Attic Insulation, Engineer's Perspective?

random.stranger

Hi All,
I know there's threads running on this in the DIY section, but I would like to get input from fellow engineers, if possible.

I live in a bungalow that was built back in 1979. There was originally 100mm of insulation in the attic, this has since gone down to about 50mm.

The house gets very cold in the winter (it is double glazed, has cavity wall insulation and we just had a condensing boiler put in).

So, I would like to top up the insulation in the attic to bring it up to around 300mm & floor over it for storage.

My main concern with this is about running insulation over wiring & the possibility of wires overheating (I read about the possibility of this on the Sustainable Energy Authority's website), the cables have very little slack & could not be run over the insulation without rewiring. I would also have concerns that accessing cables, if and when necessary in the future would be difficult buried under all that insulation.

So, these are the options I have come up with so far:

1. Just insulate over the wires like most people do.
2. Put "zip on" cable tidy over the cables in the attic to give them "room to breath" & insulate over them
3. Rewire the high current sockets, such as in the kitchen, also rewire the cooker and electric shower. Insulate over remaining wires.
4. Complete rewire, including lights. (sockets are ok & fuseboard has recently been updated)

If anybody has any thoughts/opinions/alternatives that they would like to share, I would appreciate it.

bnt

Whether or not the wires overheat depends on how loaded they are: the worst case is heavy current in a thin wire. I would not want to generalise, I would want to know how hot each wire gets. They shouldn't be getting more than slightly warm, if specified correctly for the load according to e.g. this chart, but I wouldn't be happy assuming anything.

Example: a light fitting with 3 x 100W bulbs pulls about 1.2 A, which isn't such a lot, but if the wire is old, borderline or under-specified already, it could be a problem.

Mr Marri

Are the wires going to have insulation on one side or both sides?

Might be a good idea to fit a fire alarm in the attic incase we get it wrong!

Turbulent Bill

I know sod-all about household wiring, but would be inclined to go for the complete rewiring option (or at least respliced cables in the attic section). It might be expensive, but you'll get to check the 30+ year old cables, route them exactly where you want and make the attic easier to insulate/floor. Also easier to rework in future, better fire safety, easier to sell the house if you want to etc.

From what I know the main overheating problems come from older, smaller-gauge cable connected to bigger-than-design loads, which heats up the wire. This can cut through insulation like cheesewire, which again raises the wire temp etc. It's unlikely that a 1979 house would have this problem, but you never know...

Your local electrician is the way to go here.

random.stranger

Thanks everyone for your replies.

bnt:
Thanks for the link. I am happy enough that the cables have been correctly sized, I wouldn't have considered them overheating until I came across it on the SEAI website.

Mr Marri:
The wires for the lights would only have insulation going over them, but the wires for the sockets, shower and cooker would be running over the joists, so they would have insulation going over and under them. I think the fire alarm would be a great idea, I'll definately do that (& test it to check it can be heard in the living quarters).

Turbulent Bill:
I'm inclined to agree with you, I would prefer to get the job done properly and rewire the house completely. Unfortunately, money is the problem with this one, I had a look through forums and came up with a very rough figure of €3k to rewire a 4 bed bungalow (I could be miles off).

I had a look at the cables and they look like they are about 2.5mm^2 in a ring main. I really like the idea of splicing the cables, as it seems to be a very good compromise. I could also check that there is no breaks in the ring main by doing a continuity test when the cable is cut.

Anybody by any chance know if splicing the cables would void house insurance?

Thanks a million for the help.

Turbulent Bill

random.stranger wrote: »

Anybody by any chance know if splicing the cables would void house insurance?

Probably dependent on the policy (check the T&Cs), but at a guess the work would have to be done by a registered electrician. Assuming that the attic access is ok, you have the new cabling routes planned and measured etc. it shouldn't take long to do or be anything as expensive as a rewiring.

For piece of mind (and a second pair of eyes on the job) I think it's worth getting a pro to at least check out the job, rather than diving in yourself. No reflection on your skills, just prudent given the potential danger.

random.stranger

That makes a lot of sense, thanks very much Turbulent Bill.