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Should I put underfloor insulation under the floorboards inside the house or under the house?

Hi,

I have a damp problem and want your advice on the best way to insulate my home to stop moisture getting in under the floor. I live in the central coast NSW in a bush setting. My house is a the bottom of a hill and water is collecting at the front of my property and under the house - I often have mud outside my front path.

I have taken up the carpet in my house and it's currently masonite flooring. I want to lay engineered floor boards.

Should I:

1. keep the masonite and lay floorboards over the top with insulation in between the masonite and floorboards.

2. remove the masonite, install insulation with a water barrier and lay floorboards over the top of the joists.

3. keep the masonite and lay floorboards and then put underfloor insulation under the house.

I've been told poly insulation is good for wet areas, is this the best advice? Please recommend the best solution and the types of products I need.

I also was wondering how effective rain gardens are at preventing water from going under the house if they are connected to the sewer line?

Thanks so much,

Lisa

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Comments

MitchellMc

Welcome to the Bunnings Workshop community, Lisa (@LR). It's terrific to have you join us, and many thanks for your question about insulating a floor.

The main issue here sounds like the water collecting at the front of your property and under the house. Before considering any insulation or vapour barriers, the first priority should really be to address drainage so water is directed away from your home. This might mean installing surface drains or an agi-pipe that collects water and connects it to your stormwater line. Preventing water from sitting under the house will be far more effective in the long run than trying to block moisture from entering through the floor.

When it comes to installing your engineered floorboards, the location and type of insulation or vapour barrier can vary depending on the manufacturer’s requirements. Installing a vapour barrier in the wrong place can trap moisture and actually cause damage to the boards, so it’s really important to check with the supplier of your floorboards before proceeding. If you were purchasing through us, I’d be happy to reach out to the supplier directly on your behalf to clarify the best approach.

In terms of insulation under the house, products like Expol insulation are often used in damp-prone areas as they don’t absorb moisture in the same way other types might. But again, the success of this will be limited if the water problem itself isn’t solved first. Also, insulation doesn't have any particular moisture blocking properties, and a vapour barrier is likely more what you need, rather than insulation, which is used for temperature control.

As for your question on rain gardens, they’re generally used to slow and filter stormwater runoff in garden settings. What you’d likely need instead is proper site drainage that carries water away from the house and into the stormwater system, rather than allowing it to pool near or under your foundations.

If you were to install a vapour barrier, I would suggest that the most appropriate place for it would be on the underside of the floor timbers, which would prevent the moisture from reaching them.

Please let me know if you have any questions.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/56154#Comment_56154

Thanks Mitchell for the great advice and for getting back to me so quickly.

https://www.workshop.bunnings.com.au/discussion/comment/90353#Comment_90353

I've started planning my drainage project and wanted some advice. We have lawn and some small shrubs in our front yard. The footings of our house are about 75cm - 100cm lower than the street level. We have a pebblecrete driveway that leads to our garage. The garage is built on a concrete slab that is adjacent to the main house. The main house is on piers that elevated above the ground.

1. Should I cut a channel through the pebblecrete driveway for drainage all the way across the front of our house or is it OK to have drainage in front of the main house and not the driveway?

2. How far from the house does the drainage need to be?

Please let me know if there are any other products that I would need besides the agi-pipe.

Thanks,
Lisa

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/111411#Comment_111411

Hi Lisa (@LR),

If you have some photos of the locations where drainage is an issue, this will greatly help us understand what you are working with. Can I trouble you to upload some photos showing where the main areas you are having issues are?

It would be tough to say whether you need to add drainage across the driveway without seeing it, but a drainage channel that spans the driveway could certainly be worthwhile.

Generally speaking, underground drainage should be at least 600mm from the house, but the specifics of your issue could change this.

For french drains, you will need drainage gravel and geotextile mat. The trench is lined with geotextile mat and drainage gravel, with the agi pipe running through the middle. The drainage gravel allows water to drain through the gaps between it, then find its way into the agi pipe to be carried away. The geotextile mat is there to prevent soil from eroding between the gravel, which will block the channels for water to flow through.

Once we can see what you are working with, I am happy to assist further.

Jacob

https://www.workshop.bunnings.com.au/discussion/comment/123136#Comment_123136

Screenshot 2025-09-22 at 11.48.19 am.jpeg

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/130818#Comment_130818

Hi @LR,

If you find water flows down the slope of the driveway, hits the door and runs off to the sides, then I would suggest cutting the driveway and adding a grated channel right before the concrete slab for the garage would be a good idea. That will create a collection point which you can then divert into the agiline you'll be running across the front of the house.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/135542#Comment_135542

Thanks again for all your advice. I really appreciate your help.

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