"David Hearn" wrote in message
...
"BigWallop" wrote in message
...
"David Hearn" wrote in message
...
Quick question about loft insulation.
I currently have 4" of 'loose' fibre glass insulation in a 1930's
semi.
I
may add some extra joists at 90 degrees to add another 4" of
insulation.
As
I understand it, its the air that gets trapped in the insulation that
provides the insulating effect. Therefore, there's no point in buying
the
thickest pack of insulation around and squash it into 4", rather get
one
which is closest to the thickess I require. Should be cheaper (per
m^2)
and
provide similar insulation to a thicker one squashed to 4". Or am I
wrong?
Thanks
D
Why are you going to lay another layer of joists ? Why not just roll
your
new insulation at 90 degrees across the top of the existing stuff that's
already there ? The insulation doesn't need to be between joist to be
effective, in fact the looser and thicker it is, the better it works.
The
fibre insulation is very cheap to buy on its own, so please don't go to
expense of laying joists on top of joists just to be able to squeeze
insulation between them. It's just not worth it.
I guess I assumed everyone would remember my other posts! I'm planning on
boarding out the loft, and therefore need the extra joists to add height
for
additional insulation whilst hopefully providing a little extra strength.
D
The only way to gain extra strength is to make the existing joist thicker in
both surfaces. Adding joists on top of joist is only adding more weight to
the joists underneath the new ones, so that is a definite no, no. The whole
purpose of joists it take the load they are designed for, and because you've
put thicker ones on top, doesn't mean you've made anything stronger. Try
putting the ends of a matchstick across two points and pressing down on it
until it snaps. Then try putting another matchstick at ninety degrees
across the lower one and press down on them both where the meet. I think
you'll find that lower matchstick still breaks at about the same pressure.
After reading your other post, I think you find that when you take away the
joists that are already running at 90 degrees across the top of the joists
and then lay your new boards at 90 degrees to the existing joists, you're
actually doing the same job as the timber that is already there. Once
you've boarded over the joist you should actually see an increase in the
insulation properties of the ceiling because you've created a sealed unit
with the existing fibre wool that's there. So you shouldn't really need to
lay another layer of insulation on top.
If the area of the loft is boarded over right up to the edges with the new
chipboard flooring systems that are currently on the market, then they to
add some insulation properties to the ceiling because of their construction.
Try to make the job as simple and as cheap as you can. You won't gain
anything hugely different from what you already have, unless you intend to
increase the size of the existing joists and make them deep enough to able
to take two layers of insulation between them. A loft area must be allowed
to breath and move with different weather conditions, so the space must be
ventilated properly to prevent moisture from gathering and then dripping
through. So to seal the loft space to tight will mean you are actually
going to create more problems for yourself in the future.
Try doing a web search for roofing construction and get some tips on what a
roof and loft space actually do for a house. I think you'll be amazed at
what you find out.
Good luck with it all.
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