10mm air cavity?

muffler

Source.

Insulating your cavity walls and leaving a 10mm air cavity like below seems to be at odds in a way with the traditionally 40mm that was retained.



Id be interested in comments on this. Why not fully fill the cavity completely? Why not leave the 40mm standard? In fact all their rigid boards are designed to leave 10mm.

Thoughts, comments?

sinnerboy

Well arguably tradition ( i.e. the 40mm cavity ) can be countered by this

Both the TGD's and Homebond make reference to such certificates

However the most carefull attention must be paid to

( cut and paste from cert )

It is important to ensure during installation that:
a) Wall ties and fixings are installed correctly and are thoroughly clean.
b) Excess mortar is cleaned from the inside face of the leading leaf and any debris is removed from the cavity.
c) Mortar droppings are cleaned from the exposed edges of installed slabs.

Now this advice is apt for any cavity insulation boards , full or partial fill . It is way beyond absolutely vital for this board .

After 25 years of witnessing cavity wall construction the only thing it is safe to say is that this advise is a short list of what rarely occurs .....

imitation

Considering many think getting a brickie to keep a 50mm cavity clean impossible, 10mm doesnt seem that great an idea.

It was originally specced for my house, but it seems its already fallen out of favour and the suggestion was that I then pump the cavity. In the end I decided on kingspan, I'm not an expert by any means, but I know if I have moisture problems a few decades down the the line none of these guys will be around and I will have one hell of a headache.

heinbloed

USE:
The product is used for the thermal insulation of rendered walls up to 12 metres in height, unrendered brick walls up to 8 metres in height (two storeys) and is not allowed for use with unrendered masonry walls. The product also facilitates the control of surface and interstitial condensation in walls.

This way of construction causes condensation. Therefore the 10mm of drain.
Which needs to be ventilated of course. Counting the external concrete block leaf in the U-value calculation of such a wall as "dry-and-existent" is absurd.
It's wet and ventilated.
Ventilating a wet surface will cool it down further. The process of evaporation (entalphic energy) will lead to a higher thermal loss from the room, the exposed surface of the EPS is not only as cold as the outside temperature blowing along it's surface but
colder by several Kelvin. Since it is facing an energy sucking surface, i.e. the wet external concrete block wall.

Mellor

heinbloed wrote: »

This way of construction causes condensation. Therefore the 10mm of drain.
Which needs to be ventilated of course. Counting the external concrete block leaf in the U-value calculation of such a wall as "dry-and-existent" is absurd.
It's wet and ventilated.
Ventilating a wet surface will cool it down further. The process of evaporation (entalphic energy) will lead to a higher thermal loss from the room, the exposed surface of the EPS is not only as cold as the outside temperature blowing along it's surface but
colder by several Kelvin. Since it is facing an energy sucking surface, i.e. the wet external concrete block wall.

The comments regarding interstitial consensation refer to all wall build ups.
The drain is there to prevent water ingress from the wet external concrete block wall.
Arguing over the U-value of the outer leaf is in itself absurd. It's true that its not as resistant as the inner leaf due to the cavity, but to suggest that the cavity behaves as the external airspace is wrong.
Evaporation losses. You are joking right? Pedantry taken to the extreme. Also, vapour will more readily condense on the outer leaf as it is colder.
The air isn't "blowing" in a vented cavity, although there is obviously some movement.
Kelvin. :rolleyes:

heinbloed

Arguing over the U-value of the outer leaf is in itself absurd. It's true that its not as resistant as the inner leaf due to the cavity, but to suggest that the cavity behaves as the external airspace is wrong.
colder

.

I didn't. It is propably worse.
If it is damp outside the damp air will transport more energy. Away from the wall.
If it is damp inside the cavity the damp air blown out (there is always wind in Ireland blowing through the ventilataded cavities) of the holes in outer fabric will transport more energy away. Plain logic.

You are joking right? Pedantry taken to the extreme. Also, vapour will more readily condense on the outer leaf as it is colder.

How much difference in thermal energy demand for a structure comparing between a wet outer leaf and a dry outer leaf have you meassured under 'Irish' conditions? Or calculated? That would be how much in a building's lifetime?

heinbloed

?

heinbloed

With a reduced air-gap size going down to 10mm from the trad.40mm the resistance increases. Meaning that either the wetness in the wall increases as well because the drying effect of free blowing air/passive ventilation is severly reduced by the factor 8 (I'm guessing here) or the m3 per hour must now be blown mechanically through the reduced cavity.
Alternatively the number of holes in the external wall would have to be multiplied when compared with the trad. leak-systems (factor4?)....

What an intelligent system, amazing what we see the nearer we come towards the zero energy building legislation.....

Mellor

heinbloed wrote: »

.

I didn't. It is propably worse.
If it is damp outside the damp air will transport more energy. Away from the wall.
If it is damp inside the cavity the damp air blown out (there is always wind in Ireland blowing through the ventilataded cavities) of the holes in outer fabric will transport more energy away. Plain logic.

If you are going to use the word logic in a sentence, at least use it right. A statement of fact, serves no logical proof for different comparison.
Obviously, air and vapour movement transports energy.
I never said it has zero effect. I said the external airspace is worse.

The air speed in a cavity is nowhere near the same levels as the external environment. High speed directly causes a greater loss. Plain logic.

Mr Gumby

I hope you don't mind my input but this is an area that interests me greatly.
I believe you are into a very complicated area here of physics and human nature plus a few other factors.
As was possibly alluded to earlier, as the requirements for more heavily insulated and airtight structures increases so the need for better detailing increases and hence better understanding of all factors affecting the problem of all types of moisture in wall construction!
The human factor may in some ways have the greatest influence.There is little point in specifying a complicated detail that requires a degree of integrity from an operator (ie.blocklayer) that is unreasonable.This has been proven to be the case in insulated cavity construction, I believe it has been shown that if the cavity insulation is only a matter of a couple of millimetres off the inside leaf then it may as well not be there! The insulation system that started off this discussion was designed to improve on this problem by effectively filling the cavity while still preserving the air space necessary to prevent water penetration. On the face of it an admirable goal, however, there is still an onus on the blocklayer for clean construction that is possibly unrealistic. As for the sizes of air gaps, energy losses due to evaporation etc, this is where I believe the physics gets somewhat complicated. Far greater minds than mine seem to agree on only one thing in this area, there is a lot going on.
One country in particular seems to have done a lot of relevant research in this area and that is Canada and a lot of relevant and accessible information is out there. (possibly more relevant to Ireland than European Continental research)
As I said before this is without doubt a very complicated and rapidly progressing area and one to keep an eye on particularly for Technicians.
I know I haven't really aswered a question here rather raised a few more and hopefully generated further discussion.

sydthebeat

i think everyone realises that traditional block cavity wall construction is a very limited technology.

Its all very well debasing it from an academic standpoint rather than an applied standpoint, but thats off topic here. Here we understand that the technology is very prevailent in ireland and will remain until the masses are educated. But until then its better to augment than to critisise. thats what the manufacturers of the product are trying to do.

In my opinion given the choice between full fill polystyrene, or blown in bead, ill go for blown in all the time. simply because of the reasons already outlined, ie the application of a board insulation by a blocklayer is almost always not perfect.

In conclusion ive seen many a cavity wal demolished and ive never yet sen a case of mould in the cavity, not have i witnessed wet inner faces of inner leafs. This may be due to the looping air movements in the cavity or it may actualy also be due to the fact that the insulation works to an appropriate degree to keep the inner block warm.

sinnerboy

It's interesting to note that the thickest board - 140mm thick U value 0.20 it is clearly targeted for the max-cavity-without-need for structural-engineer market . The very market that is ill prepared for the realities of the building site .

So if contemplating a 350mm thick wall - why not go for 215 block + 140mm EPS . U Value 0.22 . An apparent drop in U value but with no cavity ties , stepped dpc's , thermal looping my opinion is that this U Value will be realised . The full fill will not .

We have spent 2 generations since WW2 expecting blockies to trouble themselves with cavity insulation . It's about time we copped on ... that they ain't never going to .

Then consider that an EWI wall lends itself much easier to thermal bridge free detailing you have to ask - why use full fill ? Why continue with the cavity ?

Mr Gumby

In reply to points raised by sydthebeat. I'm not so sure that I agree that block cavity wall construction is as limited as you imply. You say yourself it is very prevalent and so is likely to persist for some time, plus now with some insurance companies concerns with timberframe? The oringinal post was about the 10 mm cavity and then went on to a debate about what happened in that cavity in a some-what technical theme.

To have mould to grow there must be food for it present, for instance cellulose, absense of mould does not mean much. Nor does not seeing a wet inner leaf in my opinion, the point raised originally here was a 10mm air gap and the implications therein.

sinnerboy has brought up a possible solution that with further thought and development may provide an answer that could provide a way to achieve even higher u values.

The bottom line problem is asking the blocklayer to take on the responsibility of detailed construction. Science and precedent can provide answers that are likely to work, but provided things go on as before with no real quality control, things won't change.

I'm a firm believer in the KISS principal, Keep It Simple Stupid.

I agree with sydthebeat blown bead does seem like a better bet, provided you can trust the instalation!

Mellor

Mr Gumby wrote: »

The human factor may in some ways have the greatest influence.There is little point in specifying a complicated detail that requires a degree of integrity from an operator (ie.blocklayer) that is unreasonable.This has been proven to be the case in insulated cavity construction, I believe it has been shown that if the cavity insulation is only a matter of a couple of millimetres off the inside leaf then it may as well not be there!

This is not true.
There is a drop in performance, but saying its might as well not be there is a big exaggeration. Maybe, it was hyperbole to illustrate the point but seeing as the rest of the thread is quite technical, I thought I should highlight it for other reading who may take it at face value.

Mr Gumby wrote: »

I'm a firm believer in the KISS principal, Keep It Simple Stupid.

Hmmm, a phrase picked up in bolton street prehaps?

muffler

Mr Gumby wrote: »

I'm a firm believer in the KISS principal, Keep It Simple Stupid.

Mellor wrote: »

Hmmm, a phrase picked up in bolton street prehaps?

beyondpassive

dew point.jpg
wide cavity PIR.jpg

In extreme circumstances, such as those modelled in the attached calcs, you can get condensation on the inner face of the outer block. The attached detail is a 2010 reg compliance or passive house detail where the PIR is installed and sealed before the outer leaf is constructed. Compliance with the 2010 regs will be about 0.15 for walls.

140 EPS board as discussed previously, might work for 2007 regs, though I'd consider bonded bead a more practical solution. The 2010 regs are in terms of thermal performance of the building fabric, at passive levels. Anything built today will soon have to compete on the market with houses built after june 2011, So perhaps its best to aim for the new standard for the fabric now.

imitation

sinnerboy wrote: »

Then consider that an EWI wall lends itself much easier to thermal bridge free detailing you have to ask - why use full fill ? Why continue with the cavity ?

Are there any drawbacks with EWI though ? can you face it with natural stone for instance, and is there any risk of it compacting overtime causing cracking ?

galwaytt

imitation wrote: »

Are there any drawbacks with EWI though ? can you face it with natural stone for instance, and is there any risk of it compacting overtime causing cracking ?

...that's a good point. I don't believe there is any issue with insulation, per se, only that of it's fixing and attachment.

Imho, cavity construction was the simple way to mitigate the traditional moisture issues, from outside driven rain etc, that non-cavity construction led to, historically. The simple expedient of breaking the wall, by introducing a cavity, was effective at that. However, progression, and the need for insulation, minor at first, greater now, means this 'issue' is back on the menu again, as insulation itself may re-introduce the concerns about external damp, especially in a cavity.

I thing rigid-board insulation, with the best will in the world, in a cavity construction, is simply to difficult to work effectively, on-site. You need a greater level of care & attention than most are prepared/able to give, plus, don't forget weather. Laying damp or wet blocks in damp or wet weather, and it's going to be impossible to keep the cavity devoid of mortar snots. It's just physics, is all ! Couple that with the ever-increasing demand from clients to get work done for less, then I'm afraid, human nature being what it is, you will only get what you pay for.....for this reason, I'd be an advocate of pumped, there.

So, back to your point about EWI. And I'm thinking same as you: the vernacular around here is for stone facade's, in whole or in part, and I can't see how you would work it in. Similarly, and having seen mechanical damage on the render on an ICF house (which is essentially the same ext finish), caused by a tiny errant thump from a bucket, I'm wondering what the long-term effect is for cracks, and water-infiltration. E.g., around here, hurling is the game of choice. And young people do hit sliothar's around, and even off the gable, for practice !

Mr Gumby

It's the practical side that is nearly always the sticking point.

It's all very well designing a solution but until it's been used for some time often the drawbacks aren't as obvious as even your excellent points.

Things are changing very quickly in the Irish market place and in regulations, everyone trying to get a advantage over the next man with their solution to the new energy requirements, it may not be for some time that the possibly resulting problems reveal themselves.

A tried and tested method is often best?

Thats why I can see the benefits of a forum like this, many minds and years of experience available.