U-Value Calc - Am I doing this right?

insular1

Hi all,

I'm running through some different possible insulation scenarios before sitting down to discuss with my arch tech in the next week just so I have an idea of what he'll be talking about. Just wondering am I doing the u-values right.

So for a general scenario block build 200mm cavity with 150mm insulation and 50mm air cavity:

Outer block thermal conductivity 1.19W/mK, thickness 0.1m, r-value 0.084
Air Cavity thermal conductivity 0.025W/mK, thickness 0.05m, r-value 2.0
Insulation thermal conductivity 0.023W/mK, thickness 0.15m, r-value 6.52
Inner block thermal conductivity 1.19W/mK, thickness 0.1m, r-value 0.084
Gypsum sand plaster on inner wall thermal conductivity 0.22W/mK, thickness 0.01m, r-value 0.045.

So the total u-value of the above set up would be the inverse of the sum of the r-values or U=0.12 approx.

The few of questions I have: Firstly am I way off in what I'm doing here and how can I get it right?
Secondly assuming the above to be roughly correct how would I account for wall ties?
Thirdly is it true that basalt wall ties have a conductivity so low as to be negligible in terms of the overall U-value calculation and so wouldn't need to be included?

I pulled all the above thermal conductivity off various google sites so Im sure some are less than accurate. I hope to get a PHPP assessment of the plans done (awaiting a quote for this) so I'm sure he'll be able to explain all this to me but I would like to have a bit of knowledge myself before I start and I find it pretty interesting!

wyliecoyote1

Yeah, looks good, although you should be including a resistance for the internal and external air film resistance. Just add it to the sum of the resistances.

see link below
http://www.kingspaninsulation.co.uk/Services/Technical-Services-Department/U-value-Calculations.aspx

youll see in the image they have included an element called "outside surface resistance". AFAIK, it should very fairly negligible

sydthebeat

you cannot calculate u values without reference to this:

BR 443

Metric Tensor

Might be worth finding a few spare quid to get something like this:

http://projects.bre.co.uk/uvalues/

It's perfectly possible to do u-value calculations without it but it speeds the process up a lot and includes the tricky things like wall ties without you having to figure out the minutia of BR443.

Of course you still need to understand the inputs and outputs to make sure you are doing it right.

insular1

Thanks a million syd. Great document! Very thourough. So based on the guidance from this document am I correct in calculating my thermal resistance for stainless steel wall ties as follows:

Thermal conductivity = 17W/mK. Thinkness 0.4m (cavity 200mm+100mm internal block +100mm external block). Density of wall ties 2.5/m2. Cross sectional area of wall ties 80mm2 or 0.00008m2. So thermal resistance (r-value) = (density /m2)x(crosssectional area m2)x(thickness m)/(conductivity W/mK) = 0.0000047m2K/W

This seems really small? Am I way off?

sydthebeat

how are you working out that cross sectional area??
seems quite large, probably double what is normally applied.

have a look at pages 16 and 17 here for a working example

remember, if the wall tie correction is less than 3% of the uncorrected resistance it can be negated.

Calahonda52

Op: what's the plan for the outside finish?

It's all very well to crunch these sort of numbers at the desk, you need to think about how practical it will be to achieve these numbers on site as well as the extra cost of a 200 wide cavity with basalt ties.
All fine and dandy but if the 150mm insulation is not done right ......

insular1

sydthebeat wrote: »

how are you working out that cross sectional area??
seems quite large, probably double what is normally applied.

I took the larger of the two diameters in section 4.9.2 of original BRE document you posted thinking this would give me a worst case scenario.

Calahonda52 wrote: »

Op: what's the plan for the outside finish?

Haven't thought this far ahead yet! Could this make a large difference to the U-Value? Hoping my arch tech will have most of these details for me!

Calahonda52 wrote: »

It's all very well to crunch these sort of numbers at the desk, you need to think about how practical it will be to achieve these numbers on site as well as the extra cost of a 200 wide cavity with basalt ties.

Yeah at the moment these are purely theoretical designs so I really have no idea of costs. Based on the above calcs though it would seem that the cost of Basalt wall ties would not be worth the tiny improvement in U-Value unless it was a massive house. As for the 200mm extra wide cavity I kind of thought the biggest additional cost here would be the extra thickness in insulation as the basic construction is the same? Is this a bad assumption to make?

Calahonda52 wrote: »

All fine and dandy but if the 150mm insulation is not done right ......

Yeah this worries me as I have no construction experience at all. I'm retaining the arch tech as project manager so I'm hoping he'll be able to keep on top of this aspect. I'm also hoping to hire a builder with a proven track record although I'm struggling to identify too many local to me that I can be sure are up to it.

Thanks for all the replies!

Johnnyhpipe

As an aside.... basalt wall ties are about €1.50 per tie.... the regs give guidance on tie spacings up to a max of 150 cavity, and even at that its ties at 450mm crs each direction! 200 cavity will be more onerous again.

Just be sure to calculate how many thousand wall ties you need and multiply this by €1.50! Thats a lot of money on wall ties alone.

I only mention it as your u value calc would be pointless if you're going to deviate from design when it comes to site.

Johnnyhpipe

For clarity..thats €1.50 per tie for the 225mm long ties.

With a 200 cavity you will need the 325 long ties which are over €4 PER TIE!

insular1

Thats pricey allright! So there are definitely additional costs to be considered when widening the cavity. Looking forward to here what my arch tech has to say about all my ideas now!
Thanks for pointing me in the right direction all!

Calahonda52

+ i believe there are additional design and build costs with a wall that has a 200 cavity, maybe even structural engineer design costs

dfader

insular1 wrote: »

Thanks a million syd. Great document! Very thourough. So based on the guidance from this document am I correct in calculating my thermal resistance for stainless steel wall ties as follows:

Thermal conductivity = 17W/mK. Thinkness 0.4m (cavity 200mm+100mm internal block +100mm external block). Density of wall ties 2.5/m2. Cross sectional area of wall ties 80mm2 or 0.00008m2. So thermal resistance (r-value) = (density /m2)x(crosssectional area m2)x(thickness m)/(conductivity W/mK) = 0.0000047m2K/W

This seems really small? Am I way off?

Seems right; this website might help verify your results; www.uvaluepro.com