U-values for rubble filled stone walls

terryg17

Does anyone have any idea what values are associated with 450mm thick sandstone walls?

Mike George

Not good, around 2.1W/m2K [based on Conductivity 1.56W/mK]

tony

I could go 1.8 or even 1.5 for less dense stone with some lime mortar and a few air gaps in the rubble..

rogerwhit

Can I add a parallel but related thread to Terry's sandstone query? I'm also curious about U-values. Our walls are slate, about 600mm thick in total, of two leaves enclosing an irregular rubble-filled cavity of say 150mm. The cavity tends to be airy - cut through the inside leaf & you can feel an issuing draught.

Mike George

Ah, yet another flaw with the suitability of the u-value. It may be okay to estimate the Sandstone query, but not the slate one methinks. Any value would be meaningless

rogerwhit

Mike, are you saying that because my vented 'cavity' has unpredictable airflow?

Terry, what is the nature of your rubble fill?

Mike George

Roger, Yes, combined with the irregular fill.

U-values recognise neither of these.

1.The conductivity [k-value] and respective surface areas/ volumes of the irregular fill and any air pockets would need to be estimated in the same way that thermal bridging of wall ties is now allowed for in BR443. A complicated calculation if you are not familiar with this kind of thing and in my view not worth the effort. Better to try and agree something with your BCO
2. Air infiltration losses are not taken into consideration so the more airflow you have the less reliable any predictions are going to be.

You may get away with a simplified calculation assuming the wall is all made from a worst case conductivity [ie slate] That is the approach I would take BUT air losses are still not accounted for.

howdytom

Mike .... or anyone, is there any practical way of assessing heat loss through a wall, IE using a heat source internally and some kind of sensor outside ? Obviously internal draughts would effect the outcome. Old houses, pre cavity, must form a significant proportion of the UK stock and it would seem that a practical solution rather than a paper one would be a more useful assessment for them.

Tom, in a very old house (at the moment).

howdytom

Would it be possible to pump a lime mortar mixed with something to aerate it, say a plasticizer or bicarb of soda and use compressed air to transport it into the void, it would require loads of injection holes and a lot of hope/guess work but .."outs better than nowt"
tom

chuckey

Having had my rock'n rubble cottage re-roofed a year ago, the awfulnness of the roofer has sunk in. I too am plagued with drafts from any void in the rough old plaster inside, even though I have re-pointed the outside walls 100% (OK - 99.9%!). Well the roofer put 53 large buckets of mortar in the top of each of the two non-gable cavities on which he bedded the under eave stone flags. But he put nothing into the cavities of the gables, so these are vented via the breathing membrane and voids in the loft. I intend to inject foam into these cavities from the loft, just above the ceiling level ASAP.
Frank

Gervase Webb

You can grout voids with hydraulic lime - it's a standard conservation technique and serves both the strengthen and cut down on wind penetration.
As for u-values, there is no recognised/accepted value for rubble stone that I have been able to discover over more than two years of arguing with BCOs. Given that a quarter of our housing stock is pre-1914, and a significant proportion of that has solid walls, there's a real need for some research into this area. Current BCO thinking would seem to be to demand multifoil and drylining at the very least, and few seem to have heard of lime/hemp insulation, so heaven help anyone in an old house who wants to preserve interior detailing.

Mike George

Posted By: howdytomMike .... or anyone, is there any practical way of assessing heat loss through a wall, IE using a heat source internally and some kind of sensor outside ? Obviously internal draughts would effect the outcome. Old houses, pre cavity, must form a significant proportion of the UK stock and it would seem that a practical solution rather than a paper one would be a more useful assessment for them.

Tom, in a very old house (at the moment).

Yes, I agree with you, and thermocouples will give heat loss data in the way you suggest. I have been trying to get funding for this kind of research for years but this kind of money is very hard to come by.

fostertom

Posted By: Gervase WebbYou can grout voids with hydraulic lime - it's a standard conservation technique

Really? Tell us more - how's it done and isn't it begging for water penetration problems - accidental long distance void/passages created, as is the old danger with cavity retrofill?

timevans2000

A pal of mine had his barn conversion insulated on the inside with spray polyeuranthane foam. It was then plastered. Keepes the original feel of the barn and adds lots of insulation but is devoid of thermal mass on the inside. Seems to work ok and he got it through building regs ok

fostertom

Never heard of that - could even work, kind of, in the thermal mass area, because of the intimate contact between the layers. Spray hemcrete wd be even better.

A project that got cancelled, previous owner seemed to have injected rubble walls with urethane foam somehow - great nibs of it had exuded from many joints, where unfinished in hidden area. I dreaded to think what that might have done to wall longevity and water resistance, hence my interest in injection with lime mortar, above. If lime mortar, how about hemcrete?

Gervase Webb

Grouting can be done by gravity, by pump or by vacuum. For delicate walls and uncertain voids, gravity is best - you need a head of around 4 metres, with a hopper feeding down pipes with a minimum of 1-inch bore, leading to inlet holes around a metre apart horizontally and 50cm apart vertically. Flush the top holes with water first until it runs clear from the lower holes, and then plug the lower holes and other leak spots with oakum or shredded hessian. Introduce the grout - made from NHL3.5 lime, PFA and water in a 1:0.2:0.75 ratio, and work up a metre at a time, plugging frantically as you go
It's fiddly work and expensive, unfortunately. Particularly since vast quantities of grout can be needed, and then you find out that it's found the line of least resistance and filled up an old sump or cess-pit near the wall! However it can save a building - large parts of York Minster are standing today thanks to grouting.
It's also far greener than using polymer foam - and it still allows for breathability and autogenic fracture healing.
On a smaller scale it can be done - but not as well- with a pointing gun to fill lesser voids and cracks in domestic rubble-stone walls.

fostertom

Posted By: Gervase Webboakum or shredded hessian

I love it!

Yep, sounds good. No water penetration problems? - accidental long distance void/passages created, as is the old danger with cavity retrofill?

howdytom

Gervase, my thoughts of using bicarb or soap and air to inject the mortar, were to increase to insulant value by creating a foam type solution within the mortar, Do you think it would work, any pitfalls ? our walls seem to vary from well filled, to loads of chaff (rodent fodder) to rat runs. Anything that would fill them would make a huge difference to US and save a load of wasted energy through draughts.
tom

Joatex

Howdytom if you just wanted to measure the temperature of any surface I suugest using a simple infra-red thermometer and that at Ebay is inexpensive. (http://cgi.ebay.co.uk/Infrared-Thermometer-Infra-Red-Laser-Point-Kitchen_W0QQitemZ160224048244QQihZ006QQcategoryZ43421QQssPageNameZWDVWQQrdZ1QQcmdZViewItem)
That will give an instant surface temperature at that spot . Take the temperature on both sides of the wall, you know the wall area, the time 'is' one second, so the energy needed to maintain that wall temperature can be calculated. Of course the readings should be at regular intervals over 24 but then you would need an expensive thermocouple and data logger. The beauty of taking spot temperatures is that you can see the direction of heat flow, similar to a thermal picture but limited to a much smaller area. Simple, fast, rough and ready but saves having to consider the wall structure.

fostertom

Joatex, opposite surfaces of a body may be greatly out of phase with each other at a given moment, unless a steady state has prevailed for a very long time - in the case of a heavy wall, for days. In real-life fluctuating state, you can't place any reliance, even rough-and-ready, on taking inside and outside temps and then assuming that's proportional to heat flow, either at that moment or over a long period. The relative temps may even be reversed! It would only work with very thin materials of very low thermal mass.

As an interesting mind-experiment, what you're suggesting in fact shows up the whole obsolete myth of U-value thinking - which ignores the overwhelming effects of both thermal massiveness, and of the non-steady-state that is real life. Which is why conventional insulants give actual heat retention results, over a period, as poor as 40% of what plain U-value calcs would expect; and why old solid buildings with pathetic calc's U-values often perform well; and why multifoil insulation works so well, because it understands and exploits the non-steady-state.

Joatex

Thanks fostertom for your views with which I agree. I find it difficult to accept that the U value of a wall,ceiling etc can be accurately determined except by direct measurement. One can't see the wall contents and I am a hesitant accepting values which have numerals after the decimal dot. So U values are an assumption, broadly indicative of the insulation value and only that. Assumimg a book U value for a whole wall for example completely misses the variations, the cold or hot spots on that wall ,yet they are important in the next move of improving the insulation of that wall.

I should have mentioned that there should be a steady temperature state, that was assumed. There should be spot temps taken over the wall giving a thermal pictuure, not as bright and complete as that of a thermal camera but the infra-red thermometer remains well within the financial range of most of us.

howdytom

I'll try that on a north facing wall Joatex, then the sun can't affect the readings as much, but on walls as thick as ours 600-800mm, with loose infill and draughts down the middle of it all, I think I'm on a "hiding to nothing".
tom