loft insulation

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.
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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 insulation.
Christian.
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Of course, I meant "is more suitable for a new build".
Christian.
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I don't believe this is a significant problem unless you have lots of cracks or holes through your plasterboard and an unventialated loft.

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).
--
Andrew Gabriel

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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 barrier.

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 better.
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.
Christian.
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Christian McArdle wrote:

All good sttuff from Christian, but perhaps too good for the current problem. 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 right angles.
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.
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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.
Christian.
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Christian McArdle wrote:

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.

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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 disposal).
Christian.
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Christian McArdle wrote:

But rockwool is half the price for equivalent insulation.
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Yes, but if it means that you can board over and still get enough insulation, the price is worth paying. I bet Celotex/Kingspan is cheaper than installing deeper joists so you can board.
Christian.
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Christian McArdle wrote:

I would not be to sure of that, at least if DIY and labour costed at zero.
Celotex is improbably expensive.
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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.
Dave
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Dave wrote:

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.

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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 storage.
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.
Christian.
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enough
and
need
will
there
Unfortunately it's also got 100% of the moisture which may condense in this area - over the storage. So a good vapour barrier and a sealed loft hatch is essential.
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this
Yes, I'm a believer in installing vapour barriers below loft insulation.
Christian.
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snip

Many thanks for that, I'll take a look at the feasibility of doing the cross joisting.
Dave
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