I want to improve the insulation in our loft as I believe most of the
heat from our radiators is going straight through roof.
Some loose insulation material (rockwool?) currently exists between
the joists to a depth of approx 4 inches, though some areas have no
insulation at all.As I only bought the house this year I have no idea
how old the insulation is.
Am I better off to remove the existing insulation, which would be a
rather messy and dirty job, and start from scratch or should I lay new
insulation on top of the old stuff.
What's the best insulation to use and to what depth. I'd prefer to
over-insulate than under-insulate. It is a lathe and plaster ceiling
and part of the roof has floorboards.
Any advice gratefully received.
The best, but not cheapest method is to use rigid Celotex or Kingspan
boards. These provide much greater levels of insulation per thickness than
rockwool and don't disintegrate into a lung infesting mess. They are,
however, more expensive. Wear a respirator and gloves when removing the old
filth and take advantage of the work to thoroughly vacuum the loft.
Consider what your vapour control strategy is. When insulating, you should
have a vapour barrier beneath, to prevent hot moist air becoming cold
saturated air and leaving mould causing condensation soaking the insulation.
The problem would be much more apparent with fibre based insulants.
Normally, this is done using foil backed plasterboard, but this isn't
suitable for a new build. The best solution in retro-fit applications is
often to lay large sheets of plastic sheet between and over the joists,
taping up any holes you need to cut around struts etc.
Finally, consider the effect on electrical circuits. If you only have
lighting circuits up there, these aren't likely to be a problem, even if
derated 50% for insulation burial. This is because lighting cable is usually
seriously overrated to begin with. However, 2.5mm cable for ring mains is
much more marginal to begin with and may be a failure if buried in
I don't believe this is a significant problem unless you have
lots of cracks or holes through your plasterboard and an
I think this is well OTT. Also, I would not put a vapour barrier _over_
the loft timbers. Seems like a recipe for them getting wet and being
unable to dry out.
If you have problems with a damp loft, look for draught holes through
from the house, lack of loft ventilation, and pumping over in the central
heating header tank (making it warm/hot/steaming).
The problem is that at 21C, the air can hold a lot of water. With a well
ventilated loft, at, say, 5C, it can hold considerably less. Plasterboard is
not gas proof, so the air rises up through the board and into the loft. As
this air cools, the humidity rises. This water has to go somewhere. Without
a vapour barrier, it will dump itself out as soon as relative humidity
reaches 100%, which will be inside the insulation. The solution is to make
the system completely gas tight on the warm side. This is the vapour
To keep the wood good, it needs to be either warm (i.e. under the insulation
and thus part of the house's heating envelope) or dry (i.e. above the vapour
barrier). This condition is satisfied when sheeting over the joists. If you
use foil backed plasterboard, then it is both warm and dry, which is even
Obviously, the vapour barrier prevents water escaping from the house,
raising humditiy there, instead of within the insulation. The solution to
this is ventilation. In a well sealed house, heat recovery ventilation is a
good idea, as it saves pumping expensively warm (but too wet) air out.
All good sttuff from Christian, but perhaps too good for the current
Ifte attic is a 'cold roof' i.e. has fairly decebt ventialtion, then
condensation should not be a problemif teh rockwool type slition is used.
What I would do is to seal up ancy celing cracks for draightproofing,
lay existing rockwool back , couner batten, and lay another layer at
This is all breathable stuff, and moisture percolatuin through existing
plasterboard will not meet utterly cold air till the top layers of the
second layer of insulation, and that is now above the timber level. In
practice air movement of cold dry winter air (its pretty hard to have it
cold and wet at the same time) will dry ofthis tempioary moistening.
A little moisture is not a poblem: Its permanently wet timber that rots.
The fungi simply die when the timber dries out, and unless there is a a
sufficiently long period of damp to allow new spores to germinate and
produce spores in their turn, the rot doesn't ever take hold.
All my rot problems have been in timber sealed in walls where leaks
allowed water in, or rising damp got trapped. Unless you have a leaking
roof, or zero ventilation, roof and ceiling timbers seldom rot.
But what is the hassle in putting 10 quids worth of plastic sheeting down
under the insulation? You'll kick yourself if you don't do it and the
condensed water drips back onto the ceiling and causes black mould stripes.
That's fine IF the majority of the insulation is above it. AND if the
joists are below it. BUT if there IS a roof leak, you won't notice it
until too late maybe...
My argument against your soulutin was moe against expensive celotex and
foil backed board.
The advantage of using Celotex/Kingspan is that you can get worthwhile
insulation just between the joists and board over. If this is not an
advantage in the particular application, then there's nothing wrong with
rockwool (apart from its general nastiness in installation, use and
How does this cross battening affect the strength of the ceiling joists?
Elsewhere in the ng there is a suggestion that the original ceiling joist
(name?) are only just capable of holding up the plaster board ceiling.
Reason I ask, is that I am about to put more loft insulation and less junk
up there myself, at long last. :-))
I have also talked someone else out of boarding his modern loft with chip
board, but to use normal floor boarding instead if he must go ahead with
this idea of boarding. After all, it is a lot lighter.
Well leaving aside the issues of floor weight, if you cross joist the
loft with substantial timbers (at least 4x2. maybe 5x3 or 6x3 with long
section vertical) and actually screw (DO NOT NAIL into a flexible
ceiling - it WILL crack the plaster) every joist where they cross, you
will immensely strengthen the structure, even more so if the cross joist
can be rested on the walls at the ends, and even better if they are
connected by hangars and braces to the ridge and rafters.
Effectively you may double, treble or quadruple the stiffness, and
prevent bowing in the horizontal plane. You create a huge layer of space
into which pipes and so on can be lost inside decent insulation, and can
board out to a smooth finish for storage.
I'd say its a win win in every sense except time and cost.
The thing to watch out for is not supporting the new joists at the ends
- you will lose a lot of strength if you don't. Typically they will end
up at a hip or gable end of the house, you will have to work out how to
support them there and still leave (in the case of a hip) ventilation.
Another alternative, if joists are deep enough for strength, but not enough
for your insulation is to use rigid foam insulation between the rafters and
fill the joists to whatever depth is available before boarding. You do need
to consider ventilation. The space behind the insulation in the rafters will
probably need ventilating, although this is not necessarily a given if there
is no felt or sarking.
The advantages of this method is that it turns the loft into a better
climate and doesn't require changes to the joist structure. If the rafter
and joist insulation is of the same value, the temperature will be about
halfway between the house and the outside temperature, making it ideal for
Basically, there are often loads of options for insulation and boarding out
of lofts. Much depends on the structure already present, the available time,
materials, and requirements for that storage.
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