Evidence that extra loft insulation does anything

[WabbitPoo]

I'm intrigued as to the effectiveness of loft insulation.

To get straight to the point, I'm not convinced that some woolly glass fibre resting on plastered plasterboard (which must itself be a pretty good insulator) should make much of a difference. Therefore, to add layer-upon layer of extra woolly glass fibre is very much a case of diminishing returns.

Am I just being cynical?

Has anyone ever lived in an un-insulated room and then added loft insulation and really noticed a difference?

Genuine query.

 

[JohnD]

yes, and you are wrong.

I once had a neglected old house. In winter, the bedroom was unbearably cold (uninsulated L&P ceiling) although fairly draught free and not damp. I had a 3kW fan heater, and on a frosty night it ran continuously, without making the room warm.

I fitted glassfibre in the loft. That night the fan heater went off on its thermostat, and I had to turn it to 1kW as the room was too hot. A 1200W radiator was more than enough and it was never cold again.

I expect someone can work out the heat loss through a 16m2 ceiling of L&P at 20C difference. It's a lot.

 

[endecotp]

plasterboard (which must itself be a pretty good insulator)

Plasterboard = 0.19 W/mK; mineral wool = 0.038 W/mK i.e. 5 times better for the same thickness.
So 200mm thickness of mineral wool could be replaced by 1m thickness of plasterboard! And having just spent the afternoon carring sheets of plasterboard around, I know which I'd prefer to install.

to add layer-upon layer of extra woolly glass fibre is very much a case of diminishing returns.

Yes, it's absolutely a case of diminishing returns. That doesn't mean it's pointless though.
If you have X amount of time / money / whatever to invest in insulation you should put it where it has the greatest benefit. The point about loft insulation is that it is quite low effort, compared to e.g. walls, and the materials are quite cheap compared to e.g. windows.

 

[JohnD]

Plasterboard = 0.19 W/mK; mineral wool = 0.038 W/mK

so if say 16m2 at 20C, what is the calculation of heat loss?

can't be "U" x m² x °C

for plasterboard 0.19Wx16x20 = 60.8W
for mineral wool 0.038Wx16x20 = 12.16W

so how do you do it?

Where do you put the material thickness?

 

[rsgaz]

so how do you do it?

Where do you put the material thickness?

You've jumped ahead and skipped a step, metres squared doesn't come into it, yet. The material thickness is the 'm' from the equation. So plasterboard being half an inch is 0.0127metres.

https://www.omnicalculator.com/physics/thermal-conductivity

To get the 'heat flux', you want 0.19 * 20° / 0.0127m = 299 watts lost per square metre (for plasterboard only)
OR, 0.038 * 20° / 0.2m = only 3.8 watts lost per square metre (for the mineral wool, 200mm thick)

So, in your 16 metres squared room, that'd be 4787 watts lost for a whole ceiling of just plasterboard.

 

[JohnD]

thanks

that explains why the 3kW fan heater didn't get it warm.

So ceiling only losses:

for 12.7mm plasterboard 0.19Wx16x20/0.0127m = 4787W

for 200mm mineral wool 0.038Wx16x20/0.2 = 60.8W

Can that really be right or have I made a mistake? It looks incredibly low.

 

[rsgaz]

Can that really be right

Yup! I had to double check it myself earlier because it didn't look right.

But going from 0.19 to 0.038 is 5 times better and going from 12.7mm to 200mm is 15.7 times better, so multiply those factors together and you can see why the figures show that it is 79 times better!

 

[Notch7]

I'm intrigued as to the effectiveness of loft insulation.

To get straight to the point, I'm not convinced that some woolly glass fibre resting on plastered plasterboard (which must itself be a pretty good insulator) should make much of a difference. Therefore, to add layer-upon layer of extra woolly glass fibre is very much a case of diminishing returns.

Am I just being cynical?

Has anyone ever lived in an un-insulated room and then added loft insulation and really noticed a difference?

Genuine query.

Yes loft insulation is effective over just plasterboard.

Even increasing from 100mm to 250mm loft insulation is worth it. Diminishing returns, maybe but its one of the cheapest improvements.

 

[WabbitPoo]

Right, I'll get up there....

 

[alan333]

I added loft insulation to an old flat I once owned which had an un-insulated lath & plaster ceiling. It made a noticeable difference and was well worth doing. However, I recently posted the following on another thread:

Extra loft insulation doesn't save much IMHO. I'll explain...

We have a 4 bed detached house built year 2000, with loft area of about 64m2, which already had 100mm of insulation.

We got cavity wall insulation put in a few years ago, but declined the offer to get an extra 270mm thickness of loft insulation. A few months later I got a letter telling me how great my house now was, but that it could be much better with the extra loft insulation... and I could save UP TO a whopping £9.50 per year!

I reckon that some insulation is much better than none, but lots of insulation is only marginally better than some.

I decided the hassle of lifting the partial flooring and then not having storage space and access to wiring etc outweighed the potential benefits.

Link to that thread if you want to read it...
https://www.diynot.com/diy/threads/free-loft-insulation-questions.493394/#post-4041877

 

[motorbiking]

I found just 20mm of foam backed plasterboard made a big difference to a 1920s build with solid walls. Which reminds me, I must get round to celotexing the bay window in one of the front bedrooms the dwarf bay wall is single skin rendered brick.

 

[cwhaley]

I posted a thread about this recently. Loft insulation alone is only one factor in keeping the heat inside a home.

Parents had it done ~10 years ago and I definitely noticed the difference as I still lived there then (bear in mind they went from little/nothing up to 270mm). My house has 75mm of it and it's missing in large parts of the attic so will be having it topped up very soon by my energy firm.

I found just 20mm of foam backed plasterboard made a big difference to a 1920s build with solid walls. Which reminds me, I must get round to celotexing the bay window in one of the front bedrooms the dwarf bay wall is single skin rendered brick.

Definitely recommend this. Our upstairs bay window is also single skin and was previously very cold and damp in the winter. Since battening, insulating and plasterboarding over it's now a dry, warm wall we've had no problems with. Was my first proper DIY job in this house.

 

[John D v2.0]

Loft insulation will save more money due to heat rising. However floor insulation will help comfort more because although the warm air will stay in the room, the cold air will stay at the floor.
So as above, you need to take a holistic approach of cost of improvement against saving, plus comfort, and decide the best idea.
On cost, double glazing is a terrible investment, but in reality it can improve your draughts and condensation so be worth it for comfort.