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Another insulation question

Does what it says on the tin
PinkDalek
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Re: Another insulation question

#153382

Postby PinkDalek » July 19th, 2018, 1:17 pm

Snorvey wrote:https://www.uswitch.com/insulation/guid ... te-a-loft/

The reason I ask is that the loft has a fairly large amount of crap in it and one heavy electrical shower cable that runs diagonally across the whole loft.


Not an answer to your warm/cold loft insulation question and I'm sure you spotted it but your link includes:

If there are electric cables in the loft, try to leave these exposed so they can remain cool. If there is enough slack, they can be gently raised and the insulation put underneath. ... Shower cables are most likely to need attention, although the fire risk is fairly minimal as showers tend to only be used for short periods. If you see any cables or junction boxes that appear to be in a poor state, you will need to get an electrician to put them right, anyway.

kiloran
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Re: Another insulation question

#153443

Postby kiloran » July 19th, 2018, 4:16 pm

PinkDalek wrote:Shower cables are most likely to need attention, although the fire risk is fairly minimal as showers tend to only be used for short periods

I have failed miserably in getting my wife to accept that. :(

--kiloran

Maroochydore
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Re: Another insulation question

#153466

Postby Maroochydore » July 19th, 2018, 5:50 pm

We had ours laid over the existing on one of the freebie deals.

The old insulation sat within the joists allowing me to put staging down for storage. We were instructed that the area had to be cleared for the company to do the work so we removed all the items stored up there as well as the staging.

The company slapped another 10 inches of insulation on top of the old so the loft gave the impression that you get when you look out of a plane window at 33 000 feet, just soft fluffy clouds as far as the eye can see.

The downside was there is no way we can put staging back or store junk in the loft without compressing the insulation which I guess negates it's effectiveness. There are little supports you can now buy that sit on top of the joists to take boarding but even those don't protrude above 'the clouds' so effectively we've lost the loft as a storage area although the difference in heat loss makes up for the inconvenience.

gryffron
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Re: Another insulation question

#153479

Postby gryffron » July 19th, 2018, 6:37 pm

Snorvey wrote:is it better/easier to add new insulation under the roof itself (i.e. what they call a 'warm loft' - I'd still leave the original stuff between the joist down though)

Rafter insulation is MUCH more expensive. And only really worth the cost if you want to use the loft space for something.

Gryff

pochisoldi
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Re: Another insulation question

#153481

Postby pochisoldi » July 19th, 2018, 6:43 pm

Maroochydore wrote:The downside was there is no way we can put staging back or store junk in the loft without compressing the insulation which I guess negates it's effectiveness. There are little supports you can now buy that sit on top of the joists to take boarding but even those don't protrude above 'the clouds' so effectively we've lost the loft as a storage area although the difference in heat loss makes up for the inconvenience.


Loft legs/Loft storage stilts. Either screw or clip and screw onto the joists at appropriate intervals, then put loft boards on top.
The clip and screw ones are more expensive.

PochiSoldi

midnightcatprowl
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Re: Another insulation question

#153506

Postby midnightcatprowl » July 19th, 2018, 8:03 pm

We had ours laid over the existing on one of the freebie deals.


I'm interested in what you said as I had never realised that any of the freebie deals would be available to add to insulation rather than start from scratch. Like Snorvey I live in a relatively modern house built around 1983 or thereabouts and I moved in when it was just a couple of years old. By the standards of the time the loft insulation was good i.e. there was loft insulation! By the standards of now it is inadequate.

I'm quite content not to use the loft for storage but a DIY approach is beyond me. N.B. A friend who moved to a 'modern' though by no means new house in Wales found it impossible to get any company to quote for adding to loft insulation, they would only consider 'starting from scratch' type jobs.

Dod101
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Re: Another insulation question

#153511

Postby Dod101 » July 19th, 2018, 8:21 pm

What is a shower cable? Does that take the electricity to an electric shower? If so I can relax as I do not have one.

Furthermore, I really wonder about keeping electric cables in general cool/cold in the loft because there must be loads of electric cabling around my house, not in the loft, which will get warm or cold as the seasons progress so what is so special about electric cables in the loft?

What thickness of insulation is recommended in a loft?

Dod

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Re: Another insulation question

#153521

Postby MyNameIsUrl » July 19th, 2018, 8:49 pm

Regarding laying new insulation on top of old, clearly there is no need to fit the insulation between joists, but where can I get rolls of insulation which isn't pre-cut? Last time I tried this the roll of insulation just fell apart into narrow strips and it was a very difficult job to get it laid neatly without gaps.

AleisterCrowley
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Re: Another insulation question

#153544

Postby AleisterCrowley » July 19th, 2018, 10:49 pm

From what I read, it (shower cable) may present a fire risk if routed under the insulation (mine isn't)

News to me, as a lot of power cables are embedded /buried
I'm sure the wiring regs cover this. If only we knew a highly skilled electrician.... (CHRIS!!)

JMN2
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Re: Another insulation question

#153562

Postby JMN2 » July 20th, 2018, 6:58 am

Insulated loft hatch! Of course! I need to do that + some kind of a gasket around the hatch frame first before I start looking into if extra insulation is needed in the loft. Neighbour told me the previous owner did carry tonnes of insulation into the loft so my problem might be just the hatch. Thanks Snorvey for the idea.

pochisoldi
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Re: Another insulation question

#153620

Postby pochisoldi » July 20th, 2018, 10:22 am

AleisterCrowley wrote:From what I read, it (shower cable) may present a fire risk if routed under the insulation (mine isn't)

News to me, as a lot of power cables are embedded /buried
I'm sure the wiring regs cover this. If only we knew a highly skilled electrician.... (CHRIS!!)


You choose cables on the basis of current carrying capacity.

The current carrying capacity is down to two factors - resistance (thickness and length) and the ability to dissipate the heat caused by that resistance.

The thickness bit of the resistance factor is understood by most people - they know that if they want to wire a 32A ring main, they need to use 2.5mm sq twin and earth, rather than the 1mm or 1.5mm they might use for a lighting circuit.

Less well understood is the length factor - a long run requires the derating of the cable. (e.g. treating a 2.5mm cable as having a rating of 20A rather than 32A).

Even less well understood is that a cable generates heat, which needs to be dissipated. You can get heat dissipation where you might have thought it impossible. Sticking a cable between two layers of insulation means you really have to derate the cable, as the heat dissipation becomes negligible.

When I reinsulated my loft the lighting cables were across the insulation. I worked on the principle that a cable nailed to a joist dissipated a lot more heat than a cable sandwiched between insulation. I also worked on the principle that an upstairs lighting circuit designed to take four 100W lightbulbs, wasn't going to struggle with 4x 10W CFL/LED bulbs. So I moved the wiring onto the joists and put the insulation down.

This approach will NOT work for a shower cable.

There are many shower installations that were designed and the cable sized for a 7.2kW shower.
Then the shower needs replacing and the homeowner thinks "I don't want a tepid dribble, I want a 8.4kW or 10kW shower".
A naive homeowner or a feckwit installer then looks at the current install, sees 6mm cable and thinks "Woohoo - we're good for 10kW here".
They don't realise that it's 6mm instead of 4mm because the cable run goes halfway around the world to get to the bathroom and needs to be thicker because of the length (more L = more R = more heat)

So before they've started, the cable in the loft is already pushing the envelope, but given that one side is lying flat in free air, and they don't have a teenager who takes 20minute showers, they get away with it.

Sandwich the cable between two layers of insulation...

I would suggest that a shower cable running on top of insulation needs to be rerouted, noting that if the drops to the shower unit and consumer unit are retained, care needs to be taken when using junction boxes to connect the replacement cable - otherwise you replace the risk of overheating cables with the risk of a high current resistive connections causing smouldering/burning/fire in the junction boxes ("Bright and tight" is the rule).

For the record, I don't have an electric shower - that circuit was repurposed to power for a shower pump and the original MCB was replaced with a 6A jobby (looks a bit incongruous with 4mm cable!)

PochiSoldi
(I am not a qualified electrician disclaimer applies here..)

AleisterCrowley
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Re: Another insulation question

#153623

Postby AleisterCrowley » July 20th, 2018, 10:35 am

If you have a long cable run, isn't the dissipation per metre going to reduce?
The cable resistance will be higher, so more energy dissipated in cable, but current lower so in a unit length the I^2.R loss will be lower

pochisoldi
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Re: Another insulation question

#153645

Postby pochisoldi » July 20th, 2018, 11:59 am

AleisterCrowley wrote:If you have a long cable run, isn't the dissipation per metre going to reduce?
The cable resistance will be higher, so more energy dissipated in cable, but current lower so in a unit length the I^2.R loss will be lower


Yes - but in this case the amount of current flowing is dominated by the load, not the resistance of the cable.

From a fag packet calculation, comparing a 7.2kW load (having a nominal resistance of 8ohms), with a 50m or a 25m run of 4mm cable the heat generated by a metre of cable is 6.82W (in a 50m run), and 7.18W (25m).

The delta is 0.36W, IMHO it doesn't make a marginal long run of cable any more safer than a marginal short run of cable.

PochiSoldi

AleisterCrowley
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Re: Another insulation question

#153652

Postby AleisterCrowley » July 20th, 2018, 12:21 pm

Yes, take your point. Cable resistance is lower than I remember (used to dealing with 7/0.1 wire to Def Stan 61-12!)


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