Building an extension - solid blockwork and EWI?

[phatboy]

Hi all,

Our current bungalow was built on the 1950's, and has 12" thick block walls with no cavity (6" thick blocks laid on their side), which we had 90mm of EWI added to a few years back.

We are now looking to extend, and considering construction types.

Would it be sensible to just continue to build the same way as the original house, and use EWI again? I read this is becoming a popular way to build as it can be cheaper and so quicker?

Or should we be looking at a traditional cavity construction? Or even timber-framed?

The design we like so far adds rooms to the outside of the existing footprint and converts windows to doors (roughly), so a change in wall position on the inside would be of no consequence.

Hopefully that isn't too vague for anyone to cast an opinion on.

We are in Jersey, so the temperature has barely ever reached freezing.

Thanks!
Tim

 

[^woody^]

Yes, match the existing unless there is a specific and necessary reason not to.

 

[phatboy]

Yes, match the existing unless there is a specific and necessary reason not to.

Thanks Woody.

Would you agree that it should be a cheaper quicker method of construction? I've no doubt it would be quicker. I'm not sure if the costs of EWI cancel out any savings though....

Thanks
Tim

 

[^woody^]

Costs can vary. If the EWI would be fitted by specialist and not general builders, then there could be a premium with that, and likewise for the render.

The actual masonry wall cost for cavity or solid wall should be more or less identical.

The other option to perhaps build a conventional cavity wall and not render externally (ie brick) could reduce longer term maintenance costs and may be a bit cheaper to build. But it will need proper costing if cost is an issue.

The other thing to potentially consider is that the benefit of EWI is in a compromise for insulating existing walls. It will mean that the walls get heated up first before the insulation does it's job. Also it could stop the room benefiting from solar gains - ie rooms are cooler in summer (advantage or disadvantage). With cavity walls the central insulation can mean the walls are more responsive to heat from inside and outside and more efficient.

So on the basis that you already have EWI, then it makes sense to build the new extension in that too, because you are already living with the advantages and disadvantages. But if other factors are important, then it may be that a more traditional cavity, or even internal insulation should be considered as this is a fresh slate for that room. So first thing to do (ie for your designer to do) is consder your actual requirements and room usage.

 

[phatboy]

Thanks @^woody^ , been thinking about the whole project whilst we can't go out much.

Another consideration is that 1 wall would be a boundary one, and so maybe a cement render finish towards the neighbour would be preferable, as you say this is less maintenance.

We will have our potential designer back out once freedom is restored, and can go over it all.

 

[dontbelieveawordofit]

insulated cavity and /or internal insulation with external render is most likely the quickest , most cost effective. as woody says ewi is very dear.

 

[Leofric]

If the walls are rendered I would go for cavity wall construction ,but full fill cavity insulation is not recommended for rendered walls so partial fill with an insulating blockwork inner leaf with 12.5mm plasterboard and skim. Depending on the space available you could have a thicker than 100mm inner leaf.
ps there are numerous permutations for external wall construction !