Building advice - floor insulation

[BogMonster]

To avoid hijacking the other thread, thought I would ask this in a new one as there are obviously some knowledgeable builders on here.

Our house has next to no under floor insulation - it is a timber framed house sitting on piles. Built in the mid 80s the 'insulation' is like a foil backed bubble wrap about quarter of an inch thick stuck to the bottom of the chipboard flooring, which means the floor is always cold and it costs us a fortune to heat in the winter.

I want to insulate under the floor but it will be a faff. There are basically two options I am aware of

- Rockwool type insulation. Each bay between floor joists to have a sheet of OSB supported by a batten at each side along the bottom, and the rockwool will be between the ply and the floor making a snug sandwich.

- One or other of the types of rigid foam sheet insulation that can be ordered to size (or cut to size) and fitted solid against the underside of the floor (in fact it would be against the existing foil/bubblewrap insulation).

The first one would be my preference and is how we insulated the extension, but as space is tight under there it will be immensely difficult to fit under at least half of the house as we obviously have to work from underneath.

The second one would probably be easier to fit but I am wary of it, as unless it is a very good fit, the slightest air gap around it would appear to render it completely useless as the cold air just flows around it and gets to the floor anyway. And, due to the bulk of it, shipping it all down here makes it very expensive as freight is by the cubic metre.

Are there any other alternative options around that I should investigate?

Thanks

[Mollers]

Rockwool underslung would be vermin heaven.

My choice would be a Celotex type product. Could you possibly raise the thresholds and lay on top?

[slate1234]

If you lay on top you will be loseing part of the skirting's cutting the bottom of the doors and when you get to the front and back door you would have a detail that would be hard to weather proof, full fill with kingspan / celotex, or there is a company in south wales that sell seconds that would do for a floor the boards are a coupple of mm either way in thickness also have a look at Knauf Insulation web site
for rockwool.

[Mollers]

Originally Posted by slate1234

If you lay on top you will be loseing part of the skirting's cutting the bottom of the doors and when you get to the front and back door you would have a detail that would be hard to weather proof,]

Chopping doors and replacing skirtings may be easier than going under, if the external door detail will allow. If access to under floor is restricted, it would be more than difficult to carry out an efficient job.

A room of slate or any other hard finish may scupper the 'on top' option if you don't want a step.

[Erin]

There's no reason why you couldn't staple up another layer or two of foiled bubble wrap or multi foil sarking insulation such as the Triso mentioned on the other thread. Probably easier to handle in a confine space and more forgiving of irregularities between joists. Seal the joints with foil tape to keep it air and vapour tight.

[Mollers]

Originally Posted by Erin

Seal the joints with foil tape to keep it air and vapour tight.

https://www.iso-chemie.eu/uk/home/

Above's tapes are superb! Bloody expensive! I used their foil backed butile for the MVHR ducting and airtight green for sealing plasterboard. Both are there forever.

[Siochair]

What about polyurethane spray on foam? It. Should be a lot easier to get an airtight finish with this.

[Dan F]

Where I live (Oregon - cold and wet all winter) the most popular option by far is fiberglass insulation batts. After my house had a flood from above (overflowing washer) we had to replace all the underfloor insulation. It took two guys about half a day. The insulation comes in rolls, and is relatively easy to staple into place using the paper backing that comes on the insulation. Typically you can get a much higher R-value for your money from fiberglass than foam board as well, but that might not be true in your location.

[Dan F]

Also, from my experience, airtight=mold. Good insulation should "breathe" to avoid mold/dryrot.

[Erin]

Originally Posted by Dan F

Typically you can get a much higher R-value for your money from fiberglass than foam board as well, but that might not be true in your location.

Be careful as a higher R value actually equates a poorer U value. R is the conductivity and obviously the more it conducts the poorer it is at insulating. Per mm thickness foam boards are almost twice as effective than loose quilt.

[Poly]

If I lived in the Falklands and faced the costs of shipping lots of fresh air there I'd be wondering whether there was a local material like sheep fleece that might be practical?

[willk]

Did someone drop this?

[willk]

Originally Posted by Poly

If I lived in the Falklands and faced the costs of shipping lots of fresh air there I'd be wondering whether there was a local material like sheep fleece that might be practical?

Or you might p-p-pack it with penguins?

[C-NUMB]

I would do it with xps insulation, working from top of the floor. Its such easy to cut, clean job and finishing using pu foam(the botle stuff) it will be air tighter than any other solution. Would remove any other old/previous insulation. Easy to make flush with existing floor structural beams, so no change inside the house.

If using rockwool You need to build "carrier" planking to keep it in place(not needed with XPS), that need to be done from the underside. Then You need breathable wind stopper material beneath the rockwool if done properly.

Guess 100 mm would be a great improvement so for an 100m2 floor, you need almost
10m3.....(less the beams), might hurt for the transport cost, don't know would rockwool be much cheaper though.

Know that in some houses insulation improved by 2K PU foam, but defo not a DIY job, very risky business, would not be my choice in any case.

[ShinyShoe]

Originally Posted by Poly

If I lived in the Falklands and faced the costs of shipping lots of fresh air there I'd be wondering whether there was a local material like sheep fleece that might be practical?

Or a way to put the air in yourself like a PU spray...

Can presumably be applied right to keep airflow under it and stop condensation.

[slate1234]

if useing rockwool take up floor boards and nail chicken wire to on side of the of the joist and up the other side and form a cradle for it to liy in thats how I would do it.

[jwalker]

I'd be making some holes in the floor and fill the cavity with vermiculite. I've done this as wall insulation between a stone cottage wall and plaster board and it's been in place for many years without a problem.

With a bit of ingenuity, I'm sure you could simply blow it in to fill the space.

[jwalker]

Here's a wee link. Download the Construction Market PDF then read the last paragraph.
Home - The Vermiculite Association

[slate1234]

He would be filling the sub-base under the floor and the price of that stuff he would have to sell his RIB lol

[Pikey Dave]

Originally Posted by BogMonster

The second one would probably be easier to fit but I am wary of it, as unless it is a very good fit, the slightest air gap around it would appear to render it completely useless as the cold air just flows around it and gets to the floor anyway.

I've seen a few references about getting a good fit with rigid foams & I can't see the problem. We built a very substantial extension including a cellar in 1999-2000. We used Celotex & Kingspan throughout for floors & ceilings respectively. Cut the foam with an old bread knife or hand saw, fit it in place. Hold it with a couple of galvanised clout nails into the joists & get jiggy with the expanding foam, we used gun grade foam, much easier to control & cheaper. Result, perfect airtight self supporting insulation. Add under floor heating & whole house heat recovery system, toasty & cheap to run.