Having taken down all the lath & plaster in the bedroom in my 1936 house, I want to put insulation above the ceiling in the bay before putting the p/b up. The loft is insulated but there was no access to the bay until now. I intend to put 2 inches of polystyrene in but I also have some foil backed bubble wrap from Wickes left over from another job so I was going to use that up on this job aswell. I can't remember if insulating to keep heat in (as opposed to out) is the foil face outermost or the foil face innermost. I was also going to put the polystyrene in first, then staple the bubble wrap to the noggins the the board. Thanks
If the foil is intended as a moisture barrier you will need to put the foil on the warm inside. That way, the warm air inside will be stopped by the warm foil, so won't condense.
If you did it the other way, the moist air from inside would permeate as the barrier foil on the outside, which would be cold, so you'd get condensation.
Foil downwards to stop water vapour rising. It isn't part of the insulation properties, but is basically to stop damp air condensing on the roof structure. The foil edge should be butted up to the plasterboard and any joins taped. If there is any insulation below the foil, it will act as a condensation generator and it will all drip down, damaging the plasterboard.
Do it properly. Forget the bubble wrap. Put as much solid insulation up there as you can. 50mm of polystyrene isn't really enough, although it will be vastly better than what went before. I'd use 100mm of Celotex/Kingspam, or 200mm of polystyrene if there was room and the polystyrene turned out cheaper.
I think we should point out to you that if there are electric cables with pvc insulation in the loft, then keep the polystyrene away from them. Poly affects pvc insulation.
I am looking to insulate my walls cavity walls, as I build them, I have now priced the options, initally I was going for put as much of the best stuff in as possible.
Is there any rough forumla for how much I can save in my heating bills by going over the minimum recommended ammounts of insulation ?
You can begin by doing the calculations as though you are installing a heating system - i.e. take U values for each surface and multiply by area and temperature through it. Then you add up the losses in watts. There are PC programs from the radiator vendors to do this as well.
The assumption for a heating system is to design for a worst case temperature outside of -3 degrees.
When I redid my heating system, I did this exercise to make sure that the radiators were adequately sized, so I know the calculated heat losses reasonably well.
Subsequently, I have taken the average monthly temperatures and looked at gas consumption. I'm fortunate that I have a constant level of gas use for the range so I know that everything else is heating. Using the basic efficiency (not SEDBUK) figures for the boiler, I was able to work out the effective heat output into the house. This correlated pretty well to calculated energy requirements against average monthly temperature.
Therefore, to a first approximation, you could work backwards. Calculate the heat losses now and with insulation and you should be able to get quite close to a figure.
On a simpler level, you could assume that the walls account for about half of the heat loss of the house - generally it is with uninsulated walls plus/minus 10-20%. You can then work it on the basis of reduction in U value for that part,
'The Whole House Book' recommends roof 300-450mm timber walls 200-300mm masonry walls 150-250mm solid floor 150mm susp floor 200-250mm but says that these are very approximate rules of thumb as there are so many variables - the main one being space obviously! Basically the more the better and the marginal cost of extra insulation may not be that high compared to the overall cost of putting it in in the first place, especially if new build. Even small amounts are effective - the conductivity of 1 inch of exp. polurethane is equivalent to an astonishing 33inch thickness of brick or 46 inches of granite wall!! A graph in the book shows the cost effectiveness of insulation approaching zero at above 400mm - again, a sweeping generalisation but worth bearing in mind. And it depends on what pay-back time you expect.
I think those figures are for Rockwool and also to get the highest SAP figures. SAP is useless and insulation to eliminate a full heating system should be the aim. If you put in the walls 300mm of Kingspan the U values would be a lot lower than Rockwool.
EcoHouse - A Design Guide does not give maximum levels. It says put in as much as you can. As oil prices rise, the cost of the insulation will too, as oil is used in the heat process to make insulation. Then there is the home fuel bills too on top. So, the payback, if any at all on a new build as you save in not putting a full heating system (and reduced services charges on the heating in the future). So you have a double benefit for the future. Putting in more insulation is an insurance for your old age. It doesn't cost anything to run.
The cost of the oil used in making the insulation is a very small percentage of the oil saved in heating the house so although 'whole life cycle costs' should be applied to things, in this case it is almost pointless. As you sort of said afterwards, more insulation is better, much more is much better.
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