Insulated panels internally -v- pumped cavity

Quazzie

Ok, so to start off I will give my wall spec as it is at the moment.

19mm External Render
100mm Concrete Block (Outer Leaf)
50mm Cavity
100mm XT/CW Xtratherm Insulation board
215mm Concrete Block (Inner leaf)

Advice from Xtratherm has lead me to believe that this will give me a U-value rating of 0.18 W/m2 K

My target was 0.17 W/m2 K and so I am left with the task of reducing that ever so slightly to reach my target. I originally thought of adding 37.5mm Xt/TL on dabs to achieve a U-value of 0.15 W/m2 K, but am worried that by doing this, that I will remove a considerable amount of the thermal mass of the building, thermal mass that I need to help store the heat from my UFH. All internal walls downstairs are 215mm so them in themselves would offer considerable thermal mass, so do I need the external walls too. I know this is a bit of a blind question and I apologize for that so I'd like answers that are more general.

Is there a case for pumping the remainder 50mm cavity instead of the insulated panels, and would/should this suffice to drop my U-value to suffice. Sorry about the specific questions but I've no experience and only limited knowledge of pumped insulation.


TL;DR

Which is a better use of insulation/money to decrease external wall U-values marginally, Pumped wall -v- Insulation slab panels?


Disclaimer:
I realise and accept fully that how the construction method is implemented is more important than that actual method used, and that workmanship of either method is crucial to its success of failure.

Jimbo

Some quick u value calcs:

Existing:
Wall Insulation

Fabric Thickness (m) ÷ Conductivity (λ) = Thermal Resistance
External resistance - - 0.0400
External Render 0.019 0.57 0.0333
Block 0.1 0.57 0.1754
Cavity - - 0.1800
Insulation 0.1 0.023 4.3478
Block 0.215 0.57 0.3772
Internal Plaster 0.0125 0.43 0.0291
Internal insulation 0 0.021 0.0000
Internal resistance - - 0.1300
∑ Thermal Reistances = 5.3129

U-value of wall = 0.188
W/m2/K


50mm cavity fill (Silver bead):
Wall Insulation

Fabric Thickness (m) ÷ Conductivity (λ) = Thermal Resistance
External resistance - - 0.0400
External Render 0.019 0.57 0.0333
Block 0.1 0.57 0.1754
Pumped Insulation 0.05 0.033 1.5152
Insulation 0.1 0.023 4.3478
Block 0.215 0.57 0.3772
Internal Plaster 0.0125 0.43 0.0291
Internal insulation 0 0.021 0.0000
Internal resistance - - 0.1300
∑ Thermal Reistances = 6.6480

U-value of wall = 0.150
W/m2/K



37.5mm internal drylining (PIR):
Wall Insulation

Fabric Thickness (m) ÷ Conductivity (λ) = Thermal Resistance
External resistance - - 0.0400
External Render 0.019 0.57 0.0333
Block 0.1 0.57 0.1754
Cavity - - 0.1800
Insulation 0.1 0.023 4.3478
Block 0.215 0.57 0.3772
Internal Plaster 0.0125 0.43 0.0291
Internal insulation 0.0375 0.024 1.5625
Internal resistance - - 0.1300
∑ Thermal Reistances = 6.8754

U-value of wall = 0.145
W/m2/K



The difference is negligable.

I'd say go with the pumped- less labour intensive (& probably cheaper?) and, as you say, you'll retain the thermall mass which helps with underfloor heating.

Quazzie

Thanks a million Jimbo. I wish I knew (had the software) to do that myself.

sydthebeat

Jimbo...

your block thermal conductivities are way off.

use 1.33 for most dense concrete blocks.
youre not also making allowances for wall ties.

have a look at this:

http://www.bre.co.uk/filelibrary/pdf/rpts/BR_443_%282006_Edition%29.pdf

Quazzie

sydthebeat wrote: »

Jimbo...

your block thermal conductivities are way off.

use 1.33 for most dense concrete blocks.
youre not also making allowances for wall ties.

have a look at this:

http://www.bre.co.uk/filelibrary/pdf/rpts/BR_443_%282006_Edition%29.pdf

Am i correct in assuming that makes his figures lower than they should be?

And assuming that it does, well then does his point about the scale of difference between internal lining and pumped insulation still stand true? or are his calculations about that also wrong?

sydthebeat

Quazzie wrote: »

Am i correct in assuming that makes his figures lower than they should be?

And assuming that it does, well then does his point about the scale of difference between internal lining and pumped insulation still stand true? or are his calculations about that also wrong?

the final u values are lower, but i suppose your correct to say the difference would be the same, theoretically.

ironically, i used a figure of 1.15 for your original calculations.

Quazzie

Both methods bring me below my target of 0.17 (0.162 compared to 0.161 based on a very basic understanding of it) so the main question is can pumped bead, or blown rockwool be trusted to be as competent as internal lining. God knows its a good bit cheaper but is it selling out by cutting back for the sake of a grand or two.

Jimbo

sydthebeat wrote: »

Jimbo...

your block thermal conductivities are way off.

use 1.33 for most dense concrete blocks.
youre not also making allowances for wall ties.

have a look at this:

http://www.bre.co.uk/filelibrary/pdf/rpts/BR_443_%282006_Edition%29.pdf

Your right, did those calculations in a bit of a rush.

Quazzie wrote: »

Both methods bring me below my target of 0.17 (0.162 compared to 0.161 based on a very basic understanding of it) so the main question is can pumped bead, or blown rockwool be trusted to be as competent as internal lining. God knows its a good bit cheaper but is it selling out by cutting back for the sake of a grand or two.

I dont see why not. Internal drylining has its own issues such as detailing around first floor joists.
Filling the cavity will also reduce the risk of thermal looping and won't eat into your floor area.