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How to retrofit underfloor insulation?

Hey all,

I am hoping to retrofit our townhouse with underfloor insulation as a summer project, and had a couple of questions after having a look underneath the house today.

1. The floor joists we have are a bit of a mess in terms of spacing (putting it nicely). There are various sections where joists run parallel for a couple of metres only spaced by 10mm or so, and other areas where joists intersect into the gap where the insulation should sit (attached photos). What would be the best method and/or product to approach this? The polystyrene insulation panels only seem to be able to be cut nicely at the edges, and I imagine Earthwool might be hard to cut a chunk out of in the centre...

2. Our bathroom is on a concrete floor, and some more complicated joist/piping work near it - should this area also have insulation installed underneath it for best effect or is not worth the effort for this area?

I have attached some photos underneath to hopefully make my questions a little clearer.

Any advice would be much appreciated! Cheers

Joist overlapping into insulation spaceSmall gap between joists and concrete floor

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Comments

Jason

Hi @lm4,

Apologies for the slow reply. Unfortunately one of our D.I.Y. experts Mitch has had to take leave unexpectedly. I'm sure his colleague @EricL will be happy to help as soon as possible.

In the meantime, please check out a few previous discussions we've had about installing underfloor insulation:

Underfloor insulation by @Diannelin

How to add underfloor insulation? by @edechalain

Underfloor insulation by @RKH

There is also a helpful guide from the Bunnings team How to install underfloor insulation batts.

Thanks for your patience,

Jason

EricL

Hello @lm4

Thanks for sharing your question about underfloor insulation. The key to cutting the insulation easily is to use a brand-new snap off knife like the Craftright 18mm Snap Off Knife. In order to cut into the meat of the insulation the knife has to be long and sharp so that you can cut channels into it. A dull short knife will only deform the insulation. Once you start to feel that the knife is struggling or is producing an unsatisfactory cut change the blade to a new one.

I've installed both types of insulation in various houses and between the Earthwool R2.5 90mm x 420mm x 1160mm 5.4m² Insulation FloorShield Underfloor Batt and the Expol 1200 x 470 x 60mm Polystyrene Insulation Panel. The polystyrene insulation is easier to cut, but both provide adequate insulation. It will be a bit of trial and error on your part when installing in between the non-standard gaps. But should the gap become too small for the polystyrene, I suggest using Expanding Foam Filler. This will let you cover those small-unusual gaps that can't be covered by standard insulation.

In regards to the bathroom floor, I suggest covering it as best you can so that it will be insulated as well. It will prevent the floor in your bathroom from becoming cold.

My best advice is to take your time when cutting the insulation, there is no need to rush and getting good precise cuts will give you a snug fit when installing the insulation. Any gaps that do appear can be filled in with expanding foam filler.

Here are some handy step-by-step guides:

Please remember to wear personal protection such as appropriate clothing, gloves, goggles and a proper mask when working on this project. Please remember to follow all the safety guidelines printed on the packaging of the insulation bag.

If you have any other questions we can help with, please let us know.

Eric

lm4

https://www.workshop.bunnings.com.au/discussion/comment/32051#Comment_32051

Hey @EricL and @Jason

Thanks very much for your responses. At this stage I think I am leaning towards the Polystyrene panels mainly for the ease of cutting, especially for the weird and wonderful joist spaces we have. Thanks for the tip about the expanding foam filler, that seems like the best bet for the long narrow channels!

If I was to go for Earthwool instead (the higher R-value is tempting), how would you suggest insulating the overlapping joists (photo #1) and under the concrete pad where wood/piping is going through the section the insulation should sit in. I'm mindful that its 90mm thick when settled, so don't think squeezing it up against the wood or around the pipes is a good idea.

Also I am assuming that I should leave a good amount of space between the insulation and the ventilation grates (photo 2), but also don't want to leave gaps in the insulation for pests to nest in - how much space is recommended and how is this done?

Appreciate your help a lot!

Cheers,

Lachlan

EricL

https://www.workshop.bunnings.com.au/discussion/comment/78445#Comment_78445

Hi @lm4

When using Earthwool, it is much more forgiving when it comes to fitting it into irregular shapes. However, the same rules still apply, you can cut the Earthwool to make space for the irregular space but you 'll need to add extra strapping support so that those parts don't sag.

Some parts will stick out as it is unavoidable, but as long as the insulation is secure and uncompressed it should provide adequate coverage. In regards to the ventilation, let me make some inquiries for you regarding distance. Once I have more information, I'll get back to you.

If you have any other questions we can help with, please let me know.

Eric

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/78445#Comment_78445

Hi @lm4,

I'll just add that for those overlapping joists, cut slit the insulation where the joist in the middle is and insert the batt around it; no need to remove an exact section from it. Ideally, it's best not to over-compress batts, but the width of that joist is negligible.

For the vent to work at the same efficiency, you'd need to leave a gap of the same surface area as its opening. So, set the insulation back from the wall, say around 100mm, and leave a decent gap to allow the same amount of air to flow around the batt. However, ideally, in this situation, you'd remove the vents, replace them with standard bricks, and install new ones directly below the insulation. That way, you get the full insulation effect and maintain the airflow through the vents.

Please let me know if you have any questions.

Mitchell

lm4

https://www.workshop.bunnings.com.au/discussion/comment/103454#Comment_103454

Hey @MitchellMc

Thanks very much for that, will be a lot easier to just cut a slit for the joist and fit the insulation in that way.

Unfortunately the vents are currently as low as they can go, so we won’t be able to move them further down below the insulation. I’ll make sure to leave enough of a gap to allow airflow, and try to secure the insulation as flush as possible against the floor to prevent pests burrowing in to the gap.

Cheers,

Lachlan

DIYGnome

https://www.workshop.bunnings.com.au/discussion/comment/118765#Comment_118765

Hello @lm4 did you consider some type of remote void venting?

Some type of DIY wooden boxing/framing contraption (or utilize PVC/Corrugated piping) would work admirably, I also wished to point out that the polystyrene insulation panels aren't considered to be an attractive proposition to rodents (for nesting) unlike the other "fluffy" alternatives.

lm4

https://www.workshop.bunnings.com.au/discussion/comment/127876#Comment_127876

Thanks @DIYGnome, I’m definitely considering some sort of structure to direct airflow from the vents and also block off the edge of the insulation. Probably not as complex as that diagram, maybe just a flat section of plastic which I can install at a slight angle with some brackets on the joists.

The pest concern is definitely feeding in to the polystyrene v earthwool considerations. We already need yearly pest inspections due to the area we’re in, so I’m thinking that this is atleast a mitigation against pests getting a foothold. But will have a bit more of a think about it.

Cheers,

Lachlan

Chris1256

Looking at this i need to consider this as well being down south, need to also consider stopping debris from entering the subfloor space as well.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/39893#Comment_39893

Hi @Chris1256,

A warm welcome to the Bunnings Workshop community, it is fantastic to have you join us.

It's great to see @lm4's post has inspired you to get started planning a similar project.

I notice you've made a post of your own, so I will jump over and help you out there.

Jacob

Chris1256

Was there a completed photo after this was all finished?

I am thinking of insulating and fireproofing under my house.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/40092#Comment_40092

Hi @Chris1256,

Allow me to tag @lm4 so they are notified of your question.

Hopefully they will be able to assist with some photos shortly.

Jacob

lm4

https://www.workshop.bunnings.com.au/discussion/comment/40092#Comment_40092

Hi @Chris1256

Unfortunately we haven’t gotten around to doing this yet. Good luck!

Cheers

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