Underfloor insulation. A question for builders, diy'ers or insulation freaks.

[andyfromotley]

Hi, live in a mid 1980's house with a suspended timber floor (chipboard on top of joists), it has no underfloor insulation so whilst i am replacing kitchen etc thought i would insulate the underfloor.

I think i understand the principle, support the insulation in between joists, using either netting for wool type or nails to rest on for kingspan type. Question really is which would be the best material to use, if its wool can i just use loft insulation? (and if so can i use the foil contained type) which would be cheapest? Is there a minimum u rating i should be looking for> Any other general tips?

I know its a bit OT but you really are a clever lot so thought i'd ask anyway.

cheers

Andy

 

[slowmotion]

My 2 cents. Rockwool or glasswool will be a lot cheaper than foam boards. One of the problems with rigid boards is that you need to make them a tight fit between the joists in order to prevent air "shortcircuiting" the insulation where there are gaps. With wool, you can just stuff in more insulation.

Whatever you do, do not block any air bricks that ventilate the underfloor space. If you do, you may get rot problems.

 

[User482]

You can use ordinary rockwool, but you may find that space blanket is a better width for your joists. But in the long term, I would worry about the netting holding up - what happens if the blanket falls down? You're fecked unless you have a cellar.

I think I would go for rigid board, friction fitted and resting on battens or screws driven in at the base of the joists. As slowmotion says, you'll need to fill in the gaps, but on the other hand, you could probably get away without lifting all the boards.

You might also want to think about a vapour control layer between the top of the joists and your floor covering.

 

[montage]

I'm an insulated freak

 

[Dewi]

Just about to do my ground floor - there's a crawl space so not too bad - going for rockwool and nets as trying to friction fit celotex etc around radom joist centres etc is hard work - celotex works in theory if the centres are standard, 80s house you might be ok

 

[Over The Hill]

Do you get mice or anything down there? If so, you are giving them lots of nesting material if you use rockwool.
Also watch if you have any water pipes down there. The air space will be much cooler below the insulation so more likely to freeze. If you have then bring them up near the floor and put the insulation under the pipe.

My DIY book gives both as an option so I dont think you can go far wrong.

 

[asterix]

Foil? I have used this in part of the roof of my barn conversion where it has proved effective. I was advised by a builder to get the better quality stuff as the cheap versions aren't worth the money.

Foil Types

Foil insulation works as a convectional and radiant barrier. It has been difficult to get the best performance out of traditional foils due to building construction methods and strength of products, however newer more advanced types on the market offer some fantastic performance features. The primary advantage of foil is that it acts as a complete vapour barrier stopping rising dampness coming into the home and they are also great for working in tighter areas. Foils are installed in blanket format making them quick and easy to install but there is risk of electrocution in using them so please read and watch the installation instructions carefully.

editted to remove formatting stuff that appeared from nowhere!

 

[User482]

Do you mean multifoil, or conventional insulation wrapped in foil? Multifoil is quite controversial (for those who care) as all sorts of claims have been made for it, none of which are supported by independent tests. Not to say it doesn't work, just not as well as claimed.

 

[Globalti]

Yes, make sure any pipes are well protected with wrap.

Wickes are selling four rolls of 170mm glass wool at the moment, 8.1m2 each, for £29.97. Insulation has never been cheaper. We've bought 8 rolls so topped up the whole house for £60. Crazy.

 

[asterix]

Do you mean multifoil, or conventional insulation wrapped in foil? Multifoil is quite controversial (for those who care) as all sorts of claims have been made for it, none of which are supported by independent tests. Not to say it doesn't work, just not as well as claimed.

Multifoil. I am aware of the controversy and at the moment the only bit of my barn that has roof insulation is the bathroom! When I return to it in March it will almost certainly be cold enough to get some idea how good it is and retaining warmth.

However, i'm more hopeful than sceptical about thin layers as insulators. For example this year I put in 4 velux windows and for the bedroom one I went for slightly more expensive glazing that prevents energy transfer (not available in the UK, it seems). In the summer the contrast between windows on the same slope was very considerable.

The standard glazing we used was already supposed to stop heat from the sun to some degree:

..standard glazing comes with a toughened outer pane. In winter, it reflects 60% of the heat normally lost through glass, back into the room. During the summer, it works in reverse by reflecting excessive heat from the sun to help prevent the room from overheating.

Of course there are differents ways that energy is lost/gained so that's not the whole story.

 

[asterix]

Yes, make sure any pipes are well protected with wrap.

Wickes are selling four rolls of 170mm glass wool at the moment, 8.1m2 each, for £29.97. Insulation has never been cheaper. We've bought 8 rolls so topped up the whole house for £60. Crazy.

Yes I have been buying rockwool in the UK and shipping it France where it costs 7 to 10 times as much! Pays for the journey..

 

[Globalti]

Multifoil. I am aware of the controversy and at the moment the only bit of my barn that has roof insulation is the bathroom! When I return to it in March it will almost certainly be cold enough to get some idea how good it is and retaining warmth.

However, i'm more hopeful than sceptical about thin layers as insulators. For example this year I put in 4 velux windows and for the bedroom one I went for slightly more expensive glazing that prevents energy transfer (not available in the UK, it seems). In the summer the contrast between windows on the same slope was very considerable.

The standard glazing we used was already supposed to stop heat from the sun to some degree:

Of course there are differents ways that energy is lost/gained so that's not the whole story.

Isn't this K glass? It has a slight mirror to it, which makes it more reflective.

 

[asterix]

It has slight colouration but is not mirrored. It has 77% heat shielding compared to the standard 60% glazing but I can't find out exactly what the coating is.

In all other respects Velux windows in France are the same as UK ones.

 

[akaAndrew]

Multifoil is quite controversial (for those who care) as all sorts of claims have been made for it, none of which are supported by independent tests. Not to say it doesn't work, just not as well as claimed.

I noticed here in France that last year's DIY catalogues were claiming r values for multifoil. This year, they're gone. They still sell the stuff, and we've used it in my wife's workshop. It's better than nothing.

In our ceiling, we've used a loose fill insulation made from recycled paper/pulp. It really is good and, added bonus, the rats/mice hate the stuff. We've had no rodent noises above since installing it.

 

[Globalti]

It has slight colouration but is not mirrored. It has 77% heat shielding compared to the standard 60% glazing but I can't find out exactly what the coating is.

I believe the coating is a very thin layer of gold, applied by the same vacuum mirroring technique as aluminium for mirrors. It reflects certain wavelengths back but is too thin to be visible unless you view the glass from an angle, when it looks more reflective.