Cheapest practical way to insulate under EXISTING wooden floor?

andrewmoorcroft

I'd like to insulate under my existing downstairs wooden floor. It has 400mm spaced joists covered with boards. I can access the underside so I dont want to bother getting the boards up if possible. The area is well ventilated and dry. There are a few areas where i will have to crawl around underneath.
What is the best and economical insulation to use? As i will be in an enclosed area when fitting I was thinking of the 'space blanket' type insulation or polystyrene blocks but not sure how to make it stay in place. I suppose i could silicon or staple them in place.
Anyone got any ideas on the best stuff to use and how to do it.
Thanks.:beer:

bennymac

the normal way is if the floors are up you lay down some netting over the joists and the insulation rolls are suspended in the netting.
i suppose you could put the netting on the underside after fitting the blocks but it all depends on space.

Errata

Been there done that ! Did it from underneath using loft insulation and garden netting fixed to the joists with battens.

economiser

The best stuff would probably be Kingspan or similar. Its rigid, a good insulator and easy to support but expensive. Fibreglass would be cheap but difficult to support and difficult to handle in a confined space. Polystyrene would be easy to install, cheaper than Kingspan but not so effective as an insulator. Kingspan or polystyrene could be simply supported by resting it on nails in the joists under the sheets.

ormus

ive used chicken wire before now.
almost anything that holds it up, and is free or cheap, will do. simple wooden laths nailed up every few feet, will do it.

evilgoose

I've been considering doing simular, I'd suggest the rockwool option and battons to hold it in place.

Could a 'solid' insulant such as the kingspan or polystyrene sheet cause a bit more noise from foot steps and the like? The rockwool has some accoustic deadening properties.

Canucklehead

Hi

http://www.selfbuilder.net/products/insulation_pic_non-itch_loft.html


This is Roger Bisby (on the right) using Non Itch by YBS insulation.http://www.ybsinsulation.com/home.htm
He stapled the empty bags across the joists to prevent sagging .
The article was in a freebie trade mag but I can't find it online.
It made a huge difference to the warmth of the house.

Corgi Guy.

27col

The main thing, is to ensure that whatever insulation you use is held tight against the bottom of the floor boards. If it is allowed to slip at all there will be a draught of moving air against the bottom of the boards. This will completely negate the effect of the insulation. The principle of insulation is to maintain a mass of still air. I would recommend this basic precaution to anyone else who is proposing to insulate under a floor.

andrewmoorcroft

27col wrote: »

The main thing, is to ensure that whatever insulation you use is held tight against the bottom of the floor boards. If it is allowed to slip at all there will be a draught of moving air against the bottom of the boards. This will completely negate the effect of the insulation. The principle of insulation is to maintain a mass of still air. I would recommend this basic precaution to anyone else who is proposing to insulate under a floor.

Not according to this website! http://www.secondsandco.co.uk/page_show/read_solidground/index.html

27col

If you want an insulated space for pipes and services, then ok. If the ends are open there will not be an insulated space, just a flow of air. Which is not what you are trying to achieve.
If the ends are closed then possibly the flow of air will be reduced and there will be an insulating effect. I know which I prefer.

andrewmoorcroft

27col wrote: »

If you want an insulated space for pipes and services, then ok. If the ends are open there will not be an insulated space, just a flow of air. Which is not what you are trying to achieve.
If the ends are closed then possibly the flow of air will be reduced and there will be an insulating effect. I know which I prefer.

And me ... an air gap with capped ends as advised to me by building consultancy.
I'm using corriboard (plastic cardboard) stapled underneath the joists (wall to wall), 6" enacapsulated insulation in the 8" joist space (wall to wall and curled up at the ends by the wall), and my insulated heating pipe work is in the 2" air gap.