Underfloor Insulation

[Al1234]

So now that winter is kicking in it is becoming apparent that my new house is lousy at retaining heat. It's had loft insulation redone in recent years, cavity wall insulation put in and new double glazing. However, it does have a very large void underneath, the property is on a hill and therefore there is a very large volume of cold air sitting underneath.

Property is a 1930's detached house.

Having looked at various options I think I am decided on something like Knauf Earthwool as an insulating material held up with plastic strapping and staples. The logic behind this is the in the varying conditions, at the front the void is about 4 foot, at the rear less then 2. Rolls of chickenwire, as suggested here would be a nightmare.

The joists are unevenly spaces as well, various between 300mm and 400mm so solid boards or batts will give potential for lots of waste.

Where I get confused is when sites talk of vapour barriers/mebranes, air gaps etc. Am I wrong to think I just pin the insulation up between the joists, obviously making sure all the air vents and unobstructed?

I'm aware I need to try and keep cables the void side of the insulation, is there anything else to be aware of?

 

[JohnD]

do you have carpets, or bare boards?

How old is the house, and how draughty?

Do you close the internal doors during the heating season?

Which rooms are coldest?

Some varieties of Knauf mineral wool are treated with Ecose, I strongly recommend it, it prevents them shedding irritant dust and fibres. The wool is brown, not yellow, and the wrappers state "Ecose" even if they are sold as own-brands.

Insulate all pipes while you are down there, and clean up rubbish.

Cables can be above or below the insulation, exposed to the air on one side. They must not be encased in insulation.

 

[Al1234]

Thanks, carpets throughout the ground floor, except the extension at the back which is solid.

House is 1930's and the two main reception rooms are the ones with the void underneath. The ground floor is quite open though, through fireplace between the two main rooms.

Had planned to have a good tidy up whilst I was down there, lots of old cables and pipes still in situ or just dumped.

Heating pipes are insulated but may well replace these as well as they are looking pretty tired.

 

[JohnD]

losses through ground floors are worst with bare boards, because it blows in as cold draughts. There is no convection downwards. So carpets usually block it. Look out for draughts especially round the skirting, and if you want to make a start, begin with mineral wool round the edges where there may be gaps. Fireplaces encourage draughts so use a balloon in any that are out of use. Draughts coming through the floor show up as black dust stains on the carpet.

Open plan encourages draughts, and lets warm air rise upstairs.

It's a good time to add an access hatch in a corner away from the door. You can install a floor safe too, if you like.

 

[Al1234]

Thanks, don't need an access hatch as there is a door under the house, as I said it's on quite a slope.

Any advice on the membranes, some say yes others say no, I only intend to do this work once so want to get it right first time.

 

[Al1234]

So I've stripped out the old pipe work and parts of at least two previous electrical installations, measured up and am thinking about ordering insulation.

So the depth of the joists is 5 inches, so about 120mm however most insulation is either 100mm or 170mm. Practically it makes sense to use 100mm but how to fix it, flush with the bottom of the joists (A) or flush with the floorboards (B). I would have thought it probably more effective flush with the boards, but a bit more of a faff to install/

 

[endecotp]

You should be able to find 150mm - squeeze that in. It's very compressible, and/or it can hang out a bit.
Rockwool flexi slabs are supposed to be available 120mm thick but may not match your joist spacing.

 

[pilsbury]

I too am looking to insulate my ground floor as I will just be having floor boards. Thinking of going the rock wool route like you but don't have the luxury of the space you do. I recon under 2' so it's gonna be a squeeze.

What are the plastic straps you are thinking of using? A roll of chicken wire is no good for me.

 

[Al1234]

Hi, I'm going use polypropylene strapping like you get on the outside of packing boxes, can be bought on the reel.