Insulating my Victorian suspended floor

[liam546]

I have an Victorian suspended floor which sits about 2 feet off the earth. I am looking to insulate beneath the floor boards.

My sub floor is well ventilated with some recent additions of a few more air bricks. I've looked into using celotex between the joists but am worried that thus material may not be breathable and therefore moisture and condensation could penetrate the joists. From what I've read I need to be using a breathable material like mineral wool as this is permeable and will let the floor breathe. Although I have still read that if I use wool I should also install a breathable membrane or permable vapour membrane.

If you can offer any expert advice on the best practice to insulate a Victorian suspended floor I would be very grateful [/list]

 

[noseall]

The idea is that all the moist air (which is internal by the way) stays on the warm side - i.e. internally.

Drape some breather membrane between the joists, friction fit the foil faced celotex then foil tape or mastic and/or foam all the joints.

Anything that gats past the vapour barrier will be dealt with via the sub floor vents.

 

[liam546]

Is the celotex a permable material then? From what I have read you should use mineral wool or hemp as this is breathable and permeable.

Can I ask why you chose to use Celotex board and what it's properties are?

 

[TheVictorian]

I would use mineral wool (and have done so) personally as it is cheaper and also will allow any spillages to pass through it. Only a small percentage of heat is lost through a floor so whilst it is worth doing there is no point going overboard.

I used chicken wire with 170mm of wool on top.

 

[liam546]

Once we have insulated beneath I plan to put a new wooden flooring over the existing Victorian floor.

Can you foresee any issues with this?

 

[TheVictorian]

I wouldn't bother relaying the original floor in that case and would fix sheets of ply down to give you a better more level base, it's also quicker.

 

[geraldthehamster]

Is the celotex a permable material then? From what I have read you should use mineral wool or hemp as this is breathable and permeable.

Can I ask why you chose to use Celotex board and what it's properties are?

You don't want the insulation to be permeable, you want a vapour barrier to prevent moisture in the warm air inside the room penetrating the insulation and condensing in it. Celotex is usually foil covered, which provides the vapour barrier.

Cheers
Richard

 

[nebjamin]

Drape some breather membrane between the joists, friction fit the foil faced celotex then foil tape or mastic and/or foam all the joints.

Why would you bother with the breather membrane and not just fit the celotex directly?

Can't see what benefit it would add since the Celotex is foil backed anyway.

 

[liam546]

Am I right in saying celotex isn't breathable either? ...and that's why hemp or wool is recommended for Victorian houses with these suspended floors that need to breathe?

 

[nebjamin]

Am I right in saying celotex isn't breathable either? ...and that's why hemp or wool is recommended for Victorian houses with these suspended floors that need to breathe?

As long as you have good enough airflow (via airbricks) under your floor, then IMHO you don't need to allow any 'breathability' between above and below the floor.

I think rockwool is generally specified because it's cheaper than Celotex/Kingspan.

I am intending to do this job to my own 1930s suspended floors, which get bl**dy cold when it's cold outside despite thick underlay and thick carpet. With all the disruption necessary to do it (remove furniture, remove carpet, etc), I am sure as hell going to put the best stuff I can get down there, even if it is over specified.

IMO rockwool is not the best solution unless you have a breather membrane underneath it to stop the cold air wicking heat away from the wool.

I have found that houses vary wildly. Our floors get freezing, whereas a neighbour up the road also has suspended floors but they are usually warm. Neither are insulated. All I can assume is that we have more airbricks.

As I said - I am intending to insulate my floors with celotex/kingspan. Definitely do not want to be having to redo that job any time soon so the extra cost of rigid insulation over rockwool (a few hundred quid?) is insignificant compared to the time it'll take to do it.

 

[geraldthehamster]

I am intending to do this job to my own 1930s suspended floors, which get bl**dy cold when it's cold outside despite thick underlay and thick carpet. With all the disruption necessary to do it (remove furniture, remove carpet, etc), I am sure as hell going to put the best stuff I can get down there, even if it is over specified.

It's what I used. Are you going to need to remove the floorboards, or do you have enough space to work underneath?

Cheers
Richard

 

[nebjamin]

It's gonna have to be a full pull the floorboards up jobby. Not looking forward to it. Will probably be next summer now!

 

[liam546]

Im going to remove all the floor boards too, as I need to run plumbing and electrical cables too.

Once I remove the floor boards ill be able to see the condition of the joists, is it worth trating them whilst the floor is already up? Im worried about potential dry / wet rot or woodworm

 

[geraldthehamster]

Im going to remove all the floor boards too, as I need to run plumbing and electrical cables too.

Once I remove the floor boards ill be able to see the condition of the joists, is it worth trating them whilst the floor is already up? Im worried about potential dry / wet rot or woodworm

Probably not, if you have decent ventilation via air bricks. Have a look once the floor is up.

Cheers
Richard

 

[liam546]

I did a bit more investigating today and removed a few boards.

My void is only 22cm deep measures from ground to the bottom of the joist. This is less than a foot, is this sufficient?

The joists look ok considering they are over a 100 years old although some have signs of woodworm and some are a little rotten but they feel dry. See pic below.

In still undecided on celotex or a mineral wool but which ever I use I'm worried there will be insufficient space to run elec and plumbing above the insulation so can I run this beneath the insulation in the sub floor and then up through the insulation? Wound this cause a fire risk as it would be coming through insulation.

Any advice much appreciated

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• liam546
• 12 Sep 2013
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• liam546
• 12 Sep 2013
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• liam546
• 12 Sep 2013
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