Double Brick Wall Cavity Insulation - Builder Wrong Product?

slothicious · Apr 21, 2023 1:04 am

We're building a new house in WA where double brick wall is dominant, and the builder has today put in different wall cavity product to what was agreed during the pre-start.

We were told the cavity wall insulation will be EPS bead blow-ins during the pre-start, however the insulation company today rocked up and blew-in mineral wool into the cavity.

To make matters worse, the cavity insulation went in before the roof cover (with roof cover not looking like it'll be installed for at least another two weeks with rains forecasted).

Has anyone had bad experience with the mineral wool blow-ins for the wall cavity in a new build? I know it's caused damp & mould issues for some people. Is it possible to vacuum these out so it can be replaced with the correct product (EPS beads)?

Ashington Homes · Apr 21, 2023 9:11 am

slothicious

We're building a new house in WA where double brick wall is dominant, and the builder has today put in different wall cavity product to what was agreed during the pre-start.

We were told the cavity wall insulation will be EPS bead blow-ins during the pre-start, however the insulation company today rocked up and blew-in mineral wool into the cavity.

To make matters worse, the cavity insulation went in before the roof cover (with roof cover not looking like it'll be installed for at least another two weeks with rains forecasted).

Has anyone had bad experience with the mineral wool blow-ins for the wall cavity in a new build? I know it's caused damp & mould issues for some people. Is it possible to vacuum these out so it can be replaced with the correct product (EPS beads)?

Are you talking about filling the cavity between the inner and outer bricks with insulation?

If so, I always though that this cavity had to be kept clear for moisture? I must be wrong.

I am sure there is probably someone that knows more than me, but here in Sydney we always leave that cavity free

Cheers

Simeon

Stewie D · Apr 22, 2023 7:55 am

Apparently it is common now but I was taught ( like you probably were Simeon ) to leave an air gap between the two so that any moisture penetrating the outer skin wouldn't be able to cross the cavity to the inner skin. I'd prefer either the old way of a gap or if improvements to the R-rating for that wall was necessary then something like 40mm foil board against the inner skin would be better.

Stewie

alexp79 · Apr 22, 2023 2:40 pm

Yes, I would also recommend rigid insulation plus consider wider gap between the bricks to accommodate insulation + air. Brick ties will still be acting as thermal bridges. Best option is to use external insulated facade though, this is how they do it in colder countries.

Ashington Homes · Apr 23, 2023 12:22 pm

Stewie D

Apparently it is common now but I was taught ( like you probably were Simeon ) to leave an air gap between the two so that any moisture penetrating the outer skin wouldn't be able to cross the cavity to the inner skin. I'd prefer either the old way of a gap or if improvements to the R-rating for that wall was necessary then something like 40mm foil board against the inner skin would be better.

Stewie

Stewie

Thanks for bringing me up to speed with what the kids are doing

What I don't understand is how do you deal with the moisture that gets into the cavity if it is full of insulation?

Cheers Simeon

Stewie D · Apr 24, 2023 1:08 pm

I'd say that the insulation would have to be non-absorbent but I would still prefer a cavity with solid sheet insulation against the inner skin.
Below are a couple of drawings I did a while back as we debated this as well for our place. In the end we are going for "improved brick veneer ". We were aiming for a total R value for the ground floor around R4.5. If you play around with the thickness of the EPS sheets and also the cavity you can achieve this but at the expense of having a thicker wall.
You can see in the third drawing that without any cavity insulation how poor a double brick wall rates.
Also, making the 30mm air gap or cavity sealed or enclosed takes the R-value from R 0.12 to R 0.68. A relatively easy and inexpensive but effective exercise.

Here is one of the original drawings I did but given the total thickness of the wall and the fact that we live in a temperate climate here only 1.4 km from the coast it was deemed to be a bit of overkill.

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Next we have what we are aiming to have.

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Finally, how I would improve your bog standard double brick wall.

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As you can see, it still lags way behind an improved brick veneer wall at half the R rating while being about another 30mm thicker.

Stewie

alexp79 · Apr 25, 2023 1:05 pm

It is usually a very bad idea to put EPS in front of batts insulation. EPS has low vapour permeability, while mineral wool and gyprock have high vapour permeability. This means that excessive moisture will be trapped in between and will impact not only the insulation performance levels, but potentially will be promoting mould behind your gyprock and can even eventually impact the structural stability of the framing (in the long run).

Also, depending on your location, mass enhanced R value for your double brick design, shown above, will be in R3.3-4.1 range, which is more than enough for the absolute majority of the Australian climate zones.

Stewie D · Apr 29, 2023 9:56 am

So prior to designing our place I invested quite a bit of time in research on all components of our build. I looked at the temperature range for our climate ( temperate and 1.4 km from the coast ) the expected comfortable temperature inside the house - summer and winter, the type of insulation, building methods as per my drawings above, dew point etc and have concluded that we should be fine. We never have frosts and it rarely dips below 8ºC in winter. All rooms have opening windows over the minimum recommended for ventilation. If we lived in a colder climate then yes I may alter how we insulated the house but I'm pretty happy that for most of our walls we will achieve an R-rating of around 4.5.

Also I don't know where you get that R rating range for that wall above? A standard double brick cavity wall only has R 0.69. Adding 40mm or 50mm EPS or similar adds another 1.8 only for a total of 2.18. Making it a still air gap adds another 0.68 of course.

Stewie

alexp79 · Apr 29, 2023 1:12 pm

Hi Stewie,

I am talking about moisture trapped from shower/bathroom and from cooking mainly.
This has to be very well mechanically ventilated every time, plus the effect of accumulation is rather long term.

As for R values, I am talking about mass enhanced R values - double brick is a high density wall and has thermal mass effect, this is why you can't use standard way of calculating thermal efficiency.

For the detailed explaination, pls take look at this article here, it is written mainly for the precast concrete, but the idea is exactly the same:

https://www.nationalprecast.com.au/wp-c ... ator11.pdf

Stewie D · Apr 30, 2023 7:26 pm

Well, all bathrooms/ensuites and the kitchen will all obviously have some sort of mechanical ventilation. You'd have to be an idiot not to. In all my years of building and doing lots of renos, additions and new houses I couldn't name any that we did where those rooms weren't ventilated.All the other rooms will have opening windows and doors as well.
I'm not really worried about the thermal mass effect either. Our design and those walls as above will be more than enough to cope with temperature fluctuations given our particular temperate climate and minimal energy needs. The thermal assessor for our NatHERS certificate rated our house at 6 stars so that will do me.

Stewie

alexp79 · Apr 30, 2023 11:56 pm

I am saying that double brick has similar thermal performance (or may be just slightly worse) due to thermal mass effect.

It will be still very interesting to see the state of your framing after 10-15 years, considering the above mentioned construction.

May be it will be fine, but I am still worried that even small doses of trapped vapour might eventually cause issues in the longer term.

Rotten frames is something they dealt a lot with at the UK where they have used to put in a OSB board (also very low vapour permeability) in front of mineral wool insulation and even though they generally use vapour barrier on the internal side of the wall it was oftentimes damaged/punctured.

BTW I hope that your wall sarking is vapour permeable though.

As for NatHEARS, 6 stars is everyone getting these days, now 7 stars will be mandatory from September. But frankly, I wouldn't generally trust NatHERS calculations as they usually take 'perfect' conditions into consideration, e.g. not considering moisture inside the walls, thermal bridging etc.