What's more important for comfort U value or 'thermal mass'

[SteamyTea]

1 minute ago, Gone West said:

Presumably a waterproof additive would solve the problem.

My chemistry knowledge about concretes is very limited.

You get waterproof concretes, and you get ones that set under water, then you can get additives that repel water, not really sure what the actual differences are.

The little bit of research I did shows that the mix is very important to get the desired properties.

 

The common statement that cement based, usually ordinary Portland cement, don't breath and are waterproof I think is just wrong.

In practice, asking a builder to decide the correct mix and apply it properly is possibly not the best route to take.

Asking the chemical engineer at a cement factory probably is the first port of call.

[IanR]

9 minutes ago, SteamyTea said:

The common statement that cement based, usually ordinary Portland cement, don't breath and are waterproof I think is just wrong.

 

In absolute terms you are right, but, on old solid walls with poor or no DPC, they will draw in damp from the ground by capillary action and need to be able to transfer that moisture to the outside. Cement based renders and plasters are known to restrict that transfer of moisture to the outside so it builds up to a level that then shows damp issues. Lime based renders and plaster are more vapour open and allow the moisture out so that it doesn't build up.

 

48 minutes ago, Gone West said:

So a cement rendered stone wall will keep rain out, as long as it's not got cracks.

 

Earliest DPC's were late 19th C. Injecting a damp proof course to remedy older walls helps but is not robust and has a finite life. Unless you are sure of a robust DPC you shouldn't use cement based renders and plasters.

 

I'm currently planning my next project where I've taken on an old farmhouse that was renovated in 1980.  The newest parts are mid C19 with solid masonry walls and in 1980 had a DPC injected and were cement rendered on the outside and plastered on the inner. The walls now show typical rising damp damage, where there's a tide mark at about 1m up the inside of the wall and both the plaster and render have blown.

 

There are older walls that have got away with being cement rendered and plastered, some without any foundation at all, just sitting on an aggregate layer on earth, but the side of the house close to a water course is where most of the problems have occurred, so there are a number of factors to this.

[SteamyTea]

6 minutes ago, IanR said:

on old solid walls with poor or no DPC

Should that not be part of a renovation though.  Hoping that the wall coating can allow water to evaporate faster than than it can get in is rather hopeful, and probably not that effective in reality.

 

I still think that there is very little difference between lime or OPC mixed when it come to the g.m^-2.s^-1 numbers.

 

What tends to happen with old buildings is that we remember the ones that are in good condition and have stood the test of time, and we also remember the ones that have been renovated badly and caused problems.

We then attribute the reasons from common gossip and anecdotal evidence.

There are millions of houses that have been rendered in OPC mixes that give no problems, we never hear about them.

 

Now I am not in the least bothered if people want to cover their buildings in lime bases or whatever, but I do feel it is necessary to dispel some myths that may be covering up a larger problem.

[ProDave]

1 hour ago, Gone West said:

So a cement rendered stone wall will keep rain out, as long as it's not got cracks.

NO

 

You only have to look at Harled (rendered) Scottish houses in exposed locations.  When you get driving rain, it is clear to see the render colour on the wall darkening as it absorbs rain, and then takes days or even weeks to dry out again.

 

This problem shows itself clearly on rendered garden walls where they get cold, freeze and crack and the render and it falls off in lumps. A rendered wall here is lucky to last 2 winters.  This does not happen on rendered houses, because there is always just enough heat escaping through the wall to prevent the wet render from freezing.  (though my findings with my new house are that may become a problem when walls are really well insulated)

[Gone West]

25 minutes ago, IanR said:

Earliest DPC's were late 19th C. Injecting a damp proof course to remedy older walls helps but is not robust and has a finite life. Unless you are sure of a robust DPC you shouldn't use cement based renders and plasters.

I would be very suprised if the old parts of the house have a DPC. The whole house, original 1840s and 'modern' 1970s is cement rendered and there is no visible damp on the inside walls. That's why I was carefully considering whether to cover the inside of the old walls with insulation, in case that takes heat away which is helping to keep the walls dry and therefore the room comfortable.

[Gone West]

18 minutes ago, ProDave said:

This does not happen on rendered houses, because there is always just enough heat escaping through the wall to prevent the wet render from freezing.

We get a lot of driving rain but we don't get many freezing nights here. The render is well painted and is at least fifty years old. I am reluctant to alter anything that has worked for fifty years, so covering the inside of the old walls with insulation requires a lot of thought.

[SteamyTea]

8 minutes ago, Gone West said:

so covering the inside of the old walls with insulation requires a lot of thought.

Can you find a bit of free wall and insulate that, then monitor it?

[Gone West]

1 minute ago, SteamyTea said:

Can you find a bit of free wall and insulate that, then monitor it?

I'll think about that. It needs to be big enough that the surrounding area doesn't have an effect.

[saveasteading]

14 hours ago, Roger440 said:

Theres heaps of evidence

There is loads of info from Scottish heritage groups. Some of it it technical and convincing.

Referring both to how 3 layer walls work and lime.

There is also loads of evidence of the problem with cement render.

There will not be a damp course.

Get googling. 

 

I can't report fully yet but report so far.

Our project is a farm building, not built as a house but fof cattle and grain. 600th walls in traditional 3 layers.

It sits on granite ashlar at only 400 depth. 

We have built an inner 100mm stud (min 25mm off the stone) with vcl on the outside and mineral wool infill. Air gap,  stud, polythene, service gap,  plasterboard. An area of a few m2 is left exposed for heritage/aesthetics.

The large windows are probably the biggest heat loss proportionally.

That insulation level is decent, not stunning  but the floor and roof are heavily insulated to new build standards.

2 nights ago it was minus 7 outside and plus 22 inside. Result.

No damp worries yet.

 

 

.

[SteamyTea]

2 minutes ago, saveasteading said:

15 hours ago, Roger440 said:

Theres heaps of evidence

There is loads of info from Scottish heritage groups. Some of it it technical and convincing.

I think was was really directed to me.

 

I have been looking into this for years, I once found a chart that put some numbers on it, but never seen it again.

There is loads of anecdotal evidence, as I refereed to earlier, but I have not seen any real scientific evidence about different breath abilities of different render systems.

 

While I cannot prove anything either way, my experience in composite plastics means I can draw some parallels I think.  Concrete and renders are just composite materials after all.

This is why I think that the overall mix is the important bit, not the binder.

[SimonD]

49 minutes ago, SteamyTea said:

This is why I think that the overall mix is the important bit, not the binder.

 

Yes, this is key. The complete buildup of a multi-layered wall is also critical - you have a nicely designed breathable wall and then someone goes and applies an acrylic based paint, or uses a bonding coat of something - suddenly the whole thing has become a plastic bag and the main material gets the blame. It's one of the reasons I think people incorrectly think gypsum plaster, for example, isn't breathable and it gets a bad name.

[SteamyTea]

23 minutes ago, SimonD said:

It's one of the reasons I think people incorrectly think gypsum plaster, for example, isn't breathable and it gets a bad name.

Yes.

I also think the term breathable is a bit misleading.

Why numbers are so important.

 

I wish I was still at university then I could have used the building department to try a few experiments.

Vapour and water transmission and absorption studies really are very basic, be nice if it was a second year module on a BTEC.

[saveasteading]

1 hour ago, SteamyTea said:

think was was really directed to me.

Not really.  I'd look again myself but we've made our decisions, incl BH advice,   plus I'm researching and doing other things atm.

 

It crossed my mind while doing something else that thse 3 layer walls were built for structural efficiency but happened to keep the rain and most damp out. Then much the same technique was used on very posh Georgian  houses. Eg Edinburgh and Bath, and all qualities inbetween.

These houses have inner liners. Most survive.

[Alan Ambrose]

Is it possible to chuck in a somewhat relevant question here? Re:

 

 

What's the target for phase shift / decrement delay?

 

I can't decide whether the optimum is 12h based on the idea of 'cancelling out' some of the daily temperature swing or 'very large' on the basis that the interior temperature swing will be super low?

 

 

 

  

[IanR]

5 hours ago, SteamyTea said:

Should that not be part of a renovation though.  Hoping that the wall coating can allow water to evaporate faster than than it can get in is rather hopeful, and probably not that effective in reality.

 

On my specific example I'm not aware of a better technology than injecting a damp course, which I know isn't robust. So damp will get into the wall and needs to be permitted to get back out.

 

For me this is also somewhat under the control of the Conservation Officer at the LPA since it is a Listed Building, so removing sections of wall and installing a modern damp course isn't an option. The renovation completed in 1980 using cement render was also under the guidance of the Conservation Officer, and the guidance was wrong then, so may well be again, but lime render and plaster is now what is prescribed, and is backed up by English Heritage.

[SteamyTea]

21 minutes ago, IanR said:

6 hours ago, SteamyTea said:

Should that not be part of a renovation though.  Hoping that the wall coating can allow water to evaporate faster than than it can get in is rather hopeful, and probably not that effective in reality.

 

On my specific example I'm not aware of a better technology than injecting a damp course, which I know isn't robust. So damp will get into the wall and needs to be permitted to get back out.

 

I did that on my listed place in Weymouth, in 1991. I am not sure how effective it was as I sold it 3 years later.  I did get the chance to see inside the place about 4 years ago (it is now a holiday rental) and I did not see any evidence of damp.

The wattle and daub walls where a different matter.  I was told not to treat them at all, they were dreadful for damp, but they basically had no solid foundation and the place sat on sand (was near the beach).

That place put me off having a listed building for ever.

[SimonD]

3 hours ago, SteamyTea said:

Yes.

I also think the term breathable is a bit misleading.

Why numbers are so important.

 

I wish I was still at university then I could have used the building department to try a few experiments.

Vapour and water transmission and absorption studies really are very basic, be nice if it was a second year module on a BTEC.

 

This study is quite a good one where they've built some real buildings at Bath Uni https://purehost.bath.ac.uk/ws/portalfiles/portal/218688237/PhD_Thesis_Cascione_compressed.pdf

[SteamyTea]

16 minutes ago, SimonD said:

This study is quite a good one

Think I may need to print some of it out.

Shame there is no concrete mixes and it is to do with internally applied coatings.

But the testing methods are interesting and this line.

 

"Alternative experimental studies have attempted to investigate the actual performance of materials in real buildings, but
there is no standard methodology yet and no established relationship between moisture buffering and building performances."

 

Probably why I can't find any real numbers, without a standards everyone is talking a different language.

[Roger440]

On 10/01/2024 at 13:22, SimonD said:

 

Yes, this is key. The complete buildup of a multi-layered wall is also critical - you have a nicely designed breathable wall and then someone goes and applies an acrylic based paint, or uses a bonding coat of something - suddenly the whole thing has become a plastic bag and the main material gets the blame. It's one of the reasons I think people incorrectly think gypsum plaster, for example, isn't breathable and it gets a bad name.

 

Pretty much everything is breathable to some extent.

 

Gypsum plaster is breathable. Just not as much as lime. If the moisture absorption into the wall from the ground exceeds the abilty of the wall to shed it, then you have a problem. Gypsum is less breathable than lime. Thats just a fact.

 

Will it be a problem? Depends on the moisture. And that can vary hugely depending on build, materials, place etc.

 

If you have no DPC, then lime plaster gives you the most likely good result. Will fitting gypsum give a problem in any specific setting? Who knows. 

 

At a practical level i had an issue with damp pushing the gypsum plaster off. There were other factors i dealt with to reduce likely damp in wall.

 

But at the point of plastering i can either do lime, or gypsum. Its highly likely Lime will give me no further trouble, a higer risk of a negative outcome with gypsum.

 

Others are free to make their own choice, but ill stick with the lime in such circumstances.

 

As an aside, and as a result of practical experience, the ability of gypsum to breate appears to dimish very significantly when its "polished". Which, of course, it always is. I wonder if the "tests" take that into account?

 

As you though, even if gypsum was breathable enough in a particular application, thats rather worthless if you stick a less breathable paint over it. And most modern paints are pretty impervious.

 

[Roger440]

On 10/01/2024 at 09:53, SteamyTea said:

Should that not be part of a renovation though.  Hoping that the wall coating can allow water to evaporate faster than than it can get in is rather hopeful, and probably not that effective in reality.

 

I

 

Lets be realistic, how are you going to retrofit a DPC to a house? (please dont say a chemical DPC, thats utter bollocks and nothing but a scam)Especially one of stone and rubble infill construction? Please, come back to the real world.

 

Having the wall allowing water to evaporate faster than it gets in is how old houses with no DPC work. Sadly, of course, it didnt always work. There were no calculations to do that way back when. It was hit and miss. The really bad stuff has invariably long since gone.

 

Short of demolition, you have to accept and work WITH it if you want a damp free house.

 

When i did my last house, i admit i was sceptical too. But removing the inappropiate material completely fixed things. When i started the walls were so wet, the socket back boxes had corroded away (in 10 years) and if you drilled a hole in the wall it came out as a slurry.

 

Stripped the plaster off, repointed the outside in lime to replace the cement, removed the concrete slab on mud. In a matter of weeks, it was completely dry, to the point drilling a hole in the wall at ground level produced dry brick dust.

 

Frankly, it was quite amazing to watch over the weeks.

 

At the new place, which is stone with much thicker walls, we uncovered the fireplace early on. It had been pointed in cement. The bottom 2 feet some of the stones were dark and wet to touch. Chipping out the motar revealed the lime behind that in places was so wet, you could push your finger into it. Its taken 6 months this time (and the outside is still painted and pointed in cement) its dried out. The motar is hard, well as hard as lime ever is, stones have all dried out. Ive only repointed a small section so far.

 

Again, practical real world experience. Which for me wins over hypothesis.

[Roger440]

On 10/01/2024 at 08:36, Gone West said:

So a cement rendered stone wall will keep rain out, as long as it's not got cracks.

 

Yes, it will.

 

Problem is, it always cracks, as it not very flexible and houses move.

 

It slightly change a well known phrase, theres only 2 types of cement, cement thats cracked, and cement thats going to crack.

 

As pointed out earlier, you can often see the damplines on rendered houses after heavy wind blown rain.

[saveasteading]

3 hours ago, Roger440 said:

ability of gypsum to breate appears to dimish very significantly when its "polished

That's consistent with what is said of lime. Lime plaster should be stippled to increase the surface area.

Smoothed and polished surfaces  therefore have refuced surface area and be less breathable. Poishing will remove all texture and decrease the surface area. Will it also force dust into crevices, closing porse and making it denser?

 

[Gone West]

Many thanks to all the contributors. This is what makes this forum so useful, a lot of varied information from knowledgeable people.  I have decided not to insulate the wall and risk any deterioration. There is no sign of damp or mould, even near the bottom of the wall, so I shall keep it as it has been for the last fifty years even though I shall lose the advantage of having insulation.

[SteamyTea]

5 minutes ago, Gone West said:

varied information

You mean contradictory information I think.

[SimonD]

15 hours ago, Roger440 said:

Gypsum is less breathable than lime. Thats just a fact.

 

This is exactly mine and @SteamyTea 's point - the term 'breathable' is actually completely meaningless and shouldn't really be used at all. But with respect to the facts about gypsum v lime you should read the paper I linked but the TL;DR is:

 

Quote

Clay and lime presented mainly macro-pores, which have an average diameter of around
125 nm. Gypsum also had macro-pores of a significant bigger size (365 nm average),
but it also presented micro-pores, as shown in Fig. 3-2b. Gypsum plasterboard had a
more accentuate micro-pores presence and a significantly higher average pore diameter
(631 nm) than standard gypsum. Overall, the gypsum and plasterboard showed a
significant higher pore volume than clay and lime. Due to the more complex pore
structure of gypsum and plasterboard, both vapour and liquid transport take place
into the materials, when exposed to a RH and vapour pressure gradient, while in clay
and lime only vapour transport occurs. Water vapour transport can take place in
the macro-pores and its driving potential is the water vapour pressure, whilst liquid
transport takes place in the micro pores, where the driving force can either be the
relative humidity and the capillary pressure.

 

But the fundamental problem you're talking about hasn't really got anything to do with lime or gypsum, it's down to excess moisture. Excess moisture isn't good for any structure, so deal with the moisture.

Edited January 13 by SimonD