Advice please - will these BLOCKS be OK for local conditions

[zappastix]

Hi

Just some advice on "blocks" for 100mm cavity (outer and inner skins),
and your experienced views.

The specs for full fill 100mm cavity is: Knauf Earthwool DriTherm 32 Ultmate

Thickness - 100mm
Thermal conductivity - (W/mK) 0.032
Thermal resistance - (m2K/W) 3.10

The property is 300 yards (rough guess) from sea front, but high dunes in front.

Forceful gales and heavy wind driven rain is not uncommon.

The extension is going to be built with blocks. The original property is brick & block cavity.

The whole property is to be rendered, to disguise the blockwork, and because the 'Heather'
bricks are showing their age without any character. The render finish: smooth, with bellbead.

On this site I have read that blocks 'other than concrete' are not recommended as they crack.

The manufactures state that the blocks we had in mind for the outer leaf, are suitable for complete
build including foundations and above d.p.c. up to two stories high. Our extension is a single/chalet
bungalow.

These are the blocks we would like to use;

H+H Celcon Block Standard Grade 3.6N 440 x 215 x 100mm
Block weight (kg) 7.3 (100mm)

Have you known them to crack?

If not, will the H+H above be OK?

If so, is it advisable to use the same blocks for the whole cavity (both leaves)?

If not, what would you recommeded for the inner leaf - a regular lightweight 'aerated block' ?
Even though they probably would not have the same Newton 3.6N.

I found a post from 'oldun' who suggested Aglite block - these seem very good, especially
for rendering, and they state 'easy saw' that are heavier than the Celcon blocks at 11.5kg
BUT I CANNOT SEEM TO FIND THEM.

I'm in East Sussex, Google is not showing for Kent or Sussex.

Just one other question . . . . H+H datasheet state that the
Celcon Block Standard Grade 3.6N that have a 'scratch marked' finish IS NOT FOR RENDERING.

That is not to say you cannot render, but the 'scratch marks' are simply down to the way the
manufacturs "cut" the block.

So if I run with Celcon Block Standard Grade 3.6N would you use the 'scratch marked' blocks?

First time we have done this, that why there are so many questions.

Many thanks in advance.

Stephen

 

[tony1851]

The lightweight blocks are prone to shrinkage cracks, particularly on long stretches of wall, and also if the mortar is too strong. Movement joints are generally recommended at 6m intervals.

I would check with Knauf, but I doubt full-fill insulation will be appropriate for your degree of exposure. You may need to consider a partial-fill system, retaining a 50mm cavity.

 

[zappastix]

Thanks Tony1851,

The insulation has been specified, originally a 75mm full fill cavity but this was back in 2008 (don't ask why it's taken so long!), but the foundations were built with a 100mm cavity, thinking the extra would help keep heating costs down.

It will be rendered (looking into lime, as I read it does not shrink).

Can you recommend a strong block that will not shrink?

Thanks.

 

[tony1851]

All concrete blocks shrink, though the higher density ones shrink less than the aac-type.
The 7N blocks might be better; also don't have the mortar too strong.
As regards the lime render, may be best to check with a block manufacturer.
Still not sure about the full-fill. It may have been 'specified', but I would still check, considering your degree of exposure.

 

[simonjay]

Stay away from aircrete blocks on the outer skin - you are asking for trouble. Aircrete inners OK, inner and outer do not need to be the same.
For the exterior you want medium density concrete blocks. They look slightly porous - like a "flapjack biscuit/cake" (they are !) and give a good key to render - they do not shrink much. If you were keeping bare blocks or below ground, use the dense concrete blocks - which are heavy.

 

[simonjay]

Stay away from aircrete blocks on the outer skin - you are asking for trouble. Aircrete inners OK, inner and outer do not need to be the same.
For the exterior you want medium density concrete blocks. They look slightly porous - like a "flapjack biscuit/cake" (they are !) and give a good key to render - they do not shrink much. If you were keeping bare blocks or below ground, use the dense concrete blocks - which are heavy.

Actually maybe more like rice crispy cake (I'm getting hungry).