i wouldnt bother because what you gain in infiltration losses, you loose many times over in thermal bridges.

Granted the block is an awful thing thermal bridge wise (and every other wise except price). The point is that I wouldn't be gaining by rendering the inside over and above what rendering the outside does.

MicktheMan

antiskeptic wrote: »

what is it about a scratch coat on the inside that adds to air tightness that outside render wouldn't?

Air leakage is rarely direct. The internal scratch coat is proven to be an efficient way of sealing an external wall. Cavities are never airtight and therefore external render cannot be relied upon.

Btw, the wall scratch coat is only one element of an airtight strategy. An airtight strategy/plan ought to be specified and budgeted for before a block is laid. From experience of testing for airtightness, it is often an afterthought and results are poor.