Meeting building regs requirements for cavity insulation

[ik1]

Hi,

As I understand from the building regs my external walls should be atleast 0.030 W/m2K.

My builder is building the extension walls with outer brick leaf, 100mm cavity, inner blocks are concrete the heavy type . and we will use bonding and plaster inside not plasterboard.

Now I have used the cba-concrete blocks association website (http://www.cba-blocks.org.uk/tech/tech_uvalue.html) to get an idea what type of insulation I need and as long as I use 100mm insulation with 0.034 W/m2k value I achieve 0.30 for the wall.

My question therefore is that is this correct? The concrete block thermal values I dont have so I have used the default ones in the calculator but I would like a second opinion from one of you knowledgeable people whether I would achieve 0.30 with concrete blocks and the insulation I intend to put in.

Many thanks for your help

 

[Fluffster]

What cavity insulation are you using as rockwool and celotex have diff values. If you get stuck for cavity space a thermal laminate plaster board would bulk up your u values.
Pete

 

[freddiemercurystwin]

What manufacture and model of block are you using? This would be crucial to be able to answer your question 100% accurately.

Near as dammit though you are looking at 100mm full fill cavity insulation Crown DriTherm Cavity Slab 32 or about 45mm Celtex/Kingspan and a 50mm air gap.

 

[tony1851]

Hi,

As I understand from the building regs my external walls should be atleast 0.030 W/m2K.

Your U-value should be no worse than 0.28, not 0.3
You are looking at 100mm Dritherm 34 or 32 to achieve that rating.
Don't waste time with partial-fill boards, they're difficult to fix and can lead to damp penetration problems if not installed correctly.

 

[ik1]

Thank you all for the replies.

I don't know the make of the blocks, I will find out. I have asked the builder to use the solid concrete blocks rather than ashlite or thermalite due to hearing stories of heavy objects falling off such as kitchen units.

The current insulation the builder is suggesting is Isover CWS which has a rating of 0.037 W/mK. But this is ok to use with Thermalite I believe due to its better heat retention however with solid concrete on the online calculators it appears I would need atleast 0.032 or 0.034 if I can get an accurate u-values for the Blocks.

Although it seems from the builders experience the inspectors hardly check this but thats not the point...I'd rather have a warmer house but the cost of dritherm 32/34 is about 3 times the cost of Isover CWS.

 

[RonnyRaygun]

I have asked the builder to use the solid concrete blocks rather than ashlite or thermalite due to hearing stories of heavy objects falling off such as kitchen units.

This only happens if the wrong fixings are used. There are fixings available for AAC blocks which can carry surprisingly high loads.

 

[^woody^]

This only happens if the wrong fixings are used.

It only happens if the wrong carpenters are used!

 

[theoldun]

For some unknown reason most builders, and come to that most architects know f**k all about K. R and U values.
As stated by Tony current U value for walls is 0.28W/m2K.
Take blocks first. Most dense concrete blocks have a lambda (K value) of 1.13W/mK with the exception of Hanson Evalast at 1.31 and Topcrete Dense at 1.28..It is an easy matter to work out the K and R value of a block, but with respect, we feel that you would not understand it if we tried to explain it to you.
If your blocks have K value of 1.13, including ext and int air pressure,. 102 ext brickwork, 100mm 0.037 Isover insulation and light weight plaster, then you will not achieve 0.28W/m2K Allowing for cold bridge factor you will only achieve 0.34.
What you need is 102 ext skin brick, 100mm Dritherm 32 Ultiate full fill cavity insulation, 100mm block with K value of 1.13W/mK and 12.5 dot and dab plasterboard,
Allowing for cold bridge factor, will give you an R value of 3.51m2K./W and U value of 0.28.49, call it 0.28W/m2K
If it was our house we would dot and dab with 27mm Thermaline Plus and bring U value down to 0.25.W/m2K.
Your choice. You gets what you pays for.
Regards oldun