25 year old 100mm thick fibreglass insulation... keep or replace?

I have a house that currently has 100mm thick itchy fibreglass insulation in the loft which was put in 1988 when the house was built.
I have bought 8 rolls of 170mm ecose insulation to add as a second layer to the existing layer at right angles. This then will result in a total insulation of 270mm in total.
My question is this:
Is it worth my while replacing the current 25 year old 100mm thick layer of insulation with a new layer of 100mm ecose insulation before I place the 170mm insulation on top?
I'm thinking moisture absorption, age related ageing and whether the existing insulation has a higher thermal conductivity figure than newer modern equivalents?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tuesday, November 12, 2013 11:06:32 PM UTC, Stephen wrote:

Probably not.
In any case it will be a horrible messy job.
If you want to buy an extra 100mm new insulation just add that on top of everything else, although you are probably at the point where returns start diminishing.
Owain
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Well everyone I asked about this some years back told me not to bother about taking out the old stuff, and indeed when I finally had it done for free under a scheme, the guys said the difference was going to be negligible and the mess caused in doing it not trivial, so they did not bother. Brian
--
From the Sofa of Brian Gaff Reply address is active
"Stephen" <i_love snipped-for-privacy@spam.com> wrote in message
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 12 Nov 2013 23:06:32 +0000, Stephen wrote:

I doubt it and it'll be a horrible job, lifting and shoving into lots of black bags for disposal and filling the car with glassfibre dust...

Glass? Absorb water?

If it's still "light and fluffy" not collapsed down or squashed it's fine.

Like for like I doubt there is much difference, certainly not enough to justify removing/disposing. If you were going to use 100 mm of PIR foam in that would be a different matter.
--
Cheers
Dave.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 12/11/2013 23:06, Stephen wrote:

Probably not. If the current stuff is flat(ish) and covers all the bits it should, leave it alone and add more.

Can't see there being much in it.
--
Cheers,

John.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The ichy stuff is glass wool. Not quite as effective as the modern mineral wool. No reason why you can't top it up with somemore.
The problem with enormous thicknesses of soft insulation is you loose the storage space. You need to consider some sortof raised platform to access/make use of the loft.
Be aware that there is a suspicion that glass wool is a health hazard in a similar way to asbstos. Could be the next big dramatic lung cancer thing. http://en.wikipedia.org/wiki/Glass_wool#Health_problems The debate is ongoing. We are in the denial phase at the moment. May not be a problem.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wednesday, November 13, 2013 4:57:43 PM UTC, harry wrote:

Glass fibre was found to be biosoluble in the lung, so safe.
NT
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 14 Nov 2013 12:01:08 -0800 (PST), snipped-for-privacy@care2.com wrote:

Sure, sure. Not a problem then. Come back in twenty or thirty years and let us know.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sunday, November 17, 2013 5:55:26 PM UTC, Grimly Curmudgeon wrote:

I think the issue has been examined over a longer time frame than that
NT
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

You need to e a bit careful about covering electric wiring. Heavily loaded circuits can overheat if buried in insulation. In theory anyway.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 13/11/2013 17:00, harryagain wrote:

That can do it in practice as well. Fortunately most circuits in the loft are lighting ones, and there the cable is sufficiently over specced for it to not usually be a issue.
You need to pay more attention with power circuits, and shower feeds. Try not to cover them, but if you must, then clip them to a joist first.
--
Cheers,

John.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 13/11/2013 17:40, John Rumm wrote:

How will that help? Wood isn't going to conduct heat away and the insulation is still covering the cable. They would be cooler under the insulation on the plasterboard as its a better conductor of heat.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 13/11/2013 17:46, dennis@home wrote:

It acts as a heat sink to a certain extent, that is why its recognised as a different installation method from simply covered in insulation.

Yup in contact with PB is better as well - although it counts the same as in contact with the joist as either method 100 or 101 (depending on the insulation thickness). 101 is generally ok on ring and radial circuits with no other de-rating factors in effect. 101 Usually tips it over the edge.
http://wiki.diyfaq.org.uk/index.php?title bles#Installation_Methods
Note that I was thinking particularly of cables that are already over existing insulation - you are better off fixing them to something rather than simply entombing them. (which makes them method 103 with a 50% de-rating)
--
Cheers,

John.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 13/11/2013 22:37, John Rumm wrote:

AFAICS putting them on a joist that is then covered with insulation will have zero benefit to keeping the cable cool. Wood is not a good heat sink or a good conductor.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 13/11/2013 23:00, dennis@home wrote:

It has enough thermal mass and conductivity to make a difference. (reflect on the fuss made over timber studs and thermal bridging when talking about insulation properties).
Remember also that the installation methods described in the regs are based on detailed empirical studies.
--
Cheers,

John.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 13 Nov 2013 23:00:34 +0000, dennis@home wrote:

But it's better than having the cable surrounded by insulation...
--
Cheers
Dave.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 14/11/2013 09:29, Dave Liquorice wrote:

Wood is an insulator so clipping it to a joist before covering it in insulation is no better than covering it in insulation and is worse than putting it on the plaster and covering it with insulation.
Generally there is no need to do either and it doesn't really matter with lighting circuits as long as someone doesn't fit a few heating lamps.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 14/11/2013 13:20, dennis@home wrote:

Well its a point of view that does not tally with the results of experiment. On a PB and covered with insulation, and clipped to a joist and covered with insulation are both notably better than buried directly in insulation.
This should be fairly self evident from the nature of the materials though:
Softwood has a k value of around 0.14, compared with fibreglass wool at 0.04, and PIR 0.025, so its conductivity is significantly higher.
More significantly, wood also has a fairly substantial SHC at around 1700 J/kg/c.
So if you take the density of say Douglas Fir at 530 kg/m^3, a 1m length of 100x50 gives you 4500 odd J of heat capacity per deg C, additional heat sink on the cable.
So if we take the load on a 2.5mm^2 T&E at say 21A, that's a dissipation of 21^2 x 0.0148 = ~6.5W, that would be just less than a 1 deg rise in temperature of the joist section in 11 minutes if perfect adiabatic conditions existed and all the heat ended up in the joist (which of course won't be the case)

Even then its probably not an issue for lighting circuits, as even method 103 (fully immersed) only reduces 1.0mm^2 T&E's capacity to 8A which is still adequate.
It can be a problem for others though. A normal ring or radial circuit wired in 2.5mm^2 will start to be out of spec under more than 100mm of insulation even if it is sat on the PB or clipped to a joist.
--
Cheers,

John.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tuesday, November 12, 2013 11:06:32 PM UTC, Stephen wrote:

I've never heard of glass ageing before. You should take it out for a meal after showering and shampooing it. Blow dry & put it back. It'll then perform exactly the same.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.