How to seal subfloor from ‘crawl space’ moisture below?

Hi Everyone, first post for me! I’m about to kickstart our downstairs renovations (kitchen and engineered hardwood flooring). A particular issue we have is moisture rising from below crawl space. I’m about to have subfloor ventilation installed. However, as a belts and braces approach id like to seal the subfloor (Particleboard Yellow Tongue flooring), especially as I plan to lay engineered hardwood flooring on top. What would be suggested to seal 65m2 of Particleboard Yellow Tongue subfloor as a preventative measure acting as a moisture barrier? and what approach would you take? Thanks for taking the time reading and responding! Much appreciated.

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Comments

MitchellMc

Welcome to the Bunnings Workshop community @Malcy. It's terrific to have you join us and many thanks for your question about sealing a subfloor.

Particleboard has a wax coating on it to prevent deterioration prior to the roof going onto your property. Due to this coating, it is hard for products to adequately bond to it. You can sand the coating off which will allow you to use a paintable product. Once the wax coating is removed, you could use a product like Cabot's CFP to seal the floor.

The easiest option might be to lay a vapour barrier down onto the particle board prior to the hardwood floor. We can order you the Aquabar moisture vapour barrier from our supplier QEP. It consists of two layers of Kraft paper with asphalt laminated between them and it's an extremely effective moisture vapour retarder.

Please let me know if you have any questions.

Mitchell

Malcy

https://www.workshop.bunnings.com.au/discussion/comment/19582#Comment_19582

Thank you for your response Mitchell, very much appreciated. I engaged a builder to take a look yesterday, luckily he’s had plenty of experience in this space. He has suggested that we install/ glue down 6mm Villaboard to the particle board. This serves 3 purposes. 1st floor height will come up slightly in laundry and WC - As the floor height in laundry and WC will be brought up by screed and tiles to create a fall in case of a leak - with the villaboard, engineered hardwood etc the floor heights will be roughly the same +— mm. 2nd supports creating a firm, flat level surface for hardwood flooring. 3rd and final is it’s a great way to prevent moisture getting underneath the hardwood.

Thought I’d share this information, in case someone else has a similar scenario.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/72459#Comment_72459

It's pleasing to hear you've engaged a professional to assist @Malcy.

My only concern would be whether this method is going to address any ongoing rising moisture, which appeared to be your original question. Having the layer of cement sheet is a great idea for the height difference. However, given that cement sheeting is water permeable and has no water stopping characteristics, I'd be worried about whether this would be a practical solution. Any rising dampness will travel through the chipboard, through the cement sheeting and into the hardwood flooring.

If you are able to rid the particleboard of moisture and you are confident that the ventilation is effective in preventing the moisture from returning, then you can use underlay. Villaboard is not suitable for use on floors as it's a wallboard.

Please let me know if you have any questions.

Mitchell

Leena2

https://www.workshop.bunnings.com.au/discussion/comment/19582#Comment_19582

Hi Mitchell

I hope you are well.

I have an old double brick house with a wooden floor around two feet above the dirt subfloor.

I am wanting to put something over the dirt subfloor as a vapour barrier (particularly under one room). This is to prevent moisture rising up.

I have heard that in the 1960s in the UK they would put down tar.. prior to building, as it is always soggy there.

I would appreciate any insight/advice.

Thanks

Leena

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/84865#Comment_84865

Hi @Leena2,

A warm welcome to the Bunnings Workshop community and thank you for your question.

Using tar or bitumen is less common today due to its labour-intensive application, potential fumes, and difficulty removing it if repairs are needed later. While effective, it’s not as practical as modern solutions.

The modern solution is Polyethylene Film laid over the ground to act as a barrier to prevent moisture from rising from the ground into the subfloor.

You can lay it out and join the runs with GRUNT 48mm x 30m Builders Film Joining Tape to create one solid surface.

Let me know what you think and if you have any further questions, please don't hesitate to ask.

Jacob

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