Cavity Wall Air Tightness Layer

[pdf27]

From what I've read, it appears that the normal way of doing airtightness in a cavity wall Passivhaus is to use wet plaster on the inside of the inner leaf. This makes sense (you want the airtightness barrier to be on the warm side of the insulation, so it needs to be part of the inner leaf), but one thing that is really bugging me is why they don't do it on the outside of the inner leaf as is done with externally insulated solid walls. It means an additional process, but that should be outweighed by the fact that the airtightness layer is much less exposed to damage and there are far fewer details to worry about (wall/floor junctions, etc.).

 

What am I missing? Is it just that 100mm blocks aren't stable enough to support the roof, etc. sufficient to air seal everything without the outer leaf in place?

[nod]

Your best way of making sure that block work is airtight is making sure they are pointed up correctly 

You can add a Parge coat for very little expense an afternoon for a couple of plasters on a 5 bed detached 

Or wet plaster the lot 

 

I’m not sure how or why you would want to coat the internal face 

It would mean leaving the outer skin down and coating round the cavity ties 

[Mr Punter]

2 hours ago, pdf27 said:

they don't do it on the outside of the inner leaf as is done with externally insulated solid walls.

 

There is no inside and outside leaf with solid walls.  Just the one.

[pdf27]

36 minutes ago, Mr Punter said:

There is no inside and outside leaf with solid walls.  Just the one.

Well yeah. The point is that there appears to be nothing wrong with putting a render airtightness barrier on the outside of the blockwork but inside the insulation. Thermally, that's the same as what I'm suggesting above, with the difference being the outer leaf which is effectively a very thick render layer (even the wall ties are replicated with the EWI fixing bolts).

[A_L]

3 hours ago, pdf27 said:

This makes sense (you want the airtightness barrier to be on the warm side of the insulation, so it needs to be part of the inner leaf), but one thing that is really bugging me is why they don't do it on the outside of the inner leaf as is done with externally insulated solid walls. It means an additional process, but that should be outweighed by the fact that the airtightness layer is much less exposed to damage and there are far fewer details to worry about (wall/floor junctions, etc.).

 

As long as the air barrier layer is correctly water vapour permeable the air barrier could be anywhere in the wall cross section. The outside of the inner leaf could be coated with a brush/spray on air barrier as the wall was constructed.  The only issue would be penetration by the wall ties,

 

https://www.blowerproof.co.uk/

 

https://www.intelligentmembranes.co.uk/product/airtight-white-brush/

Edited July 26, 2020 by A_L
to add 'water'

[pdf27]

1 hour ago, nod said:

Your best way of making sure that block work is airtight is making sure they are pointed up correctly 

You can add a Parge coat for very little expense an afternoon for a couple of plasters on a 5 bed detached 

Or wet plaster the lot 

 

I’m not sure how or why you would want to coat the internal face 

It would mean leaving the outer skin down and coating round the cavity ties 

My understanding is that the floor/wall junction is the main problem area, and putting on a parge coat would presumably be tricky around there. That, and my understanding is the parge coat needs to go on before the services or they'll be the source of even more leaks. It clearly works, it just seems really complicated and I'm trying to understand why they don't do it differently.

[Mr Punter]

If done on the inside, if the ground floor is concrete there should be no issues there.  The first floor depends on how it is built.  You will need to detail carefully around hangers.

 

With services, I would think it was easier to parge after all required holes in the fabric have been made.

[epsilonGreedy]

1 hour ago, pdf27 said:

My understanding is that the floor/wall junction is the main problem area, and putting on a parge coat would presumably be tricky around there. That, and my understanding is the parge coat needs to go on before the services or they'll be the source of even more leaks. It clearly works, it just seems really complicated and I'm trying to understand why they don't do it differently.

 

 

You need a better sense of proportion on this subject, airtightness is a percentage game. The effectiveness of your air tightness envelope is not deemed to have failed the moment a three year old rams his tonker truck into the plaster and chips it.

 

Your idea for parge coating the outer face of the inner block wall would add £5,000 to build costs for a saving in heating costs of £1 per year. The result might be worse than a standard internal parge coat because once the brickies have twanged the wall ties while laying the facing bricks there would be hundreds of holes.

[Iceverge]

I considered this but was turned off the idea as it would have been tricky to manage economically.

 

The single 100mm inner leaf would have been too unstable to take it to wall plate without the outer leaf. This would have ment parging it's external face as it was built which would inevitably  have ended with a cavity bottom full of sand cement parge coat and a load of half dislodged cavity ties. 

 

Now if you were to build the inner leaf as a 215mm " block on flat" wall with cavity ties built in, use a spray on airtightness paint and build the external leaf afterwards you might be on to something.  It is the ideal place for the airtightness layer as it's away from all internal services yet still at the warm side of the insulation.  With passive insulation levels you might be heading for a rather Bastille like 650mm wall though.......

 

[pdf27]

Bumping this as we've just got the PHPP model back for a design nearly ready to go to planning, and cavity wall is looking a lot better than we thought it would. We're planning on a brick outer skin no matter what build system we use (cost + blending in with the rest of the village), and PHPP says that 200mm of Dritherm 32 plus aerated concrete inner blocks and a brick outer leaf is good enough to hit the targets since we've got a very good form factor (~0.15 W/m²K gets us to Passivhaus - the form factor is 2.4 which is one of the best the architects have seen for a Passivhaus). That starts to get interesting as any timber frame system with a brick outer leaf rather than slips is looking like >500mm rather than ~400mm.

 

Airtightness is a significant concern if we go down this route because unlike the timber frames where there are a large number of companies with a track record of delivering a package which meets the airtightness requirements. A 200mm cavity is actually pretty close to a conventional build (150mm), so if there is a builder-friendly way of putting the airtightness barrier inside the cavity that would make getting good airtightness with non-Passivhaus focussed builders a lot easier since we'd only have to control penetrations and roof/window/door junctions.

 

I'm aware of the use of painted on liquid membranes, and our architects have been playing around with some peel-and-stick membranes intended for this sort of application (probably something like this), as well of the use of a parge coat/wet plaster on the inside for airtightness which I'd prefer to avoid. Is anybody aware of any alternative solutions to achieve the same thing?

[Iceverge]

Your build sounds similar to @joe90 I think he used wet plaster internally. 

 

I'd avoid using aerated concrete if I could. They only reduce your U value by 0.05 W/m2K, they're twice the price and difficult to fix to and prone to cracking. I'm sure @tonyshouse can elaborate more.

 

You could get the same benefit from making your cavity 5mm wider. Also what cavity ties are you using? Basalt are the best but very expensive. We just used SS ones and widened the cavity to 250mm. This gave us some breathing space if we didn't hit airtightness targets and were able to cut down on attic insulation later on when we did. ( This was the theory at least, I still put 400mm cellulose up there out of stubbornness!) 

 

 

We used dense concrete blocks throughout, wet plaster internally and skim. 0.31 ACH50 so if done properly there's no issue with that method. 

 

35 minutes ago, pdf27 said:

is there is a builder-friendly way of putting the airtightness barrier inside the cavity that would make getting good airtightness with non-Passivhaus focussed builders a lot easier since we'd only have to control penetrations and roof/window/door junctions.

 

It would need to be a paint on one as any membrane would require sealing around every cavity tie. The airtight layer is also in the wrong place to allow for any test before completion using a DIY fan or otherwise. This is the only reason we were able to get such a tight house.  If you have non passivhaus builders they can't and won't reach a good airtightness result. 

 

43 minutes ago, pdf27 said:

I'm aware of the use of painted on liquid membrane

 

It's very expensive. Good for junctions though. 

 

44 minutes ago, pdf27 said:

as well of the use of a parge coat/wet plaster on the inside for airtightness which I'd prefer to avoid

 Why is this?

 

The only alternative solution I thought of when doing ours was to inject closed cell foam into the cavity. It was almost 5 times dearer than bonded EPS bead though.

 

[joe90]

18 minutes ago, Iceverge said:

Your build sounds similar to @joe90 I think he used wet plaster internally. 

Yup, not difficult, not expensive, just need to pay attention to details like joists, electrical penetrations etc etc.

[pdf27]

21 minutes ago, Iceverge said:

Your build sounds similar to @joe90 I think he used wet plaster internally. 

 

I'd avoid using aerated concrete if I could. They only reduce your U value by 0.05 W/m2K, they're twice the price and difficult to fix to and prone to cracking. I'm sure @tonyshouse can elaborate more.

 

You could get the same benefit from making your cavity 5mm wider. Also what cavity ties are you using? Basalt are the best but very expensive. We just used SS ones and widened the cavity to 250mm. This gave us some breathing space if we didn't hit airtightness targets and were able to cut down on attic insulation later on when we did. ( This was the theory at least, I still put 400mm cellulose up there out of stubbornness!) 

 

 

We used dense concrete blocks throughout, wet plaster internally and skim. 0.31 ACH50 so if done properly there's no issue with that method. 

 

 

It would need to be a paint on one as any membrane would require sealing around every cavity tie. The airtight layer is also in the wrong place to allow for any test before completion using a DIY fan or otherwise. This is the only reason we were able to get such a tight house.  If you have non passivhaus builders they can't and won't reach a good airtightness result. 

 

 

It's very expensive. Good for junctions though. 

 

 Why is this?

 

The only alternative solution I thought of when doing ours was to inject closed cell foam into the cavity. It was almost 5 times dearer than bonded EPS bead though.

 

1. Assumption is that if we go for cavity wall we'll end up using standard concrete blocks and a wider cavity - 250mm is pretty straightforward and would leave us at something like 10 kWh/m²/year keeping the other assumptions the same. Aerated concrete blocks are what the architects had as their default in PHPP and because it worked really well first time they didn't worry about optimising it. We've clearly got plenty of headroom if we go down this route.
2. No decision on cavity ties yet - not really needed at this stage, will look at this in detail later on if required. Because it isn't a tight site thicker insulation versus stainless ties is an easy cost tradeoff to make.
3. Expecting to use wet plaster internally anyway rather than dot-and-dab, simply because it will feel more solid. Thinking is that this layer will end up with lots of holes in it (sockets, floors, etc.) and an additional airtight layer on the outside of the blockwork leaf will make life a lot easier - more upfront cost with reduced cost of rework later. Realistically I'm not going to be able to be sitting on them every day, so a more robust design is important.
4. There's a trade-off here between cost and performance, which I'm trying to understand. You aren't going to have air moving up and down in a blockwork wall, instead it'll be pretty directly in to out through cracks, and two imperfect airtight layers are very unlikely to have the faults lining up with each other. For instance this means that there is no real need to seal around cavity ties perfectly with a peel-and-stick membrane if the rest of the wall is well covered. This increases the chance of getting things right first time.

[joe90]

I used standard blocks and 200mm cavity full fill with rockwall batts. I also used stainless ties as difference In U value nit justified by the cost of basalt. I also had first coat plaster done in sand and cement (much cheaper) and final coat in plaster (very solid sounding walls and can hang anything). Make sure you parge coat between floors (between floor joists where plaster does not go.) simple to use silicone where cables go into back boxes to make airtight.

[Iceverge]

4 hours ago, pdf27 said:

There's a trade-off here between cost and performance, which I'm trying to understand. You aren't going to have air moving up and down in a blockwork wall, instead it'll be pretty directly in to out through cracks, and two imperfect airtight layers are very unlikely to have the faults lining up with each other. For instance this means that there is no real need to seal around cavity ties perfectly with a peel-and-stick membrane if the rest of the wall is well covered. This increases the chance of getting things right first time.

 

What I did was decide on a projected cost of heating ( direct electric in my case, €0.10) and then look for a break even period with the insulation, in our case 25 years.

 

I chose 25 years as it seemed to tie in pretty well with achieving passivhaus targets. Much less would suggest just building to Bregs.  

 

To take an example I began by plugging the backstop building regs floor U Value of 0.21W/m2K ( in Ireland) into PHPP.  This equated to 150mm EPS at a cost of €1250 euro. To increase to the next step up in insulation was 200mm EPS at a price €415. It reduced our annual heat demand by 175kWh or €17.50 or in other words itwould pay itself back in 23.7 years. ( I didn't take into account the increased cost of credit as the money was likely to be spent elsewhere anyway if the builder didn't get it!)  Clearly increasing the insulation in our case to the next step 250mm wouldn't pay back in less than 25 years.

 

 

I repeated the process for the walls and roof. 

 

This allowed me to arrive at a ratio of where money was best spent with regard to insulation in terms of thickness. 

 

We ended up at 200mm Graphite EPS in the floor 250mm EPS bonded bead in the walls and 400mm blown cellulose in the roof. 

 

Had we dropped to 200mm cavity wall insulation we would have needed 300mm floor insulation and 600mm attic insulation. It would have cost about €2800 more in insulation.

 

 

Having 2 imperfect airtight layers is a recipe for disappointment.  IMO success can only be had by employing very good professionals or else building in a fashion that allows your airtight layer to be remedied before being hidden in the structure of the building. I like the idea of an airtightness layer away from the following trades but if it can't be tested and fixed you're goosed! 

 

 

 

 

 

[tonyshouse]

For me comfort was more important than payback. I like your levels of insulation and wish others would do similar.

 

energy is only ever going to get more expensive and so payback calculations will go out the window and lots including the regulators will regret that they didn’t go further with hindsight. 
 

 

[Iceverge]

I agree, many people live in houses hundreds of years old. Payback is oft too taken for only the period of first owner occupation. There is a practical limit somewhere. Our ground floor walls are over 600mm thick, I think someone would have had me committed if i wanted even more insulation! 

 

As we don't have any central heating I didn't want to take too many chances with missing our performance targets. 

 

I allowed for 1 ACH in PHPP and still meet the ability of a post heater in the MVHR to supply heat to the house. As it ended up I think our heating demand should be something like this. This winter will tell how it really performs. 

 

  

[pdf27]

Will have a think - initial assumption was MBC or similar timber frame would be required and that cavity wall would be bunker-like. With the design evolving and the realisation that render boards are much more expensive than we thought, we've ended up with a brick outer skin and cavity wall is back on the table, so I'm only just starting to think through some of the issues.

 

Insulation values are being driven by comfort rather than energy cost - the alternative Passivhaus criteria of 10 W/m² is essentially a comfort one (drafts and cold internal surfaces translate into feeling cold), that ends up with a low energy cost as a side effect, but that isn't the primary driver. This leaves us a heating load of 1.8kW and cooling load of 650W if we don't open windows at night - so essentially representing a heat wave. Similarly, in addition to comfort airtightness is associated with moisture movement within the structure and thus in the long term to durability, so it makes sense to minimise it. It doesn't force us to go Passivhaus - indeed we are explicitly not committed to it - but it's a decent quality control system and I suspect the majority of people with the skills we're after will be in the ecosystem in some way.

Because cooling is a hard requirement for us (my wife is from the US) we're essentially tied in to a heat pump solution - this means heat is very cheap, so we're unlikely to be at the most cost-effective point, but since we aren't trying to go for maximum value-engineering this is acceptable to us.

 

As it happens, the form factor helps us a lot - the cavity wall option has 200mm of Dritherm 32, 150mm PIR under the floor and 350mm cellulose in the pitched roof which gets us to 8.6 W/m². Since we want a heat pump and a fair bit of PV, we have a strong suspicion that we'll end up hitting Passivhaus Plus without much if any effort so may end up doing that.

 

 

If we want to reduce heating demand, it's pretty clear that thicker cavity insulation is the best place to go - which happens to be relatively cheap, so means we've got a decent ability to tweak the performance if required.

 

If we do adopt the two layer approach then it's worth noting that the inner layer **is** accessible and so can be tested and fixed, and that the starting point for finding any leaks is inherently going to be lower than for a single layer as the majority of leaks going through one layer will be stopped by the other. I'd agree that the testing requirement makes a single layer in the cavity very difficult to test however, at least for a 100mm inner wall which can't be built up unsupported.

The concerns in relying on a single inner layer are that it requires quite a lot of detail work around any wiring chases, etc. and that it is accessible and hence vulnerable to degradation over the lifetime of the house. It is also potentially quite a slow and thus expensive process to chase down leaks - as I won't be doing it myself that's a significant concern. In either case a track record in high performance building will be required - it's possible we may end up choosing the build system based on what the selected builders have experience with.

[Iceverge]

It sounds like you've a very good handle on things. 

 

Cavity wall is no doubt the cheapest method and probably the thinnest wall assuming you've excluded the possibility of brick slips. However it's very workmanship dependant. For instance I spent ages with a long stick cleaning mortar off the cavity ties after the bricklayer was gone and then using a home made thin brush and scoop cleaning the bottom of the cavity also. Probably 3 days of work and very skinned knuckles. I was paranoid about the airtightness and put about 200 man hours into it. You couldn't pay someone to spend ages chasing every tiny pinhole leak.

 

If i was to do it all again I'd build timber frame. 

 

In your situation how about the below. Plenty timber framers this side of the water doing it. Don't know about the UK. You could even stick-build on site. 

 

Robust taped OSB layer for airtightness. Fully breathable. excellent decrement delay.  Woodfiber layer deals well with thermal bridging. Not the cheapest option but far more likely to perform as well in reality as it does on paper.

 

 

 

[pdf27]

There's about a million ways of doing it with a timber frame - the standard MBC package works nicely with some margin as does a 300mm I-beam build, and using a brick wall as a rainscreen is pretty simple once the foundation details are taken care of. At the moment I'm mostly trying to work out if cavity wall is realistic for our situation or not a good fit.

[Iceverge]

Thumbs up for cavity with diligence and oversight. 

 

Otherwise timber frame. MBC is the most sorted package I've seen in this regard. Not the cheapest mind you. About €35k more for our house.