Which robust blocks to use to achieve U-value 0.18 as part of CWI on extension?

[ab12]

Hi All

 

Having a rear single storey extension built and need to achieve U value of 0.18 for CWI.

Build up will be brick, followed by 150mm DriTherm 32 slabs, then blocks and then ideally wet plaster on blocks internal face although architect has specifies dot an dab to the blocks.

 

Have been eyeing the following blocks:

Plasmor Fibolite 7.3N (U value 0.28) and Plasmore Fibolite 3.4N (U value 0.24)- both of these are lighweight aggregate blocks.

 

Alternative options include Thermalite Hi strength 7.3N (u value 0.19) which is Hi Strength aircrete and Thermalite Shield 3.6N (U value 0.15) which is Standard aircrete

 

Looking for a block which is sturdy and robust not going to crumble and fail when drilled into as the rear storey extension will house a kitchen with lots of wall fixings.

 

From the above options which blocks would you recommend which would satisfy both achieved combined u value of 0.18 and is robust.

 

Thank you

[Gus Potter]

To my mind you are hoping you get a good brickie that can execute all this properly..and for that you will need to pay for a professional brickie @Canski may be able to give you some realistic rates. To be frank you'll need to stand over them all day long as fitting insulation properly into a masonry cavity is like buying a second hand car! Give it a miss unless you have the time! Fitting the any kitchen units for example is the least of your worries.

 

A safer bet is to go for a TF which you can inspect, the air tightness and so on.

 

Mind you if you have the capability to build this yourself then go for it. The big secret is to take care of your aerated blocks and condition them on site.

 

 

[nod]

A dense concrete block will be fine 

Most Architects specify dot and dab It does have a warmer fell if sealed properly 

Using an insulated plasterboard for dot and dab on the perimeter walls will up rate your U values and give you a very warm build 

Timber Frame is a very good option 

But expect to add about a third to your build cost 

[torre]

We're using the Fibolite 7n. When handling they're much more robust, feel more like a dense block than the thermalite 7, but still a decent weight and u value. Temporary timbers have held with frame fixers into the blocks very well.

[Canski]

I’ve laid a few dozen thousand Fibolite blocks as these are a preferred block for one of my developer customers. They are a good block and if you can get the required u values using them then go for it. 
 

Ytong blocks were my preferred option for my build though. A good solid aerated block that can be cut easily and nice and square using an old blade in a battery powered circular saw.

 

Dritherm 32 is my preferred insualtion as well. Easy to cut neatly and doesn’t over expand when you get it out of the pack like some other makes.  I used it on my build with my methods posted on a previous full fill cavity hater thread. Just make sure to keep it dry. 

Edited January 20 by Canski
Added comments regarding dritherm 32

[ab12]

Hi Canski

Following your comments: ''my methods posted on a previous full fill cavity hater thread. Just make sure to keep it dry.'' I have been searching for the thread but cant seem to find it.

 

Can anyone post a link please.

 

Follow on question you mention about keeping the insulation dry, can you please expand on this- is that whilst it is being installed and overnight when it is in the exposed wall cavity ?

Thanks

[Andehh]

Our expanded even more when wet! When in the cavity didn't seem an issue, but if outside it made it impossible to fit between the blocks.

 

 

Caught the brickies tearing it in half as they got to the top, to save climbing down for more, so ended up daily inspecting to ensure they didn't! As I didn't trust the sods.

[SteamyTea]

Can you find the k-value of the materials the blocks are made from?

Failing that, what thickness are the blocks, then it is possible to calculate the thermal resistance of the wall build up.

 

  

On 19/01/2025 at 23:57, ab12 said:

Looking for a block which is sturdy and robust not going to crumble and fail when drilled into as the rear storey extension will house a kitchen with lots of wall fixings.

I am not 100% sure, maybe @Gus Potter can clarify, but I think the N number is the compression strength (number of newtons) and different block types may behave differently when drilling and screwing into them.

Edited January 21 by SteamyTea

[Gus Potter]

On 21/01/2025 at 17:38, SteamyTea said:

I am not 100% sure, maybe @Gus Potter can clarify

I'll try as best I can. The capital letter "N" denontes Newtons (Isaac Newton the man). In this case it is the compressive strength of the block in Newtons per square mm. Now for all.. the compressive strength of the block is not the compressive of flexural ( often used for wind loading that bends the wall sideways or floor joist loads that are applied outwith the centre of gravity of the masonry leaf) strength of the wall.. this block strength gets reduced a lot depending on a number of factors; the mortar strength, the dimensions of the block, what the wall loads are.. both vertical and sideways (lateral load) and so on.

 

Now when we look at aerated concrete blocks. Simplistically they are like aero chocolate bars. The more air voids the better the thermal performance. You can see this if you look at the data sheets for the blocks. The better insulating blocks tend to be lighter in weight. This means that there is less "concrete" to carry the loads... in other words the more insulating a block is the less density it often has and thus a corresponding reduction in strength.

 

Now if you are fixing into a block you want to have more dense material so the Rawl plug etc can expand against solid material. For most fixings to work they need solid material.. not air bubbles.

 

There is no free lunch here... you seek thermal performance but when you come to fixing design.. life becomes harder and exponentially more complex.