Looking for Advice - Possible Byre Conversion

[Penners]

Geoffrey - I've never really grasped the difference between the U-value and the R-value. Would it be correct to describe one as thermal conductivity and the other as thermal resistance?

If so, which is which, please?

 

[Gervase]

The BRE calculation document is actually fairly cursory and is based on known and measured common building materials - concrete slabs, blockwork, plasterboard, gypsum plaster and cementitious mortar.
There is no mention of natural stone, or lime plaster, or cob, or wattle and daub. Hence BCOs will usually default to a caulculation based on solid brick or blockwork.

The BRE does have a catalogue of U-values, which is one of the reasons that Nigel Gervis at Ty Mawr has spent upwards of £40,000 getting his limecrete floors tested so that they can be recorded on the list. It is far from adequate, however.

Otherwise the nearest we seem to be able to get to a handy checklist of the materials commonly used in period properties is the Chartered Institute of Building Services Engineers' Guide A for designers of low energy sustainable buildings.

That includes the thermal performance and U values for many materials which are available, but they are often 'notional' - for example the thermal conductivity of lime plaster is given as 0.8 w/mk while the U value for stone walls 600mm thick with 50mm air gap with 25mm dense plaster on laths is notionally 1.38 w/m2.

The problem with thick masonry walls, however, is that an experiment to calculate their thermal conductivity would be very difficult to set up. The there's the fact that a U-value is a merely a simple mathematical calculation often extrapolated from experimental data on a very different material.

For example, a 900mm thick rubble wall will have very good insulation value provided it is solid. However, all old walls have voids and it is by no means certain what material is inside the core, so even attempting to reach a U value for such a wall would be pretty meaningless.

Thus it is highly unlikely that, in dispute, a BCO would accept a value extrapolated from that based on a millimetre of limestone under laboratory conditions!

 

[GK]

Oh, gawd. What have I let myself in for. Does it sound as though I know what I'm on about?

Yes, R value is resistance. K is conductivity and U is the composite loss of the structure.

When I refer to my little book this is what it tells me to do:

A sample calculation for a wall of mine:

External wall
350mm brick with an R value of .0012/mm = 0.42
Kingspan 75mm stated as 3.25 per manufacturer
Plywood 6mm x R of .0067 = .0402
Lime plaster 20mm x R of .0020 = .04
Outside air bounday layer = .03
Inside boundary layer = .12
Dry line cavity = .18
Total = 4.0802

The lime plaster figure is just a figure for "render". In practice altering the small numbers makes next to no difference, it's the Kingspan which is really doing the work here.

U value is 1/4.0802 which is 0.245. Pretty low, I think. It actually means that for every square metre of wall, and for every degree centigrade temperature difference, you will lose 0.245 watts through the wall.

The Kingspan on its own would have a U-value of 0.29, so you can see that having all that masonry really doesn't keep much heat in. I'm not meaning to be advertising the stuff, just telling it how the numbers say it is, to someone who's really not very good at maths.

I haven't the slightest idea how relevant this is to modern building regs- I just use this method to arrive at a radiator and boiler size in a slightly more scientific way than the Mears calculator.

Then, like any other plumber, I specify a radiator twice as big. Just in case. I may have got the sums wrong...

 

[Mark]

Geoffrey - I've never really grasped the difference between the U-value and the R-value. Would it be correct to describe one as thermal conductivity and the other as thermal resistance?

If so, which is which, please?

I hadnt really thought about this, other than I knew there was confusion about stuff. I though you were correct, but after a quick search I'm not sure I think some of it is 'just' terminology

http://www.knaufinsulation.co.uk/output/design/page_77.html#Thermal%20conductivity

Thermal conductivity
The rate at which heat is transmitted through a material, measured in watts per square metre of surface area for a temperature gradient of one Kelvin per metre thickness, simplified to W/mK.

Thermal mass
The mass in a structure which is used to absorb solar heat during the day and then release the heat in the evening.

Thermal resistance
A measure of the thermal resistance of a particular thickness of material. It is measured in m2K/W and is equal to the thickness of the material (in metres) divided by the conductivity of that material. Surfaces and cavities also provide thermal resistance and there are standard figures for these resistances that must be taken into account when calculating U-values. The resistances of each material within an element are added together to determine the overall resistance of the element. The reciprocal of the overall resistance is the U-value.

Thermal resistivity
Resistivity is a measure of a material's thermal resistance and is the reciprocal of its conductivity. It is a measure of the thermal characteristic of a material and is independent of its thickness.

U-value (thermal transmittance)
Commonly known as the U-value, it is a measure of the rate of heat loss of a building component. It is expressed as watts per square metre, per degree Kelvin, W/m2K. The U-value is calculated from the reciprocal of the combined thermal resistances of the materials in the element , air spaces and surfaces, also taken into account is the effect of thermal bridges, air gaps and fixings.


Soooooo, conductivity and resistivity are properties of the material and resistance and transmittance take into effect the thickness of the material.

for each pair the resistnace is 1/ conductance.

 

[GK]

Yeah, that last post reminded me why I just get the book out and follow it by rote, any material I can't actually calculate I add in a black magic, sorry, notional, factor.

 

[Penners]

I am definitely boxing above my weight here!

However, I reckon if I print out Geoffrey's Gervase's and Mark's last few posts, then sit down with a damp towel around my head, I might just begin to get an inkling of what this is all about. Thanks very much for your patient attempts to educate me, gents.

However, despite all the highly technical terminology, the advanced mathematics and the plethora of regulations, I sense that it all still boils down to a fair degree of educated guesswork! But maybe I'm just showing my ignorance again.

 

[Nemesis]

I just need a stiff damson gin and a lie down.

 

[biffvernon]

The calculations don't take into account the wind that howls round the gaps at the edge of the badly fitted Kingspan and it's when you add in a radiant barrier that it gets seriously tricky

 

[Nemesis]

Another stiff gin...

 

[Gervase]

Oi, you keep your bloomin' foils out of here! :twisted:

 

[biffvernon]

My new washroom has oak sawdust and shavings between the outer brick skin and the inner wattle and daub. Couldn't find it in the BRE lists.

(No foils were used in the contruction of this building.)

 

[NT]

If youre determined to find the U value of an existing or proposed construction, a heat box is simple to make, and comparing the unknown material with a known one would enable an assessment. What a BCO would think of this I dont know.


NT

 

[Gervase]

You'd need pretty big heatbox to measure a metre thick rubble stone wall!

 

[Penners]

There's a thought. Forget the building - live in the heatbox.

 

[bazza1603]

Hi,

Its may be easier to live in a heatbox!!

Just a little bit of history and update.

The farm hasnot been used for years. The land has nearly always been farmed by a third party. The last small number of sheep lasted until Foot and mouth wiped out a neighbouring farm.

We are doing this, not because its free, but my wife wants to be back in the country where she grew up.

With regards to the building I would to try and keep the front the same, but would add windows to the extention at the back hopefully given enough light as the back is south facing.

Will have a talk to the neighbour and in laws and make our decision afterwards.

Thanks everyone for your help and thoughts

Regards

Barry and Jill

 

[Nemesis]

I'm curious to know where it is - not too far from me I suspect! Lovely building though.

 

[bazza1603]

Hi,

Its near to Allendale.....

How far are you away?

Regards

Barry

 

[Nemesis]

Pretty near - Bardon Mill-ish.

The stone and detailing I recognised as local-ish! Please don't spoil with cementy pointing!


If you are Tynedale - give Sandra Robertson the CO a ring - ask about local policy now on conversion of farm buildings to residential. Local policy may not enable this.

 

[Roger H]

I think Sandra has left Tynedale. Not sure that they have a new CO yet.

Roger.

 

[Nemesis]

Oh eck. I hope I haven't driven her away...

Maybe she just got fed up with the lack of conservation.