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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
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:
There is also a helpful guide from the Bunnings team How to install underfloor insulation batts.
Thanks for your patience,
Jason
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
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
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
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.
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