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Old 19 December 2015, 04:50   #1
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Building advice - floor insulation

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
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Old 19 December 2015, 06:34   #2
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Rockwool underslung would be vermin heaven.

My choice would be a Celotex type product. Could you possibly raise the thresholds and lay on top?
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Old 19 December 2015, 07:52   #3
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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.
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Old 19 December 2015, 08:01   #4
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Quote:
Originally Posted by slate1234 View Post
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.
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Old 19 December 2015, 09:16   #5
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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.
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Old 19 December 2015, 09:22   #6
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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.
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Old 19 December 2015, 09:30   #7
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What about polyurethane spray on foam? It. Should be a lot easier to get an airtight finish with this.
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Old 19 December 2015, 09:38   #8
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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.
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Old 19 December 2015, 09:39   #9
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Also, from my experience, airtight=mold. Good insulation should "breathe" to avoid mold/dryrot.
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Old 19 December 2015, 11:09   #10
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Building advice - floor insulation

Quote:
Originally Posted by Dan F View Post
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.
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