[Sherwood Forester (OP)]

Hello, a new member here, so greetings to all, here is a question I need help with:

Question: "Why does just 1% of moisture contained inside rolls of laid fibreglass roof insulation somehow lower or destroy in some way it's alleged heat retaining and roof insulation properties ?"

2nd question; I'm an insulation engineer, I tell clients who ask that heat rises  (the heat in question is about 25 / 35 feet away from the main fireplace or main downstairs household heat source) and it gets trapped inside the fibreglass insulation we are laying in the loft, and for this reason it (the fibreglass) acts like an excellent heat retaining blanket.

The problem for me is: I have handled and laid tons of the stuff and not once has it ever felt warm to the touch, in fact quite the opposite, and if I stick my finger inside some making an hole no heat or warmth is felt or 'comes out' or escapes from it, nor does any appear to be present inside it. Why is this?

Another small but worrying question is: "the loft or attic spaces we work in are over the bedroom, in the houses, meaning in practice over the plaster-boarded bedroom ceiling/s and, the alleged underside of the heat retaining fibreglass insulation, is cold to the touch, and so are the ceiling plasterboard ceiling boards below it".   So what is going on? Where is all this so called trapped heat?

[CliffordK]

I'm not sure about the first question.  Water may degrade or cause cellulose insulation to collapse, but it should have minimal effect on fiberglass insulation.  Assuming you have good attic ventilation, and no roof leaks, the attic insulation should dry out over the summer, and perhaps even wall insulation would eventually dry out.

As far as your second question.  Many people try to keep their houses at 70°F.  Your body temperature is about 98.6°.  The question of what feels warm or cool to the touch then depends on how quickly a material absorbs heat from your hand. 

A 70° piece of metal will always feel cool to the touch.
A 70° piece of foam rubber may feel warm to the touch.
Concrete & gypsum would be in the middle.
70° water is also cool.

The air trapped in your fiberglass would be no warmer than the ambient air.  The fibers of the fiberglass would be like a glass, and may also feel cool.

You might consider a HF IR Thermometer to play with.

Here is a discussion about making an IR filter/camera, although getting a nice color IR photo may take a bit more work.

The insulating effect of fiberglass is essentially preventing air convection heat transfer, and using the high insulating properties of air.  If you want to prove its effectiveness, take a space heater or heat lamp and shine it at one side of the fiberglass, then use your IR thermometer to measure the temperature on both sides.  Perhaps leave the heater engaged for an extended period of time (an hour or two).

Ahh, perhaps that is the answer to your first question.
It isn't the fiberglass that is the insulator, but rather the trapped air.
The higher the moisture content, the higher the thermal conductivity of the air, and thus the worse insulating performance. 

[alancalverd]

1% water in glass wool is well over 100% of the weight of air in the wool - i.e. saturated at most ambient temperatures. The purpose of the wool is to prevent convection of dry air, which is an excellent insulator when stationary, but saturated air will transfer heat by water evaporating from the ceiling, diffusing to the top, and condensing on contact with the ambient air. This is the principle of the heat pipe, which is an even better conductor than copper!

Glass wool isn't a perfect insulator, so the ceiling will still feel cold, but you should be able to find even colder spots where there is no insulation (e.g. under a water tank).   

[evan_au]

no heat or warmth is felt or 'comes out' or escapes from it

You will not feel heat radiating out of the glass wool - it is not a heater. In fact it is entirely passive.

You would need to make a hole in the insulation, and leave your hand in there for perhaps 10-15 minutes before you can feel that the hand surrounded by insulation is warmer than the hand outside the insulation.

But doesn't it make you feel itchy?

[CliffordK]

But doesn't it make you feel itchy?

Perhaps lay down and throw it over you like a blanket  []

[Bored chemist]

Heat pipes are sealed.
Any moisture in the glass wool would end up evaporating and condensing out on the nearest cold surface- the inside of the roof is a likely contender.
Heat pipes only work if liquid is formed and I don't think that happens often in the roof space.

Is there any evidence that the first contention ("Why does just 1% of moisture contained inside rolls of laid fibreglass roof insulation somehow lower or destroy in some way it's alleged heat retaining and roof insulation properties ?" is true?
If not, there's no reason to spend time seeking to explain something that doesn't happen.