Breather Membrane Under Suspended Floor Insulation


Postby se4terrace » Fri Jan 27, 2017 12:05 pm

I have just moved into a classic single-width Victorian terrace, with a 5'6" nice-and-dry cellar under the hallway that provides access to the 3' crawl space under the front two rooms of the house (the living and dining rooms).

These rooms both have exposed floorboards that I have had sanded and varnished. They look lovely but they are really, really cold, with loads of drafts.

I'd like to DIY insulate underneath the floor in a cost-effective way and wanted to run my plan past you all.

The crucial thing is that, because I've already had the floorboards "done", whatever solution I use can't involve lifting the floorboards.

Here's my plan:
1. Use Gorilla tape (thick, super-sticky duct tape) on the underside of the floorboards to seal the floorboard gaps.
2. Fit 100mm fibreglass loft insulation between floor joists (they are 100mm joists).
3. Hold insulation in place with a Web UV10 breather membrane (designed as a roof membrane, but should do the job). This will be stapled into the joists.

Diagram:
Image

My thinking is:
1. The tape will seal the floorboards to prevent drafts coming up, and minimise moisture going down.
2. If moisture does go down, it will wick its way through the insulation and out the breather membrane.
3. The crawl space will remain well-ventilated (air bricks present and unblocked) to avoid damp issues.

What do you reckon?
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Postby welsh brickie » Fri Jan 27, 2017 5:56 pm

I would use a hard back insulation board like celotex or kingspan, you can cut it to fit tight between the joists, and to hold them in place you can nail a wood batten each side of the joists
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Postby se4terrace » Sat Jan 28, 2017 12:47 pm

welsh brickie wrote:I would use a hard back insulation board like celotex or kingspan, you can cut it to fit tight between the joists, and to hold them in place you can nail a wood batten each side of the joists


Thanks for this.

I'm trying to do this as cost-effectively as possible - hence thinking of using fibreglass and breather membrane.

I've also heard that the tight fit of celotex can lead to breathability problems potentially leading to damp above the insulation, below the floorboards. Is this not a concern?
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