External Insualtion - How to calculate U-value for 1930's house

[lstevensuk]

As part of my new extension I'm having the existing 1930's pebbled dashed house insulated and rendered with 90mm EPS and render. Building control have just asked me this question on it:

 

Please provide confirmation that the existing wall being insulated with the EWI will achieve the required U-value of 0.30W/m2K and will not create an interstitial condensation risk.

 

How do I work out the u-value? I tried the tarmac calculator and rough guess came out about 0.28. Also what's interstitial condensation and how do I know if I'm at risk?

[ETC]

Simple - get the EWI to do the u-value calculation and condensation risk analysis.

[Radian]

39 minutes ago, lstevensuk said:

As part of my new extension I'm having the existing 1930's pebbled dashed house insulated and rendered with 90mm EPS and render. Building control have just asked me this question on it:

 

Please provide confirmation that the existing wall being insulated with the EWI will achieve the required U-value of 0.30W/m2K and will not create an interstitial condensation risk.

 

How do I work out the u-value? I tried the tarmac calculator and rough guess came out about 0.28. Also what's interstitial condensation and how do I know if I'm at risk?

 

You could try using this online calculator as a second opinion. It also shows the condensation risk (interstitial means between layers).

[Iceverge]

EWI pretty much guarantees no interstitial condensation however at 90mm of EPS you'll probably be about 0.35. 

 

The cost to increase external insulation thickness in minimal really. Almost all of the cost is the render, fixings etc. Can you do better than 90mm? 

[lstevensuk]

23 minutes ago, Iceverge said:

EWI pretty much guarantees no interstitial condensation however at 90mm of EPS you'll probably be about 0.35. 

 

The cost to increase external insulation thickness in minimal really. Almost all of the cost is the render, fixings etc. Can you do better than 90mm? 

Ideally it needs to match adjoining property which has 90mm.

 

What happens if you can’t meet the 0.30 requirement? 

[RobLe]

I think 90mm is just enough if you get the best platinum EPS.  I tried calculating, using the best EPS I could find:

EPS lambda =0.031W/mK, so U = .031/.09=0.3444 W/m/K 

Google says a standard solid wall has U = 2.0 W/m/K, this gives a tiny improvement, giving an overall U of:

1/(1/2+1/0.3444) = 0.294 W/m/K

 

https://www.insulationshop.co/90mm_grey_polystyrene_ewi_graphite.html

 

I'm with Iceverge though:  Max out that insulation, it will hardly cost any extra - it's the labour that costs.  We went for 150mm, a friend for 200mm.