Slab loft insulation question

[Brian Lacey]

I would like to add a little extra insulation into the loft of our bungalow.
I can only access part of the loft space, not the whole lot due to the design of the building (later extensions to original build.)
The roof is a low chalet style., I am not tall but even at the apex of the roof line I am stooping over, space is very limited and difficult to work in. I will not be able to use rolls of insulation as I simply cannot get it in the very shallow space. I was wondering about polystyrene slabs (cut into strips so they fit through the hatch) and then pushing them over the existing insulation to reach the extremes. The fire hazard aspect is a concern though so any slab alternatives I should consider ?
Many thanks.
Brian.

 

[hllns]

Rolls aren't as bad as you think once they're unrolled, my eaves were very tight and cramped so I lifted up strips of loft roll sliding them sideways using long thin bits of timber. You typically have to work lying down doing this kind of job.

Polystyrene isn't great, if there's any electrical cable that it could touch it would degrade the cable making the insulation brittle and unsafe.

Rockwool batts might be a better choice.

 

[pcaouolte]

I spent some time at the beginning of last winter increasing my 100mm loft insulation to 370mm by laying 270mm fibreglass roll over the top of the existing. This has made absolutely no detectable difference to my heating bills.

 

[Brian Lacey]

Thanks all.I will have a look at options.

 

[catlad]

This has made absolutely no detectable difference to my heating bills.

Energy prices increased a lot recently.

 

[pcaouolte]

Energy prices increased a lot recently.

Agreed, and I know I said bills but I actually have a system set up to record my gas usage on an hourly basis with records going back several years. I was disappointed that topping up the loft insulation made no detectable difference to my gas usage. I expect that the first 100mm of insulation does much more than the next 270mm as very little heat makes it through the first layer to be trapped by the next layer.

 

[Dereekoo]

Agreed, and I know I said bills but I actually have a system set up to record my gas usage on an hourly basis with records going back several years. I was disappointed that topping up the loft insulation made no detectable difference to my gas usage. I expect that the first 100mm of insulation does much more than the next 270mm as very little heat makes it through the first layer to be trapped by the next layer.

Heat is constantly being lost through the insulation and it all depends at what rate, the thicker the it is the slower the heat loss and so like for like a 300 roll will pass heat through at 1/3 of the of the rate of a 300 roll. Doing a U value calc on a ceiling with 100 and 270 thickness insulation gives values of 0.41 and 0.16 respectively

 

[robinbanks]

This has made absolutely no detectable difference to my heating bills.

Might your gas prices have gone up? Did you look at gas units used usage or pounds and pence paid? It's a very short sample period so far; we may be having a much colder period than the same time last year. All in, so long as you don't have some other significant problem like excessive draughts from windows, doors, floors and eaves vents bypassing the insulation it should have had some discernible effect that a scientific approach to testing would pick up

From the reasonable figures Derekoo provided if you had 100 sqm of ceiling and a house at 20 degrees with a world at 0, your 100mm of wool loses 820w. The extra insulation drops that to 320w. In other words, it's the equivalent saving of not having to run a 500w heater constantly to replace the lost heat

 

[Brian Lacey]

Could I just ask another question please. Rather than using slab insulation would the foil backed rolls offer a more effective insulation barrier ? Should be much easier to get in place.
Bear in mind that we are in a bungalow so room ceiling is loft floor.
Thanks
Brian.

 

[robinbanks]

Rather than using slab insulation would the foil backed rolls offer a more effective insulation barrier ?

I'd go for PIR/PUR boards over. Foil backed wool isn't really any different to nonfoiled wool

The fire hazard aspect is a concern

If you haven't left the house well before a fire has burnt through the plasterboard and wool to reach the polystyrene, you'll almost certainly be dead

 

[hllns]

I'd go for PIR/PUR boards over. Foil backed wool isn't really any different to nonfoiled wool

If you haven't left the house well before a fire has burnt through the plasterboard and wool to reach the polystyrene, you'll almost certainly be dead

The fire aspect isn't a concern for polystyrene but what it does to PVC is.

Another concern with insulation and power cabling is any non-lighting electrical cabling in the loft, if it's covered by insulation it can't carry as much current, are the ring mains and other electricals fed from above?

 

[Brian Lacey]

Well, I'm no electrician but being a bungalow basically built in late '70's from what I can see it is that a lot of cable appears be under the existing mineral wool with some later cables on the top.