Extension - thermalite, trusses... or not?

[John Smith Esq.]

Hi all!

I'm getting a single storey side extension added to my bungalow. 4m x 6.5m. It will come off an existing hip roofed out rigger. Builder has said:

- he doesn't like thermalite blocks. U values are good but quality poor. They crack and the walls don't seem very strong with the poor adhesion of mortar. My plans show these being used with 100mm cavity filled with 90mm insulation. He prefers fiberlite but with lower U values, means walls have to be 2" thicker so they use cavity bats that fill the cavity. Means I would lose some space internally and pay more but have a better quality build. Thoughts?

- he has suggested a traditional roof system rather than trusses. Makes installing insulation easier and means there is some room for storage, plus thicker timbers. Says it won't make much difference to price. Thoughts on this? I don't really need storage space and don't think there would be much as it is a lower roof... then again, would male the whole roof more accessible in future.

Thank you!

 

[tony1851]

1. On the wall issue your builder is correct; 'aac' blocks crack verrtically, and also don't have good adhesion to mortar. Losing 2" is a good tade off for a better job.

2. For the roof, again go for a traditional 'cut' roof. Probably cheaper than ordering trusses as well - depending on your existing roof layout.

 

[noseall]

Hi all!

I'm getting a single storey side extension added to my bungalow. 4m x 6.5m. It will come off an existing hip roofed out rigger. Builder has said:

- he doesn't like thermalite blocks. U values are good but quality poor. They crack and the walls don't seem very strong with the poor adhesion of mortar. My plans show these being used with 100mm cavity filled with 90mm insulation. He prefers fiberlite but with lower U values, means walls have to be 2" thicker so they use cavity bats that fill the cavity. Means I would lose some space internally and pay more but have a better quality build. Thoughts?

- he has suggested a traditional roof system rather than trusses. Makes installing insulation easier and means there is some room for storage, plus thicker timbers. Says it won't make much difference to price. Thoughts on this? I don't really need storage space and don't think there would be much as it is a lower roof... then again, would male the whole roof more accessible in future.

Thank you!

Aerated (Celcon type) blocks are awful. I will avoid using them wherever possible.

We removed a thermo' type block wall recently. I asked the lads to salvage some of the blocks. Out of about 70 blocks they saved the grand total of ZERO. Every single block had a fracture in it.

 

[23vc]

I built my extension with celcons - the long “turbo” ones. Got used to working with them, tended to have a splash brush nearby and damp them down where they’d be mortared. Chose to use them as I did the whole lot on my Todd and was just easier to lift, up scaffold, gable etc, and cut.
2 storey, not a visible crack anywhere 7 years on, longest wall is about 7.5m.

Not disputing what Tony and nose are saying, they aren’t great, but just saying. I suspect they may have got slightly better over the years … maybe

 

[noseall]

I built my extension with celcons - the long “turbo” ones.

Durox Supabloc

I built my extension with celcons - the long “turbo” ones. Got used to working with them, tended to have a splash brush nearby and damp them down where they’d be mortared. Chose to use them as I did the whole lot on my Todd and was just easier to lift, up scaffold, gable etc, and cut.
2 storey, not a visible crack anywhere 7 years on, longest wall is about 7.5m.

Not disputing what Tony and nose are saying, they aren’t great, but just saying. I suspect they may have got slightly better over the years … maybe

I built mine out of (regular sized) Celcons, and my exposed garage blocks are all ok. It's usually those that have got soaked or some such, like buildings that have suffered a lot of rain during construction.

 

[stuart45]

The key with AAC blocks is not to use green or wet ones, and don't use a strong mortar.

 

[23vc]

The key with AAC blocks is not to use green or wet ones, and don't use a strong mortar.

I’d have used about 5 or 6 :1. I probably asked you at the time

 

[stuart45]

I’d have used about 5 or 6 :1. I probably asked you at the time

That must have been years ago in your apprenticeship days.

 

[John Smith Esq.]

Thanks chaps - really helpful and interesting feedback.

What are your thoughts on cavity (90mm insulation and 10mm gap) versus solid wall with cavity bats - putting the blocks issue to one side?

With these thermalite blocks, what I'm getting is that they are a bit crap but will do the job fine if stored and installed correctly? Structurally sound and sufficiently strong for my single storey extension? Will hold the roof up and last fine for the next century?

What would price difference be % wise between the two approaches?

 

[23vc]

The only difference that’s ever going to impact you with using them, is they’re different/a bit rubbish to fix things to. But even then, you can still do it fine by taking it steady with the drill (never hammer action), right hole size etc. I’ve got big radiators, wall units, wall hung basin etc all good

 

[foxhole]

I had solid blocks and insulated internally , seemed quicker than cavity method .

 

[phatboy]

I had solid blocks and insulated internally , seemed quicker than cavity method .

Solid blocks and insulate externally.... may as well make the blocks part of your 'heat store'.....

 

[robinbanks]

Solid blocks and insulate externally.... may as well make the blocks part of your 'heat store'.....

But you'd be advised to devise a robust detail to insulate them from the ground in that case

 

[noseall]

I had solid blocks and insulated internally , seemed quicker than cavity method .

Never. Lose the insulation as soon as you can. It's a ball ache and a nuisance especially when it comes to fitting anything to the walls later on. I have never seen a design or an architect specify or prefer surface insulation over cavity. The reasons for which are tried and tested.

 

[noseall]

Solid blocks and insulate externally.... may as well make the blocks part of your 'heat store'.....

Thermal mass gains are fine if the building is occupied 24/7 or if temperature consistency is your thing. Busy family homes with plenty of windows and doors - pointless gains.
There is a reason architects, house builders etc chose the cavity over any other methods.