Child Item

How to add waterproof membrane under timber floor?

Hi community,
I'm looking to add insulation and moisture protection under a timber floor for a terrace house (ground level). The timber floor will go over joists that are suspended with a small cavity of around 400mm above a dirt floor.
As well as to improve the insulation, I'm looking to stop moisture and musty smells coming up through the floor. The solution I'm thinking of is to add a breathable waterproof membrane over the joists and create a bit of a hammock to hold the earthwool floorshield insulation. Pic attached of the kind of setup:

Screenshot 2024-02-19 at 3.40.45 pm.png

The product I'm looking at for the membrane is the Ametalin VapourTech Brane

It doesn't mention floor application, so I wanted to get some opinion on whether it would work for my situation?

Much appreciated.

Find more posts tagged with

Tools and Materials

Building and Construction

Comments

Jewelleryrescue

Hi @Zawadi

That product will work great dont forget to tape up the overlaps. and around the peirs

I think a staple gun would speed up the process of attaching it to the joists.

It is a moisture barrier and will support the batts weight.

Food for thought if you attched the membraine from underneath of floor it would protect timber joists as well if you wish.

I would also consider adding cross straping between the joists to stop sagging of the batts out of the floor cavity.

Zenith 19mm x 700m Poly Pro Insulation Strapping

https://www.bunnings.com.au/zenith-19mm-x-700m-poly-pro-insulation-strapping_p0810984

I/N: 0810984

$48.25

Jason

Welcome to the Bunnings Workshop community @Zawadi. It's great to have you join us. Please don't hesitate to post anytime you need a hand or have a project to share.

@Jewelleryrescue has provided fantastic advice as always. Typically our resident Bunnings D.I.Y. experts would also be more than happy to chime in however they are unexpectedly absent from the site. Hopefully they will be back soon. Feel free to let us know if you need anything further.

Jason

Zawadi

https://www.workshop.bunnings.com.au/discussion/comment/35001#Comment_35001

Thank you @Jewelleryrescue, good advice about the strapping, I'll add that in. And interesting thought about going under the joists. That could work well especially with the strapping to help support the batts. Appreciated.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/79918#Comment_79918

Hello @Zawadi

If I recall correctly Ametalin has a dedicated product for flooring called - Ametalin 500mm x 60m SilverFloor Underfloor Insulation. It is a Heavy Duty, vapour permeable floor insulation, designed for use under floors with a sealed air space between flooring and enclosed sub-floor space.

Double-sided reflective surfaces provide increased R-value to your floor system. 5 mm weep holes at 400mm intervals in the membrane structure allow water and water vapour to permeate through, preventing moisture build-up under flooring.

In regards to the Ametalin 1370mm x 30m VapourTech Brane VHP Vapour Permeable Wall Insulation. Allow me to double check with Ametalin just to make sure you can use it for underfloor applications.

A note to our NZ members: The use of foil insulation products is prohibited.

If you need further assistance, please let us know.

Eric

Zawadi

https://www.workshop.bunnings.com.au/discussion/comment/104416#Comment_104416

Thanks @EricL, seems like another good option. Interesting reading about the electrocution risk and reason it is banned in NZ. I might stick to the other wall membrane for that reason.

EricL

Hello @Zawadi

Thank you for your patience. I've just received information from Ametalin and they suggest using Ametalin 1350mm x 60m SilverWrap Micro-perforated MD Vapour Permeable Reflective Wall Insulation - 60m instead of the Ametalin 1370mm x 30m VapourTech Brane VHP Vapour Permeable Wall Insulation. The Ametalin VHP is not strong enough to last exposed and unprotected. They recommend installing the Ametalin SilverWrap foil side down.

If you need further assistance, please let us know.

Eric

Andrey_B

https://www.workshop.bunnings.com.au/discussion/comment/35583#Comment_35583

Hi Eric

I am going to use Ametalin suggestion. I can not understand why they recommend putting foil side down? Will vapour coming from ground go through the Ametalin and make the insulation wet?

Thank you in advance

Andrey

EricL

https://www.workshop.bunnings.com.au/discussion/comment/81533#Comment_81533

Hello @Andrey_B

Welcome to the Bunnings Workshop community. It's sensational to have you join us.

I believe that the foil side is a better material to be exposed to the moisture as it it will not soak the insulation. It will technically condensate on the surface and fall away, whereas insulation material will absorb the moisture instead of turning it away.

Since it is the recommended installation procedure, I propose following the instructions given by the manufacturer.

If you need further assistance, please let us know.

Eric

Andrey_B

https://www.workshop.bunnings.com.au/discussion/comment/105505#Comment_105505

Thank you Eric, It makes sense, I will install it this weekend )))

farnie

https://www.workshop.bunnings.com.au/discussion/comment/105505#Comment_105505

I understand foil vs insulation to reduce moisture soaking the insulation, but I don’t understand why the foil side of https://www.bunnings.com.au/ametalin-1350mm-x-60m-silverwrap-micro-perforated-md-vapour-permeable-reflective-wall-insulation-60m_p0810105 should be exposed, instead of blue side, to the moist air under the joists. Is anyone able to further explain this?

EricL

https://www.workshop.bunnings.com.au/discussion/comment/122153#Comment_122153

Hello @farnie

Great question—this comes down to how vapour-permeable reflective foil membranes like Ametalin SilverWrap Micro Perforated are designed to work in managing both moisture and radiant heat.

The foil (shiny) side of the membrane should face outward (downwards if underfloor), even though it might seem counterintuitive if you’re worried about moisture rising from below. Here's why: the foil side reflects radiant heat, helping regulate temperature. The blue side is not reflective, and exposing it outward would reduce the foil's ability to reflect heat.

Now to your main concern—moisture control. This specific product is micro-perforated and vapour-permeable, meaning it allows moisture vapour to escape from the insulated cavity (above the membrane) to prevent condensation build-up, while still offering protection against bulk water (e.g. splashes or wind-driven rain under subfloors). If you were to expose the blue side instead, you'd still get vapour permeability, but you'd lose the benefit of radiant heat reflection, which is part of the intended function of the membrane.

So, the foil side is exposed because it:

Reflects radiant heat (a major factor in underfloor insulation).
Still allows moisture vapour to pass through from above via micro-perforations.
Resists water ingress from below to protect the insulation and joists.

If you're in a particularly damp area, combining this wrap with proper subfloor ventilation is key to preventing moisture build-up.

If you need further assistance, please let us know.

Eric

farnie

https://www.workshop.bunnings.com.au/discussion/comment/130167#Comment_130167

Thanks @EricL ! I had assumed the membrane was a one way moisture barrier to keep moisture to one side of it. Your explanation makes a lot more sense

Child Item