[Closed] Insulating under a suspended timber floor

I used rolls of recycled polyester wool. It's supposed to  be 120mm thick and R3 but is more like 100mm. Easy to cut and pleasant to work with. It was a bit of fiddle making up all the bits of wood to hold it in place. I hope you have easier access, digging the trenches was the hardest part.

Blimey, no need to dig trenches...the floor will come up!

Our builders have used wire mesh of some description, stapled to the joists with wool (or whatever) resting on top.

(Where they haven't used Celotex).

As above, you can use all sorts of methods but most use a chicken wire type solution to stop the insulation dropping away in parts and bing tight in others.

A celotex style board sitting on runners attached to the timbers may be easier to fit though and more robust over the years.

If it's a genuinely suspended floor i.e. with a clear void between the ground and the underside of the joists, then you don't have to worry about moisture soaking the insulation.

PIR (e.g. KIngspan) insulation will give you much better U-Values than mineral wool (Rockwool/Earthwool) and can be friction-fitted between joists, or supported on battens fitted to the existing joists. I prefer Rockwool as a product because it's essentially molten rock spun to produce the fine fibres (You get a very similar product occurring naturally at some volcanoes), rather than expanded chemicals, but they both have their uses. It may depend on the depth of your joists: If you have very shallow joists, then PIR insulation will give you the best thermal performance for the given thickness. However, deeper joists may allow you to use thicker insulation, which may suit Rockwool/Earthwool.

As others have said, mineral fibre insulation generally requires a wire mesh fitted to the u/s of the joists to support it.

The downside to PIR/rigid insulation is it needs to be cut to shape, which can be time-consuming. Mineral wool does as well but as it's compressible, it is more forgiving when it comes to cutting to fit.

Rockwool, Kingspan and Knauf have lots of information on their websites, if you want to research it a bit more.

We used a Rockwool product, "Flexi" I think it was, which has enough structure to it to sort of spring back and hold itself between the joists, as well as fill in irregularities. I think we used chicken wire beneath. We had enough of a void to slither under and didn't want to lift all the boards but it was a horrible job. I couldn't honestly say it was a better method than lifting the boards. Most were exposed anyway...

It transformed the house though.

Interested as I'll be trying the same some point this year....if you use rockwool how do you stop little furry critters making a nice warm home in it>=?

How much of a difference does it make insulating the floor? Obviously heat rises so my thoughts would be insulate the loft first then the wall, windows, doors etc then a maybe the floor but never knew if it was worth the hassle....?

Are there no blown insulation products for this? I'd like to do ours but I think our void under the ground floor is only about 6 inches deep, so would require the lifting of every single floor board.

the void is there for a reason as the house was built to "breathe" an you will just trap moisture and encourage rot.

I would not fill the void completely,  unless i was properly concreting in a whole new base.

<p>It makes a huge difference, we put rockwool under ours and it is noticably warmer.</p>

Personally i would not use PIR for under floors or in loft spaces. It can be done , but its not easy, especially retrofitting. I have seen the damage it can cause if not done correctly. Not worth it imo. Iv'e been told a few  stories lately from tradesmen and others in the industry regarding problems in this area.

Dpc run perpendicular to the joists and nailed to the underside with 500mm spaces between, Rockwool inbetween joists sitting on dpc.

Insulation material choice is not ultra important for a floor. Choose whichever you think is easiest to fit, cheapest, most environmentally friendly etc. What is super important is addressing draft proofing while you're doing it.

@supremebean - Are we talking condensation/moisture problems?  This is why I was looking at sheepswool insulation rolls with a vapour permeable membrane, not a DPC.  Wouldn't moisture from the room (kitchen) get trapped beneath the floor and the DPC (in the joists and insulation) if I installed a DPC under the joists?

@mau00149 - It's more to do with drafts coming up from underneath the floor.

If it's an old house, I'd be a litttle nervous about restricting the airflow through the building. Like many old house owners, my cottage was 'renovated' to make it look internally shiny and modern, but plugged the gaps in windows/doors/chimneys etc. Even having carpet on the downstairs floor became a problem for moisture build up internally.

If you know you've got breathability and no moisture issues then insulating under hanging floors should be OK, if there's any doubt, let the house breathe.

Thanks Lovewookie - this is exactly why I was looking at natural insulation.

What I was looking into this the ideal seemed to be to remove all floor boards and add a vapour barrier ontop of the joists / insulation and below the floor boards. Keep the joists uncovered from below so the circulating air below keeps them dry as they were before but just with insulation of whatever type stuffed between.

We had rigid insulation fitted between the joists, with a vapour barrier on top. This was about ten years ago and we've never had any problems. The floor is noticeable warmer as a result. Air can still freely circulate below the joists which I assume keeps them dry.

what about the Superquilt/Superfoil stuff?  I've been having a look at that as an option on my new build in place of PIR everywhere (not made my mind up), but in your case as it acts as a vapour barrier and you can lay floor right on top (wrap to the bottom of joists first) it could be a good solution?

We did this in our kitchen last summer, we used kingspan rigid insulation, we screwed screws into  the joists at even intervals to make a shelf for the kingspan and then just placed the kingspan on top of the screws. There was probably 12-18 inches between the bottom of the joists/kingspan and the concrete base the kitchen was built on, so plenty of airflow under the joists, all the original air flow bricks were kept in place.

Then we laid down  the floor boards on top of the joists, then used some 5mm thick insulating foam/underlay stuff before laying 20mm thick engineered oak floating planks

Its made quite a difference to the overall heat of the house, even  though we only used kingspan on a small part of the downstairs.