Under-floor insulation

[SomeUser]

Since I'm going to have the floor up anyway...

What is my best/easiest/cheapest way of insulating my floor? For reference it's a suspended floor the top layer of which I'm having to replace (see other thread). Access from above only.

 

[endecotp]

Mineral wool.
If it’s a ground floor with a void below, fix some sort of netting to the joists to hold it in place.

 

[Steven Prentice]

Where is the best (cheapest) place to buy mineral wool insulation at the moment?


Oh, I'll go with mineral wool insulation - I'll assume that the floor you are insulting under is the ground floor. The ground floor... your escape route in the case of a fire. Now if you need any convincing to use mineral wool on your excape route pop a bit of mineralwool on a fire, some polystrene, and other solid insulations, sheeps wool if you want to be green. Then have a look and consider which fumes you want to try to excape through... (hintt - the mineral wool doesn't burn, the solid insulations = lots of balck smoke, I know, I did this)

 

[geraldthehamster]

Cheapest - mineral wool. Most effective, as well as less messy to lay - Celotex or similar board insulation cut slightly under size, fixed between joists with expanding foam.

As to fire, if your ground floor floorboards are burning then the rest of the building is pretty much gone already

 

[Steven Prentice]

.. underfloor insulation on fire... I'll give you an example from my house. The Electric meter, and consumer unit are next to the front door at the base of the stairs (this cannot be to unusual). The cables go up the wall to go upstairs and down the wall.. underneath the floor to the downstairs sockets and so on, plus 1 cable that isn't protected by an RCD device to the garage. All of these cables will pass directly underneath any underfloor insulation I install.. and any seriopus overheting of these cables could start a fire.. not yet to burn the floor boards but enough to give off the nasty black smoke. Maybe I am just paranoid.

 

[oldbutnotdead]

Rockwool is the most cost effective by a massive margin (ignoring labour costs) and has the brucie bonuses of not sustaining fire and not emitting toxic fumes either on fire or not . As long as you've got the depth..... also much easier to fill odd gaps with rockwool and far more likely to achieve the design U value (the solid sheets work best in modern builds with parallel joists and plumb walls)

(Calcs. Celotex/whatever at the moment is about £30 for an 8 x 4 x 100mm, so each square metre of floor insulated to building regs minimum will cost you £10.41. Rockwool to 170mm- meets building regs minimum- will cost £3.24 (you need to add a bit to cover cost of netting under the joists). Across 50 sq m, which is about average, that comes to a saving of £350 along with the not going on fire bonus and much easier installation

 

[geraldthehamster]

any seriopus overheting of these cables could start a fire

I grant you they could. However I think that's vanishingly unlikely. Celotex is routinely used in loft conversions, and wouldn't be specified if it were unsafe. Under a suspended floor it's less messy to install, tidier and more effective. It also provides a vapour barrier, not that this is essential in a floor.

There have been many threads in this forum, discussing both methods, and how to do them. Search for suspended floor insulation