Extension - thermalite, trusses... or not?

[robinbanks]

Eh? You're each talking about the same thing

 

[noseall]

Eh? You're each talking about the same thing

The thread morphed into the positioning of insulation somehow. Arguments over thermal mass gains, v's (internal) surface insulation will always rumble on, as the noticeable differences are minimal.
The point I was making is that the insulation should be buried at the earliest opportunity. Face fixing it to walls has too many disadvantages to be useful over cavity insulation. The only time we position floor insulation above the slab for example, is when the customer wants UFH fitting.

 

[robinbanks]

Oh, I thought it was a cavity vs EWI debate, which I notionally treat as the same thing if one ignores the minor u value contribution of the external hard covering

Understand the IWI vs cavity argument, though I've IWI'd many times as a compromise in a retrofit and usually included a sheet of ply or OSB in the buildup if something heavy requires fastening, such as kitchen cabinets

 

[phatboy]

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

Absolutely, Kingspan GreenGuard is good for this. But isn't it quite common for cavity construction to fill the cavity below a certain level, thus creating a cold bridge?

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.

We have a busy family home, and with complete external insulation and modern well fitted windows, the house is a consistent 21 degrees all day (Dipping at night when the heating is off), and costs not very much.

Our original bungalow was of 1950s solid construction which we externally insulated, so when we extended we kept to the same method. It meant the footings could be a fair bit narrower, was easier to self build, and in the case of the heating failing the house remains warm for a day or 2.

Lots of Europe build this way now, cavity seems to be a quite British thing, and lets be honest, we aren't renowned for our utilisation of building technology.

 

[noseall]

Absolutely, Kingspan GreenGuard is good for this. But isn't it quite common for cavity construction to fill the cavity below a certain level, thus creating a cold bridge?



We have a busy family home, and with complete external insulation and modern well fitted windows, the house is a consistent 21 degrees all day (Dipping at night when the heating is off), and costs not very much.

Our original bungalow was of 1950s solid construction which we externally insulated, so when we extended we kept to the same method. It meant the footings could be a fair bit narrower, was easier to self build, and in the case of the heating failing the house remains warm for a day or 2.

Lots of Europe build this way now, cavity seems to be a quite British thing, and lets be honest, we aren't renowned for our utilisation of building technology.

You still have the benefit of thermal mass with cavity insulation, whereby there are gains from the inner leaf etc, but without the ball ache of thick insulation on the surface of the walls.

EWI is great if you plan ahead and are prepared for all the idiosyncrasies that go with having all that increased thickness on the face of the building, and the additional window/door reveals, soffit, drain gullies, utilities/services/cables and the fact that you have to deal with having a rendered building, whilst replacing any frames is an issue too. Not for me, ta.

Let's be honest, there is a reason why cavity insulation is always specified over the other choices and why a brick façade will always be first choice over painted or coloured (maintenance issue) render.

 

[robinbanks]

to fill the cavity below a certain level, thus creating a cold bridge?

Marmox blocks FTW

Let's be honest, there is a reason why cavity insulation is always specified over the other choices

"Always" might be a bit extreme.. There are certainly alternative build implementations in existence in the UK

Traditionalism is a big drag on change - not everything is architect designed, not every architect is progressive/innovative. It means a majority of builds follow a typical format (and end up easier to finance too); many things encourage things to be done the way they always have been, but it doesn't mean breakaways don't exist

Personally think that things like ICF and those OSB wrapped Lego blocks of insulation are really neat, solve some traditional problems really well..
..they just have other problems that don't occur with trad and those problems need solving

Like the never ending EV vs ICE thread, people from either camp can always point out deficiencies in the other because there isn't one universal solution to every problem

 

[noseall]

"Always" might be a bit extreme.. There are certainly alternative build implementations in existence in the UK

There would need to be a valid reason why a designer or architect would chose any of the alternatives, simply because those alternatives are neither beneficial to the building or the builder.

 

[robinbanks]

Might just have to agree to disagree on this one..

A passivhaus designer probably ain't gonna pick a pair of concrete block leafs stuffed with wool, half heartedly erected by Messrs Shaftwell & Leggett

 

[noseall]

stuffed with wool

You are behind the times.

 

[John Smith Esq.]

There seems to be a difference of opinion now.

One builder saying the options are:

- thermalite, 90mm cavity insulation, 10mm gap
- fibrelite, 150mm cavity filled with bats

Another builder saying:

- Thermalite and fibrelite similar u values so go with fibrelite, 90mm cavity insulation and 10mm gap

??

Are there other options? I just want building control sign off and use as little space up as possible and achieve sound construction and lowest price possible. Prefer to have a gap personally, don't like the sound of solid wall. It rains a lot here in Cheshire. How do you builders do it?

 

[noseall]

There seems to be a difference of opinion now.

One builder saying the options are:

- thermalite, 90mm cavity insulation, 10mm gap
- fibrelite, 150mm cavity filled with bats

Another builder saying:

- Thermalite and fibrelite similar u values so go with fibrelite, 90mm cavity insulation and 10mm gap

??

Are there other options? I just want building control sign off and use as little space up as possible and achieve sound construction and lowest price possible. Prefer to have a gap personally, don't like the sound of solid wall. It rains a lot here in Cheshire. How do you builders do it?

You can go Fibolites with 100mm cav and 90mm PIR interlocking (rebated) insulation such as this for example.

It's a heck of a lot more expensive to buy than say Dritherm fibreglass and lot more effort to fit.

 

[John Smith Esq.]

Thanks Nose. So what do you do when constructing single storey extensions?

 

[robinbanks]

What's your target U value?

 

[John Smith Esq.]

What's your target U value?

I don't have one. Just want building control sign off.

 

[noseall]

Thanks Nose. So what do you do when constructing single storey extensions?

We go with the expensive rebated PIR stuff, but it's a faff. It involves a lot of cutting, DPC's at the corners and abutments etc. The alternative is 150mm cav's and Dritherm, but astonishingly, I've yet to build a 150mm wide cav'. All my builds since the June 2022 rules change, have been done with the rebated gear and 100mm cavs.

It can put an extra couple of grand on an average extension, with the extra labour and added expense.