Insulation of exposed wooden floor FROM BELOW

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Crediblehippo

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Evening all,

Firstly, Happy New Year!

I’m planning on insulating underneath the floor boards in my living room. 1900s terraced house so adequate crawl space to do from below, plus I don’t want to damage the floor by lifting up.

From research I understand best practice is a vapour barrier below floorboards (warm side), insulation of choice, breathable membrane (cold side).

My question is whether or not the vapour barrier is essential? As I don’t see how I can do it without lifting floorboards (which I don’t want to do). the house has has damp/rot issues in the past. All sorted now and crawl space is well ventilated but always a concern!

Any advice on method/materials etc greatly appreciated!!

thanks
Jordan
 
Jeez what a horrible job. If you use a Celotex like board it has a foil backing and you can slightly wedge cut the boards so they fit tightly with the beams. I wouldn't over do the insulation as these timbers won't enjoy being unable to breathe. 40-50mm tops maybe, unless you are going to a massive effort. That will give you a U-value of around 0.3 accounting for the floorboards.
 
As above, not a nice job.

You could always use loft insulation, pushed up to the underside of the Floorboards, staple chicken wire to the undersides of the joists to hold it in.

No need for any membranes, as the sub floor ventilation should take any moisture away. Don’t block the air vents!

celotex is twice as efficient (thermally) as rockwool, but more expensive. (y)
 
motorbiking said:
Jeez what a horrible job. If you use a Celotex like board it has a foil backing and you can slightly wedge cut the boards so they fit tightly with the beams. I wouldn't over do the insulation as these timbers won't enjoy being unable to breathe. 40-50mm tops maybe, unless you are going to a massive effort. That will give you a U-value of around 0.3 accounting for the floorboards.
Click to expand...

thanks yep fun weekend ahead for me! My concern with the board insulation is breathability of the timbers given that moisture can’t pass through the boards, what are you thoughts on that? I’ve read the materials need to be breathable , however not too breathable so that the heat is washed away by the wind. I’m also considering laying a DPM on the dirt floor to reduce moisture evaporating up from the ground.
 
Mr Chibs said:
As above, not a nice job.

You could always use loft insulation, pushed up to the underside of the Floorboards, staple chicken wire to the undersides of the joists to hold it in.

No need for any membranes, as the sub floor ventilation should take any moisture away. Don’t block the air vents!

celotex is twice as efficient (thermally) as rockwool, but more expensive. (y)
Click to expand...

it’ll be a rubbish job I know! Swaying towards this option but only concern with the chicken wire is the ‘windwash’ effect. I’ve read that the materials need to be breathable but not too breathable so as to reduce the insulating effects. That’s why I was going to use something like Rhino Ultra breathable membrane as it also has weatherproofing properties. What do you think?
 
Crediblehippo said:
Evening all,

Firstly, Happy New Year!

I’m planning on insulating underneath the floor boards in my living room. 1900s terraced house so adequate crawl space to do from below, plus I don’t want to damage the floor by lifting up.

From research I understand best practice is a vapour barrier below floorboards (warm side), insulation of choice, breathable membrane (cold side).

My question is whether or not the vapour barrier is essential? As I don’t see how I can do it without lifting floorboards (which I don’t want to do). the house has has damp/rot issues in the past. All sorted now and crawl space is well ventilated but always a concern!

Any advice on method/materials etc greatly appreciated!!

thanks
Jordan
Click to expand...
I've recently had this done. 1920's built house, solid walls, suspended floors.

The floor needed replacing due to historic woodworm and rot. I tool the opportunity to also fully insulate it.

Floor is now fully insulated with 50mm of celotex between the joists.

Minimal, if any, difference to how warm the room feels :(
 
Last edited:
Lower said:
I've recently had this done. 1920's built house, solid walls, suspended floors.

The floor needed replacing due to historic woodworm and rot. I tool the opportunity to also fully insulate it.

Floor is now fully insulated with 50mm of celotex between the joists.

Minimal, is any, difference to how warm the room feels :(
Click to expand...

Ah, I’m sorry to hear that! Very frustrating!

Did you use any kind of membrane or foam the gaps?

I’m really hoping it will make a difference in this particular room. It’s the coldest room in the house but the only one with exposed wood floorboards, so surely it will help somewhat at least!
 
Crediblehippo said:
Ah, I’m sorry to hear that! Very frustrating!

Did you use any kind of membrane or foam the gaps?

I’m really hoping it will make a difference in this particular room. It’s the coldest room in the house but the only one with exposed wood floorboards, so surely it will help somewhat at least!
Click to expand...
Because the whole floor was replaced, including the joists, we fitted battens between the joists to rest the celotex on and then foamed all of the joints.

I'm sure it would make a difference if you don't have any carpet down. We temporarily only have a large rug on top of the floorboards where we used to have carpet while we wait for the carpet to arrive. The floor is not as cold underfoot as it was, but the room feels no warmer.
 
silly point - maybe the room feels no warmer because you have a thermostat? The key test is if your bills go down ;)

Generally speaking, its better to put 50mm in the walls than 50mm under the floor.
 
Mr Chibs said:
Then insulation is the only way to go.
It won’t make a cold floor warm, but will slow down heat loss.
Click to expand...

Thanks. I’ve started the job and as we all expected it is very bloody tough! Will let you know how I get on.
 
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