Ventilated cavity - mad idea?

[oldkettle]

As I mentioned before, our architect recommended that we upgrade our current brick and block wall by installing 100mm EWI. Now it looks like there is a significant risk on interstitial condensation in a standard buildup with full fill cavity. I used http://vesma.com/tutorial/uvalue01/uvalue01.htm and it shows condensation risk in the outer layers when the internal RH is above 60% and it is below 5C outside.

The only way I could get this calculator to show no risk was with an empty cavity. The architect also mentioned ventilated cavity as one of the options. Leaving the cavity to ventilate outside does not sound like a good idea - what's the point of EWI then. But what if we used MVHR to ventilate this cavity? Mad / useless? Other options?

 

[A_L]

Ventilating the cavity to the outside would obviously completely negate the EWI, if you could ventilate to the heated space it would solve the problem at about a 10% loss of thermal resistance for the whole wall.

 

If the cavity fill is a low vapour resistance material, such as any fibrous/cellular insulation and the EWI is polystyrene there is an interstitial condensation risk in the outer masonry layer.

 

If you could change the EWI to dense fibreglass/mineral wool batts then the reduced vapour resistance should eliminate the condensation risk (assumes render reasonably vapour open).

[oldkettle]

Thanks you, @A_L

 

When you say ventilated to the heated space, do you mean just airbricks or Mvhr? 

 

I wanted to go for Icenyne in the cavity as it seems to be the best in terms of both u-value and air tightness, but if there is a potential condensation risk I would rather leave the cavity empty and increase EWI depth. I was told it doesn't affect the price too much if I go for 150mm instead of 100.

 

 

[ADLIan]

Agree with above - vent cavity negates effect of EWI.

 

As regards condenaation it not just if it occurs - also how much, is there a net build-up over the year, are moisture sensitive materals adjacent.

 

Suggest you get the insulation or the render system manufacturer to look at this for you in accordance with relevant BSs - the Vesma calc  looks very rough and ready and is not using correct values in the bit I can see - not sure what's going on behind this!

[Dudda]

I don’t understand how you can have interstitial condensation if you’ve 100mm of external insulation. Interstitial condensation is a risk when you use INTERNAL insulation not usually external. What’s the full proposed wall buildup including thickness of the cavity and other layers? 

 

Personally I’d consider 150mm external insulation. The cost of external insulation is mostly labour for fitting the insulation and then rendering. The difference in cost between 100mm and 150mm is very small. No way you’d have an interstitial condensation risk with 150mm external insulation although I doubt it’s possible with 100mm either. I'd also pump fill the cavity to stop thermal looping.

[A_L]

2 hours ago, oldkettle said:

 

When you say ventilated to the heated space, do you mean just airbricks or Mvhr? 

 

I wanted to go for Icenyne in the cavity as it seems to be the best in terms of both u-value and air tightness,

 

If visually acceptable airbricks top and bottom would do

 

The people who supply Icenyne usually have access to a closed cell polyurethane as well. Walltite, a spray applied closed cell polyurethane has a vapour resistance sufficiently high (306 Mn.s/g.m.) that interstitial condensation would not occur. There are a small number of companies that use polyurethane for cavity wall insulation, it is usually used where wall ties have failed as it sticks the two leafs together.

 

1 hour ago, Dudda said:

I don’t understand how you can have interstitial condensation if you’ve 100mm of external insulation. Interstitial condensation is a risk when you use INTERNAL insulation not usually external. What’s the full proposed wall buildup including thickness of the cavity and other layers? 

 

Personally I’d consider 150mm external insulation. The cost of external insulation is mostly labour for fitting the insulation and then rendering. The difference in cost between 100mm and 150mm is very small. No way you’d have an interstitial condensation risk with 150mm external insulation although I doubt it’s possible with 100mm either. I'd also pump fill the cavity to stop thermal looping.

 

Interstitial condensation happens in this case for two reasons,

 

1) the lightweight block is sufficiently thermally resistive to significantly lower the temperature of the brick outer leaf.

 

2) polystyrene has a significant vapour resistance, typically 100Mn.s./g.m that it causes the water vapour to 'dam up' on its warm side in the outer masonry leaf, in comparison with high density fibre batts at <10Mn.s/g.m

 

These two effects combine to push the vapour pressure above the saturated vapour pressure for the location, causing condensation.

 

Without a cavity venting solution, which really just increases the temperature of the outer masonry layer 150mm of EWI will lower the temperature and increase the vapour pressure in in the outer masonry leaf causing even more condensation

[oldkettle]

Thanks, @Dudda 

yes, extra insulation is something I will consider.

WRT the build up, 

Plaster 

Block

Filled cavity 

Brick 

Eps

Render 

 

I have found a thread on ebuild discussing the same issue, the analysis showed similar risk apparently. I am yet to use free WUFI trial. My architect also confirmed the risk. 

[Declan52]

Is putting the insulation on the inside skin an option or would you loose to much room.

[oldkettle]

3 minutes ago, A_L said:

 

If visually acceptable airbricks top and bottom would do

 

The people who supply Icenyne usually have access to a closed cell polyurethane as well. Walltite, a spray applied closed cell polyurethane has a vapour resistance sufficiently high (306 Mn.s/g.m.) that interstitial condensation would not occur. There are a small number of companies that use polyurethane for cavity wall insulation, it is usually used where wall ties have failed as it sticks the two leafs together.

 

Thank you, yes, I met people who do Walltite as well. So do I understand you correctly that with Walltite there is no need for cavity ventilation? 

[oldkettle]

1 minute ago, Declan52 said:

Is putting the insulation on the inside skin an option or would you loose to much room.

May be not too much but would be difficult to put it everywhere (about 30m of the existing walls). Would it help significantly ? I looked at using insulated plasterboard but it seemed to make no difference in condensation analysis. 

[Dudda]

26 minutes ago, A_L said:

Interstitial condensation happens in this case for two reasons,

 

1) the lightweight block is sufficiently thermally resistive to significantly lower the temperature of the brick outer leaf.

 

2) polystyrene has a significant vapour resistance, typically 100Mn.s./g.m that it causes the water vapour to 'dam up' on its warm side in the outer masonry leaf, in comparison with high density fibre batts at <10Mn.s/g.m

 

These two effects combine to push the vapour pressure above the saturated vapour pressure for the location, causing condensation.

 

Without a cavity venting solution, which really just increases the temperature of the outer masonry layer 150mm of EWI will lower the temperature and increase the vapour pressure in in the outer masonry leaf causing even more condensation

I know what interstitial condensation is. Nowhere is it stated thermal blocks are used on the inner leaf and I presumed it was regular as thermal blocks are relatively new compared to the existing housing stock. These are what's causing the problem as these are a form of internal insulation.

 

So is this an existing house with an inner leaf of thermal blocks? It mustn't be that old a house? Why not just use a breathable external insulation?

[oldkettle]

17 minutes ago, Dudda said:

So is this an existing house with an inner leaf of thermal blocks? It mustn't be that old a house? Why not just use a breathable external insulation?

 

This is an old bungalow, c. 1955. Not sure how dense the internal block is. 

 

I thought breathable insulation has worse u-value, does it not? We are detached but the space outside is not unlimited :-) 

[A_L]

1 hour ago, oldkettle said:

Thank you, yes, I met people who do Walltite as well. So do I understand you correctly that with Walltite there is no need for cavity ventilation? 

 

Yes,  here is a condensation analysis.

 

 

1 hour ago, Dudda said:

I know what interstitial condensation is. Nowhere is it stated thermal blocks are used on the inner leaf and I presumed it was regular as thermal blocks are relatively new compared to the existing housing stock. These are what's causing the problem as these are a form of internal insulation.

 

So is this an existing house with an inner leaf of thermal blocks? It mustn't be that old a house? Why not just use a breathable external insulation?

 

Since dense concrete block does not produce interstitial condensation with a filled cavity I assumed that the OP would be referring to AAC block (lambda 0.18W/m.K) which does, after all these have been around for a very long time

[ADLIan]

In a house of that age I assume cavity to be only 50mm. With a medium/dense block the 'balance' of insulation with 100mm EPS externally should be OK to avoid condensation.

 

As stated above even if condensation is predicted is it harmful? Bear in mind amount of condensate, net build-up (if any), location & nearby materials.

[PeterW]

1 hour ago, A_L said:

 

Yes,  here is a condensation analysis.

 

 

 

Since dense concrete block does not produce interstitial condensation with a filled cavity I assumed that the OP would be referring to AAC block (lambda 0.18W/m.K) which does, after all these have been around for a very long time

 

What software has produced that as I would be surprised that all the condensation rates would be zero. 

[oldkettle]

@A_L, thank you very much, this is very helpful. The main difference may be that I used 22C/90% for the inside conditions to reflect occasional kitchen/bathroom situation. Now, the bathroom will be tiled or panelled but the kitchen won't be hence I wanted to see the worst case. 

 

Not sure about the size of the cavity, 50 to 70 at most, the whole wall is close to 300mm with plaster. And I was not allowed any demolition :-) until planning is available, so don't know what type of a block we have here. 

[A_L]

@PeterW The software operates to BS5250 1989 and BS6229, it is no longer publicly available but was state of the art till about 2007 (NHER Evaluator). Here are a couple more outputs for the same build-up.

 

[ADLIan]

That version of BS 5250 is now out of date and BS 6229 is more specific for flat roofs. Calculation method for condensation analysis now in BS EN 13788. Remember these standards/methods are basically steady state rather than transient conditions you can however change internal/external conditions to asses greater/lesser risk.

[oldkettle]

Ah, the only difference now is in my form EPS is showing 0.031-0.038 depending on the type and the resulting u-value is 0.16-0.18 for 50mm cavity. With graphite 15kg/m3 priced below £10/m2 there seems to be no reason to select the cheapest.

[Dudda]

OK hold on. It’s a 1950’s house with what will be most likely a 50mm cavity on average (it might go up to 70mm in areas like mine does due to wobbly blockwork) and dense concrete blocks so no lightweight or ACC blocks. You’re not going to have the house up at 90% humidity for long in an old draughty house unless you fit a sauna or steam room in your bedroom. Any humidity from cooking or a long shower(s) will quickly dissipate. I don’t think you’ve anything to worry about here BUT as it’s a 1950’s house you could consider putting in an airtight membrane internally and create a service cavity with 25mm battens and fix plasterboard to this. This is what I did to my 1970’s detached house along with 150mm external EPS insulation but I did it to improve my airtightness and reach the Enerphit Passive House standard. The airtight layer also acts as a vapour check reducing any interstitial condensation risks.

[Jeremy Harris]

13 hours ago, ADLIan said:

That version of BS 5250 is now out of date and BS 6229 is more specific for flat roofs. Calculation method for condensation analysis now in BS EN 13788. Remember these standards/methods are basically steady state rather than transient conditions you can however change internal/external conditions to asses greater/lesser risk.

 

A critical point that is often ignored!

 

I keep harping on about the lack of dynamic modelling in "standard" interstitial condensation risk models, but I do so because it's important.  Once water vapour has condensed in a region inside the structure that can be prone to damage induced by organisms that thrive on damp, then it takes a LOT of heat energy to cause that water to undergo a phase change back to vapour and be able to escape from the structure.

 

In effect, you can have a sort of "one-way valve", where vapour enters under a certain combination of short duration conditions and condenses inside the structure.  Insufficient heat energy ever reaches the area where the water vapour has condensed, so it stay there, and build up every time a dynamic condition exists that allows more vapour to condense.  Some of these dynamic conditions can be of short duration - for example a cold, still, wet, night, followed by a hot and sunny windless morning, where the air immediately around the external structure and in any cavity could easily reach high water vapour concentration levels, and some parts of the internal structure may well be just below the local dew point for a time.  The fact they get above the local dew point later in the day may not be enough, as there may still not be enough heat energy available to convert the condensate back to vapour.

Edited October 13, 2017 by JSHarris

[Jml]

Off on a tangent.

 

 We also have a 1950 bungalow, for which we have two current approved planning applications.  The first, which we are not going to do, is to convert it to a 2.5 storey house.  When approaching builders, most suggested knock down and rebuild, as the difference in costs in their view, was not significant, due zero rating of new builds and then the choice of insulation etc. can be made from scratch and you are not constrained by the original bungalow.  May be something to consider or not!

 

 

[oldkettle]

3 hours ago, Dudda said:

OK hold on. It’s a 1950’s house with what will be most likely a 50mm cavity on average (it might go up to 70mm in areas like mine does due to wobbly blockwork) and dense concrete blocks so no lightweight or ACC blocks. You’re not going to have the house up at 90% humidity for long in an old draughty house unless you fit a sauna or steam room in your bedroom. Any humidity from cooking or a long shower(s) will quickly dissipate. I don’t think you’ve anything to worry about here BUT as it’s a 1950’s house you could consider putting in an airtight membrane internally and create a service cavity with 25mm battens and fix plasterboard to this. This is what I did to my 1970’s detached house along with 150mm external EPS insulation but I did it to improve my airtightness and reach the Enerphit Passive House standard. The airtight layer also acts as a vapour check reducing any interstitial condensation risks.

A couple of points. 

RH inside currently stays above 70% for days. This is with very little inside clothes drying as we have a new drier with a heat pump, and with me opening the vent immediately after showers and keeping at least a night vent open. Our bedroom on the ground floor stays close to 80% - again, with prolonged periods of keeping windows open. Admittedly it happens now when it is quite warm outside, but in winter we had rh above 60 quite often. Why that happens at all - no idea. But this is why I am keen to make sure I am not going to ruin the house after the extension is built. 

 

It seems I may well have to go the service cavity way - my guess is this is what @Declan52 had in mind as well. It probably loses 50mm overall, hopefully, we can live with it. To be honest at this stage I am not sure how exactly the extension TF walls are going to be positioned with respect to the existing ones. Since they will definitely have service cavities we may have no choice but to match those. Alternatively may be we can wait and see how RH behaves with Mvhr installed. 

 

[oldkettle]

@Jml, thank you for your reply.

 

I keep returning to this idea but just can't see how the math works here. 

It seems impossible to have anything built cheaper than £1500/m2. This is by far beyond our budget. The extension is large and is actually greater than the existing building but since the foundations stay it is not that bad, at least, so we were led to believe. And we will be able to live in the house, apart from hopefully a short period of time when the roof is removed and the first floor structure and new roof are erected. 

Edited October 13, 2017 by oldkettle

[PeterW]

1 hour ago, oldkettle said:

[...] it seems impossible to have anything built cheaper than £1500/m2 [...]

 

Ok ... unless you have a particularly difficult plot, or are building something very special, you can get £1000/m2 without much effort and below that if you do a lot of the grunt work yourself. 

 

Do not be put off by builder and architect quotes that are usually wildly inaccurate - do your own sums.