Underfloor insulation: foil + Expol?

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McNulty

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#175203 21-Jun-2015 14:43

I know there have been discussions around underfloor insulation here before, but I think this is a new question.

I have a 1980's timber house on piles. The house has existing foil underfloor insulation. In some places the foil is ripped etc. and since the floor is cold underfoot on a winter's morning (even in the non-ripped areas), I want to install Expol or some other product.

Now the questions:

Since Expol is supposed to be installed closely to the underfloor, do I need to remove the foil? Or can I install the Expol beneath the foil?
How can I best remove the foil, since it was placed at time of construction from the top side? Just cut it loose?
Could I get double-insulation benefit from having both in place, or is the R-value reduced because the Expol will be less-than-perfectly situated?

Anyone done it this way before?

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#1328693 21-Jun-2015 14:47

Expol goes right under the wood, you'd have to remove what's there. If you put it underneath the current foil it may still work, but I doubt it would work as well. Does expol have both polysterene and foil?

If you do this I don't think you'll get a huge benefit, the floor will still be cool to cold. Heat rises, and given it's outside temp under the house and floor temp inside the house it will be at best half way between those. If you have carpet inside you may be better off putting in a good thick underlay and thick carpet and leaving the foil where it is, just repairing what's broken.

We have stiff polyester under the house, it didn't make much difference when it went in. The plastic ground sheet made a huge difference to the moisture levels and smell though.

McNulty

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#1328694 21-Jun-2015 14:53

I'm also considering a ground sheet.
Did you need a special ground sheet or just the stuff they sell on big rolls at Mitre 10?

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timmmay

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#1328803 21-Jun-2015 20:04

I had it done. Thick plastic, pegged down at the corners and with masses of tape to attach it to pillars. It can get windy down there apparently, or people on it moves it.

Niel

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#1328821 21-Jun-2015 20:22

Expol is installed between joists, it is okay to put it under the foil as long as it can still fit tightly all round against the joists. The issue is if it does not fit tight and has a gap to the floor, then you can get a draft around the Expol. I'd consider removing the foil, fit the Expol, and then perhaps fit foil to the bottom of the joists. Then add a ground sheet to keep the damp away (evaporating water cools down, which is why the temperature can briefly drop around sunrise).

But often a cold floor is because of a small draft around doors/windows letting cold air in which travels along the floor. Also condensation on windows evaporating results in a drop of temperature, and the cold air drops to the floor (guided by curtains instead of mixing with room air at window height).

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Aredwood

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#1328825 21-Jun-2015 20:33

Just get rid of the foil. How it is supposed to work is by reflecting back heat that is radiating from the floor boards. But it has to be really shiny for it to work. And even then it was only R1.1 at the most. It is most likely all tarnished and covered in dust by now. So won't be providing any real insulating value. And by squashing it against the floor, Any remaining insulation value will be gone. Also Expol sheets are shaped so there is an air gap between the Expol and the floor. If the foil is still there that gap will be gone. The other problem is that there were cases of that foil becoming live due to staples going through power cables.

So leaving it in will just cause problems.

timmmay

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#1328837 21-Jun-2015 20:55

I figure foil should at least block a breeze blowing up.

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oxnsox

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#1328857 21-Jun-2015 21:31

It's my understanding that when it comes to insulation (whether floor, ceiling or window), for best results always go for the product with the greatest surface area. This is why thicker Batts and lined curtains out perform thiner batts and thermo-drapes.

On this basis, styrofoam provides a good surface area (for floors) to trap more non circulating air. Foil probably offers similar benefits to ground sheets, simply preventing rising damp air from adding to air that maybe able to circulate immediately below the floor surface.

Regs

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Snowflake

#1328918 21-Jun-2015 23:23

i'd be inclined to just use a polyester 'batts' type product tacked on over the foil to get the combo protection. give an installer a call though. www.underfloor.co.nz install all three types (polystyrene, batts, foil) so might be a good one to call. they did mine - the top R value Cocoon polyester which was great in my 1920 wooden floor bungalow.

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andrewNZ

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#1328929 22-Jun-2015 00:08

If you use any polystyrene product, it MUST NOT touch wiring (the instructions should say this). The two react and the cable insulation turns to goo.

froob

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#1328960 22-Jun-2015 08:06

For myself, I would probably remove the foil. I understand it can become electrically live if accidentally stapled to an electrical cable - [url]http://www.building.govt.nz/codewords-23-1[\url]

If the new insulation had to be installed by stapling (e.g. batts), then I wouldn't take the risk.

We've had polyester insulation installed under the floor. It is not night and day, but you can notice the difference when walking barefoot on the one uninsulated spot on a cold day.

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bfginger

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#1329811 23-Jun-2015 11:01

Foil insulation only works if it is installed correct by creating a still air gap and it sounds like your foil is ripped up and not doing much.

A ground sheet vapour barrier should help reduce dampness if your underfloor is enclosed or has low air flow. Enclosed under floor cavities are supposed to be ventilated to prevent dampness build up.

I agree with Regs, the high R value Cocoon R2.6 polyester should be a good choice. The building code only requires R1.3 so most people have just R1.4 to R1.8 installed.

Polystyrene is not suitable for some houses as it is much harder to fit to arbitrary joist widths than polyester. Polyester does lose up to 40% of its R value when exposed to heavy winds so polystyrene might be better in wind if it can be installed flush against the wood without wind whistling around it. Expol is not the only company producing polystyrene. There are polystyrene products with higher R values or you can install a double layer.

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#1329841 23-Jun-2015 12:44

Good comments.
Like Timmmay, i got more benefit from ground sheet than insulation.

I would add, check house vents are clear to ensure airflow and fix floor squeaks at same time

jonathan18

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#1329855 23-Jun-2015 13:00

In our previous house we had both the polyester insulation and polythene groundsheet laid (as we've had done in our present house), and I believe the insulation made a significant difference.

The first winter after our son was born we were reluctant to let him play on the floor, as even through carpet the floor was cold to the touch; next winter - post-insulation - there was no such issue.

That's not to say the polythene wasn't also a brilliant success - where we lived (Lower Hutt) the high water table meant we were getting moisture through the floorboards, to the point one room had a significant problem with damp. Again, this was completely solved by the polythene.

A good example I reckon of how useful both products are.

Personally, until I see something better I'd stick to using this polyester product as it seems easy to fit, long-lasting, and effective, whereas foil and polystyrene seem to have their issues, as highlighted in previous posts.

mdf

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#1329865 23-Jun-2015 13:14

Regs: i'd be inclined to just use a polyester 'batts' type product tacked on over the foil to get the combo protection. give an installer a call though. www.underfloor.co.nz install all three types (polystyrene, batts, foil) so might be a good one to call. they did mine - the top R value Cocoon polyester which was great in my 1920 wooden floor bungalow.

+1 for a batts-type option. You can get foiled backed batts if moisture is an issue.

We've got expol in our 1900's cottage. It's good, but not great.

First, if you use your basement for anything, it *stinks* for about a year.

It also works by creating an air pocket. So if your joins aren't tight, it doesn't work as well. For us, we've got joists that are a bit wonky, 100 years worth of patches and changes, and plumbing and electrical that are difficult to navigate. If your house shifts, it can also fall out and you have to put it back in. You can get a cam/cleat thing to help hold it, but it's still a pain.

Finally, our floor boards were in a pretty bad state (think visible gaps). You could still feel a draft through these gaps, even after the expol went in. The draft was coming through the air pocket, which kind of undermined the point.

If it goes in, also keep an eye on any tradesmen under there. Make sure they put it back up, not just rip it down and chuck it somewhere to stand on. I speak from experience.

On the plus side, we definately noticed an improvement in warmth. It does work, but if I had my time again, I'd do it with bats.

jonathan18

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#1329872 23-Jun-2015 13:33

mdf: . It does work, but if I had my time again, I'd do it with bats.

Do you mean batts as in a fibreglass product, or the one designed for under-floor use which is (I understand) a polyester (a recycled plastic, I got told) and kind of looks like a thick felt?

I've not heard of normal batts being used under-floor, and I'd imagine they'd not have the necesary rigidity of the polyester insulation, which is far more solid but has the ideal amount of give to be able to be fill up non-straight areas and not require any form of anchoring (as it's cut slightly larger than the space it's filling).

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