Insulating Timber Floor from Cellar

[tekton23]

Hi,

I have decided to insulate the underside of our floor within our ground floor flat. It's COLD! Luckily there is a cellar beneath so I can get access to the whole area. It is a Victorian house.

I have decided to start by stapling a layer of silver foil (bubble wrap) as a (cheap version) airtight breather membrane. I will then add a layer of cheap rockwool to increase the U-value.

I know it is important to tape the joins so will be doing this, however what happens at the edges where the foil meets the brick work? Should I lap the foil up or down or what?

Also how do you deal with all the fiddly bits. For example, there are brick sleeper walls, so it is difficult to get a clear run when using the foil. Do I just cut and patch it round all the obstructions? Presumeably the sleeper walls are a form of thermal bridge to the underside of the floor, not that I am that worried - any insulation will be an improvement to none! Before I filled the gaps from above (prior to sanding) you could see the light when it came on the cellar!

Thanks,

Alex

 

[tekton23]

Untitled

• tekton23
• 5 Jan 2012
• 1

Untitled

• tekton23
• 5 Jan 2012
• 1

 

[ladylola]

I've done this in the past although never used the membrane. There seems to be some debate as to wheather it's needed or not but every architect I've worked with has gone down the not needed route.
As to the insulation itsself we've used a slab product. I can't for the life of me remember the trade name but someone may recognise what I'm on about. The slabs we used were the dark green rockwool type with a sort of black paper coating on one side. There are actually quite rigid and we cut them , shooved into place and then tacked a timber onto the joists just to secure them . The paper covering makes for a cleaner "ceiling" to the cellar too.
As to your question about the fiddly bits , you've answered your own question - cut and patch.

 

[tekton23]

Hi

Yes, the rigid board would be quicker and easier to fit, I am just using rockwool because there are so many pipes, and other bits and bobs to get around. Hardly any of the rafters are regular.

Alex

 

[tekton23]

Hi,

Has anyone any ideas where to stop the insulation? Do I tape it up to the floor, or across and against the brickwork?

Thanks,

Alex

 

[mointainwalker]

I have decided to start by stapling a layer of silver foil (bubble wrap) as a (cheap version) airtight breather membrane.

You have got things wrong. The same fabric cannot be air-tight and breathable . In any case no membrane required in this situation.

 

[tekton23]

HI,

Airtight breather membranes are extremely common in modern building design. The membrane acts to keep the house airtight from within, to prevent draughts but allows water to escape.

I have found some good help here:-

http://www.lowenergyhouse.com/timber-floor-insulation.html

http://construction.tyvek.co.uk/Tyvek_Construction/en_GB/products/vcl/airguard_reflective.html


Cheers,

Alex

 

[freddiemercurystwin]

HI,

Airtight breather membranes are extremely common in modern building design. The membrane acts to keep the house airtight from within, to prevent draughts but allows water to escape.

I have found some good help here:-

http://www.lowenergyhouse.com/timber-floor-insulation.html

You cannot have a membrane that allows water vapour to pass through yet stops cold air! Water vapour is air that contains water. If they get the fundamentals wrong I'd dismiss the whole website.

 

[tekton23]

Hi,

That is exactly what these membranes do and have done for years. You can get one for almost every situation i.e. airtight breather (meaning keeps the air still but allows vapour transmission) or airtight vapour control (which allows no vapour through (which you might use on basement walls) or wind stopping vapour impermeable (used under a roof, but above the insulation.

I agree it does not 100% STOP air movement, but the principle is to trap air as the best insulator whilst allowing vapour through.

From the research I have done I can see that I have used the foil in the wrong way and will be heading out to buy some rockwool. The foil should not touch the boards, as stated the principle is to trap air by pinning it to the underside of the joists.

I still don't know, however, if the cheap silver foil bubble wrap is or isn't breathable - I think it probably is not, otherwise they'd advertise it on the packaging.

Alex

 

[mointainwalker]

I don't know who is responsible for http://www.lowenergyhouse.com/timber-floor-insulation.html but since they name no products , I still don't know if such a concept as they describe exists.

If it does it is certainly not the Tyvek product you linked which makes it clear that it is :

DuPont™ AirGuard® Reflective is a 100% air tight air & vapour barrier

so nothing gets through.

I also think that most people on this site would tell you that it is wrong to completely enclose the floor-joists as shown by the first site.

 

[AronSearle]

I still don't know, however, if the cheap silver foil bubble wrap is or isn't breathable

The material itself is not breathable, some such products were/are used as "breather" membranes, the idea was that the reflective properties help with the insulation value, and that the sheets would be installed so that the top and sides were stapled, but not the bottom, the bubbles would then create a mini cavity behind the membrane that vapour could work out of, It sounded a bit dubious to me when I heard it.

I don't know who is responsible for http://www.lowenergyhouse.com/timber-floor-insulation.html but since they name no products , I still don't know if such a concept as they describe exists.

They are just getting the specifics slightly iffy.

The website suggest putting an airtight breather membrane under the floor, to stop drafts blowing through.

Even a standard breather membrane does significantly stop drafts (you can blow really hard through it, but not feel much on your hand), and there are breather membranes advertised as less air permeable, but still meeting the requirements for vapour permeability that a breather membrane requires. [/quote]