Which concrete blocks?

[flanagaj]

Our vague regs drawings just state the walls will achieve a U value of 0.18W/m2k and consist of high strength 7 concrete blocks.  I went back to the TA, who wasn't much help and he said "when you know what blocks you you are using, I'll run it back through the SAP calcs".  I can't be bothered with the faff and want to know if 7N blocks can be used across the whole build.  Thermalites are expensive and will they really reduce the U values, or would another layer of insulation in the loft offset their better thermal performance.

 

I need to get materials ordered as footings are scheduled to be pulled in 3 weeks time.

[JohnMo]

No one could answer your questions without more information.

 

Ideally your wall build-up drawings are needed, but you sound like you don't have the drawings down to a detailed enough spec, to even consider buying let alone building yet.

[crispy_wafer]

We used plasmor fibolite blocks , don’t seem to be bad blocks to be fair!

[flanagaj]

51 minutes ago, JohnMo said:

No one could answer your questions without more information.

 

Ideally your wall build-up drawings are needed, but you sound like you don't have the drawings down to a detailed enough spec, to even consider buying let alone building yet.

But isn't that what the technical architect is supposed to do?  Building control have signed off the drawings, but still so many questions.  Now I'm beginning to understand why the original architects wanted 10k and this guy only charged 2k

[Nickfromwales]

26 minutes ago, flanagaj said:

But isn't that what the technical architect is supposed to do?  Building control have signed off the drawings, but still so many questions.  Now I'm beginning to understand why the original architects wanted 10k and this guy only charged 2k

Depends on who's wearing the Principal Designer hat?

 

How could you think there was £8k of meat to shave of this role???

[Temp]

What have you agreed with the Architect? Some contracts are limited to getting Building Control Approval, others include construction drawings. These are sometimes the same drawings but not always. The minimum required by Building Control is less than required for construction drawings. 

 

On our house we had to use denser blocks in a few key areas like pillars supporting steel beams and between two windows where loads were higher than normal.

[torre]

You can get a good idea of the difference by going on ubakus or similar, configuring your wall build up and swapping the block type and seeing how the u value changes. 

 

Switching out thermalite for something like fibolite could take you from say 0.18 to 0.19 and if you switched to a dense block even worse. Might sound a small difference for SAP but houses have a lot of wall area so it will suffer. 

 

If you're talking about dense blocks, you'll save on materials but lose some of the difference paying more for labour.

[Iceverge]

Standard dense blocks 7.5N is the norm in Ireland. 

 

Thermalites really add almost no insulation compared to making the cavity just a few mm wider. 

 

They're lighter to lift but crack easier and don't take wet plaster as well. Swings and roundabouts. 

 

100mm dense blocks

175mm cavity batt 36 

100mm Thermalite (0.18) will get you to 0.18W/m²K. 

 

Blocks and insulation~£65.14/m² 

 

If you used 

 

100mm dense block

175mm cavity batt 36

100mm dense block

 

~£48.12 per m²

 

You'd be at 0.174W/m²K

[flanagaj]

I wrongly assumed that regs drawings and construction drawings are the same thing.  This is because my brother in law used the same chap for his drawings.

Just for my own understanding, how do construction details differ from building reg drawings?

 

when I gave our drawings to an SE, he just did calcs for the steels.  He didn’t make any mention of what concrete blocks …

 

 

 

[Mr Punter]

I like medium density aggregate blocks, like Hemelite.  They are a bit lighter than dense concrete and they cut more cleanly with the bolster.  Easy to plaster and render and can be used underground.  They won't help with u values though.

[torre]

8 hours ago, Iceverge said:

Thermalites really add almost no insulation compared to making the cavity just a few mm wider

Swings and roundabouts as you say, but this isn't quite accurate. A 100mm inner leaf of the best thermalites (0.11 W/m²K) is the equivalent of over 30mm of cavity batt 36 and the best 7N blocks (0.18) still close to 20mm equivalent. Multiply that up if you're using 140mm blocks or blocks on the outer too.

 

Also, you might not guess it but even +25mm on the cavity equates to a 2% reduction of floor area in the average size home (around 49m2 per floor versus 50m2).

[Nickfromwales]

35 minutes ago, torre said:

Swings and roundabouts as you say, but this isn't quite accurate. A 100mm inner leaf of the best thermalites (0.11 W/m²K) is the equivalent of over 30mm of cavity batt 36 and the best 7N blocks (0.18) still close to 20mm equivalent. Multiply that up if you're using 140mm blocks or blocks on the outer too.

 

Also, you might not guess it but even +25mm on the cavity equates to a 2% reduction of floor area in the average size home (around 49m2 per floor versus 50m2).

Indeed. I’d rather take the hit on value than lose GIA. 
 

When there’s a distinct lack of wiggle room for going to thicker (or much thicker) superstructures, then airtight + MVHR is the way forward every single time.

 

Airtightness trumps adding slightly thicker insulation, and should be the #1 consideration afaic.

 

Getting the best amount of heat recovery possible is second.

 

When these are in hand, the amount of heat required to keep a happy (comfortable) ambient temp is quite minimal. Then using UFH and a heat pump is a no brainer, in most situations. 

[-rick-]

3 hours ago, flanagaj said:

I wrongly assumed that regs drawings and construction drawings are the same thing. 

 

Think about going back to the AT and paying for construction drawings. You are only at the very start and have hit multiple roadblocks/issues with drawings already. You'll probably save any upfront cost with less delays/mistakes later if you have proper drawings to go off.

[LSB]

We use a combination of heavy and thermalite blocks.

Anywhere that we are likely to need to attach internal fittings to the wall (kitchen cabinets) we use heavy, but where it will be just a blank wall then we use lite.

 

This keeps our SAP lower without the risk of blocks not being strong enough.

[kandgmitchell]

4 hours ago, flanagaj said:

Just for my own understanding, how do construction details differ from building reg drawings?

I'd say that depends on the building. For a simple rear extension say,he BC drawings would expect to show the foundation arrangement, floor and wall construction and the roof structure and covering with a lot of that covered by specification notes and probably one or two sections. Most builders would know what to build to meet the regulations but would expect to be told beam sizes and any unusual requirements. Otherwise they probably wouldn't bother with the drawings once the extension layout had been agreed so construction drawings would be rare.

 

For a more complex building BC may ask for more details but again quite a lot would be covered by a written specification that showed how the building would comply. However, if the client wanted say fancy brickwork details then construction drawings would be needed to show to whoever was building it, how those details needed to be set out. In a similar way construction drawings may be needed to illustrate a complex structural layout which is beyond what the average builder would be expected to know.

 

So, in short BC drawings would cover the basics; showing how the proposal would comply with the Building Regulations. Construction drawings would go that bit further into the detail of how the building is to be put together to get what the client wants.

 

In either case I would expect the blockwork to be specified in order to show a) it's use in external walls met the thermal regulations and b) it's use was appropriate for the loadings expected from the structure.

[flanagaj]

Can anyone provide guidance on achieving a U value of 0.18 W/mK with a 7.3N block outer leaf and 150mm cavity and 32 Dritherm cavity batts.  Do I have no choice but to use expensive aircrete blocks?

 

I am now wondering whether a 90mm T&G PIR cavity would enable me to use cheaper blocks and therefore come in at a cheaper £/M2 for the masonry walls.

 

Any builders on here able to provide some options for achieving the required 0.18 U value and how costs compare for different options?

[crispy_wafer]

out of idle curiosity more than anything, why 7.3 on the outer leaf?

 

anyhow

 

here's the u calculator from celotex, I haven't double checked it.

 

 

U-value Calculator | Online

Edited 13 hours ago by crispy_wafer
added link

[Iceverge]

On 20/02/2026 at 00:49, Iceverge said:

Standard dense blocks 7.5N is the norm in Ireland. 

 

Thermalites really add almost no insulation compared to making the cavity just a few mm wider. 

 

They're lighter to lift but crack easier and don't take wet plaster as well. Swings and roundabouts. 

 

100mm dense blocks

175mm cavity batt 36 

100mm Thermalite (0.18) will get you to 0.18W/m²K. 

 

Blocks and insulation~£65.14/m² 

 

If you used 

 

100mm dense block

175mm cavity batt 36

100mm dense block

 

~£48.12 per m²

 

You'd be at 0.174W/m²K

 

 

There's a mistake here. I think I should have said 200mm cavity in the second example. 

[Russell griffiths]

If you use thermalites for your build you need to go and bang your forehead against a hard object until you pass out. 
when you come round go and get some medium density concrete blocks, you want a good quality structural block, the block should have zero relevance to the u value, you get that from the insulation and the quality of the installation of the insulation. 

[flanagaj]

11 hours ago, crispy_wafer said:

out of idle curiosity more than anything, why 7.3 on the outer leaf?

 

anyhow

 

here's the u calculator from celotex, I haven't double checked it.

 

 

U-value Calculator | Online

Is there any reason not to use 7.3N blocks for the outer leaf?  We are having vertical wood cladding on the upper floor and brick below.

 

I have plugged some options into the calculator.  If I use a medium dense or dense block for the outer leaf, I have too use a celcon or thermalite for the inner leaf.  Is that typically the norm?

Edited 2 hours ago by flanagaj

[Oz07]

Are the regs super strict now with walls having to hit .18? I seem to remember this being the case and swapping some cavities from 100 to 150 in 2015. Saying that new build sites round here still plugging away with 100mm cavity albeit with thermalite blocks. Now I'm thinking maybe you can exceed .18 if you offset elsewhere. 

[Oz07]

38 minutes ago, flanagaj said:

Is there any reason not to use 7.3N blocks for the outer leaf?  We are having vertical wood cladding on the upper floor and brick below.

 

I have plugged some options into the calculator.  If I use a medium dense or dense block for the outer leaf, I have too use a celcon or thermalite for the inner leaf.  Is that typically the norm?

Try and get away from aircrete. I think you can get .18 with medium dense and 150mm cavity from memory. Maybe dot and dab improves the score slightly 

[Iceverge]

11 minutes ago, Oz07 said:

Maybe dot and dab improves the score slightly 

 

But is very poor for airtightness. 

 

PIR boards are routinely installed terribly with lots of thermal bypass. There's plenty of threads on here to look at.

 

For the same U value ........

 

a Wet plaster + double dense block + mineral wool batts will far outperform a Thermalite, PIR, Dot and Dab Wonderwall (don't sue me Oasis)

 

 

As to the "lost" floor area just push the walls out by the same mm. 

 

 

 

 

 

 

 

 

[flanagaj]

29 minutes ago, Iceverge said:

For the same U value ........

 

a Wet plaster + double dense block + mineral wool batts will far outperform a Thermalite, PIR, Dot and Dab Wonderwall (don't sue me Oasis) 

I have put the numbers into the knauf calc and I end up at 0.19.  The other issue is that I am using timber clad and not render.

[Mr Punter]

3 minutes ago, flanagaj said:

The other issue is that I am using timber clad and not render.

 

Unless this is a strict planning condition you may find render is more acceptable to future purchasers and lenders.  Probably cheaper and quicker as well.