Acoustic insulation too heavy for steel stilts??

Hi all,

I recently had to cancel a purchase on a perfect piece of land because it had 2 restrictions:
1. Overland flow meant we had to have all habitable areas on the first floor. We found a design that suited.
2. The lot is right on the edge of a CAT2 noise corridor, and hence requires additional noise insulation.

The builder told me that the insulation would be too heavy for the structure to support. Surely it can't be that difficult or expensive to strengthen the frame? I get a feeling that it just didn't fit into their preferred style or area of expertise. Does anyone know of volume builder who do double story designs with habitable areas on the top floor?? I'm thinking a second opinion couldn't hurt.

Never heard of that excuse before?
Obviously work hasn't slowed down enough to do alternate jobs.
Most project builders have a simple sausage factory mentality
that results from the trades they hire
You may want to consider doing your own design and acoustic engineering and getting 3 Builder to quote

Thanks for the reply, kind of confirms my uneducated suspicions. I'm going to talk to 2 or so more volume builders to see if they can work within the constraints, if that doesn't work I'll go for a custom builder. Any recommendations on either are very welcome!

Hi EdB84,

Are you in Queensland? If so, the below link is the requirements for Habitable Rooms only (defined as per NCC 2019 Vol 2).

https://www.hpw.qld.gov.au/SiteCollectionDocuments/mp-4-4-buildings-in-transport-noise-corridors.pdf

Minimum requirements for each element are described in Schedule 1. Complying construction methods are described in Schedule 2. Schedule 2 is only the minimum requirements and only suggests a few compliant construction methods.

For example, looking at Schedule 1 External Walls in Category 2 calls for a minimum rW value of 41. Referring to Schedule 2, complying construction methods are as follows:

1. Two leaves of clay brick masonry at least 110mm thick with cavity not less than 50mm between leaves OR
2. Single leaf of clay brick masonry at last 110mm thick with: (i) a row of at least 70mm x 35mm timber studs or 64mm steel studs at 600mm centres, spaced at least 20mm from the masonry wall; and (ii) mineral insulation or glass wool insulation at least 50mm thick with a density of at least 11 kg/m3 positioned between studs; and (iii) One layer of plasterboard at least 10mm thick fixed to outside face of studs OR
3. Single leaf of brick masonry at least 110mm thick with at least 13mm thick render on each face OR
4. Concrete brickwork at least 110mm thick OR
5. In-situ concrete at least 100mm thick OR
6. Precast concrete at least 100mm thick and without joints.

These methods obviously do not suit lightweight construction!

There are however a few suppliers such as CSR & James Hardie that can provide lightweight systems that provide a minimum value of rW41. Below is a link to CSR's Redbook. Section G outlines several solutions for exterior cladding & insulation.

https://www.gyprock.com.au/resources/redbook

Like StructuralBIMGuy suggests your proposed build doesn't sit within your builders "sausage factory". I would also recommend engaging a building designer to design you a set of compliant plans. This would allow you to get several accurate quotes from builders.

If you have any further questions I'd be happy to help!

Cheers,

Laurence - Building Designer

Super helpful reply thanks. So basically if going single story, the one end of the house would need to be 80cm above the ground level, at the other end it needs to be about 1.3m above ground level for the entire structure to be 500mm above the 1% flood levels.

If you could give me your opinion on my biggest questions??
1. Would it be cheaper to fill/pour a slab up to this height and then build single story, or go with poles?
2. Or would it be better to go double story and only have non-habitable rooms (garage, laundry, rumpus) on the bottom floor?
3. Would it even be feasible?
4. How much would drawing up custom plans cost more or less? Would it be possible to do a custom build 4 bedroom, 2 bathroom place for under $220k? I'm pretty flexible on garage options and such.

Considering the budget I would recommend a single storey design and if the minimum floor height is approx 1.5m out of ground at the highest point it's probably worth going lightweight on steel posts however both options could be explored (built up slab & brick veneer construction). Both systems will be comparable on price.

2. Not knowing the specifics of the acoustic report they may specify a higher category for the second floor.

3. Definitely feasible however will depend on your list of inclusions and must haves. Not taking these into account I would estimate a total floor area of 150m² - 180m² including garage or carport.

4.Pricing can vary from 3.5k - 7k for custom plans and will depend on the level of service and the conditions of the lot, ie: Acoustic requirements, Flood levels, bushfire assessment ect and whether the building designer manages the other consultants such as Acoustic Engineers, Structural Engineers and Private Certifiers ect.

I believe it is a worthwhile investment getting a custom design by a Building Designer / Architect as it allows you to tender / shop around for a builder and compare quotes with a compliant design. If dealing direct with a builder and they produce a design you can't shop it around as they own the copyright. They may also only include provisional sums for things like acoustics which will only be worked out and priced up after you sign with the builder, leaving you liable for any unknown extras ect.

Hope this helps!

Has anyone had noise assessments done? Did the result come in at a lower category than the council estimate?