Loft Insulation

[Dean Richard Benson]

I have just had my loft timbers/joists treated for woodworm, and getting the house rewired soon. After that is all done, I am going to put insulation down. However I have some questions about it.

Is there a noticable difference in loft insulation (rockwool rolls) in insulating greater than a 4" depth? The reason I ask is that my joists are 4" in height, and anything greater will mean a bit more planning to ensure that I can still walk and see the joists up there.

Also, has anyone noticed a difference between rockwool and other types, such as crown wool? I have checked the u-values and such, and rockwool is a better insulator, but it is also going to cost me about 20% more than crown insulation. (b&q special offer).

I really like the idea of using natural insulation such as sheep wool :

but the price is unbelievable.

Any comments/help/advice would be greatly received.

Thanks.

[Christian McArdle]

Yes. The law of diminishing returns says that 350mm is the best environmentally speaking. Any more and the environmental cost of manufacturing and transporting the insulant is higher than the energy saved.

100mm is definitely on the low side. 200mm would be better. The difference between 200mm and 350mm isn't that great.

Christian.

[Nick Brooks]

why not use this

stuff? OK it's a bit expensive ($11/gram) but hey, when you want the best. . . . 'It's the lowest density of any solid, and it has the highest thermoinsulation properties. Though it would be very expensive, you could take a two- or three-bedroom house, insulate it with aerogel, and you could heat the house with a candle. But eventually the house would become too hot.'"

Nick

[Dean Richard Benson]

Some useful info in there. Leads me to even more questions....

- with currently having a 100mm joist height, should i either increase the joist height (cross joists) or just cross-lay the insulation over the top of the 100mm.

- If you insulation up to 350mm, how are you meant to ever reach places in your loft without falling through the ceiling ;P

- You mention the recommended above, based on diminishing returns, but what insulation material is that based on - as I understand it, different manufacturers produce insulation to different values. So my thought was, that maybe 200mm of rockwool, might equal 300mm of crown or something?

Thanks again

Dean

[Andy Hall]

You can do the sums on this.

Calculate the heat loss for the area of the ceilings.

For a pitched roof plus 100mm of insulation, the U value is 0.35

So, let's say the house is 7m square for the sake of argument.

Using conventional central heating temperature assumptions of -3 degrees worst case outside and 18 degrees in the upstairs rooms, the heat loss will be

Area x U x temperature difference

or 7 x 7 x 21 x 0.35 = 360 Watts

If you have 150mm of insulation, the U value falls to 0.25 and the heat loss will be 257W.

With no insulation, the U value of the pitched roof is about 2.0 and the heat loss about 2kW.

You can see that there is a huge difference between nothing and 100mm, but a rapidly diminishing return after that.

This is typical worst case.

However, averaged over the year, the outside temperature is probably closer to 10 degrees, so assuming continuous heating at 100mm of insulation the loss is 223Watts and at 150mm about 160W. With 200mm you would get down towards 120W.

100W of heating by gas at a price of 1.4p per kWh works out to just over £12 per annum.

In terms of the material cost, this provides a reasonable rate of return. However, if you set it in the context of whether the space is important to you and the cost of the timber and other means to access it, then this becomes less interesting.

In energy terms, if you compare with the heat loss through the walls and other means, then you realise that the focus should be on other issues than this.

.andy

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[mike ring]

Dean Richard Benson wrote in news: [e-mail]:

Can't answer quantitatively, but for the same reason I put down 4inches (HTF does one cope with a foot of insulation?)

It's made a big difference, there was already a skimpy fibregass layer there, but it's pretty well just the 4 ins.

So I think I would stick with 4 ins ant take the losses, at least I can still find joists, wiring, pipes, chimneys etc.

mike r

[Andy Hall]

It depends on whether you want to board over the top.

It may not matter, since you won't be putting many other things there anyway :-)

Before you get over-excited by this idea, take a look at how much heat is going out through the walls and windows.......

If you want a better compromise between thickness and U value, then you could use polyisosyanurate board such as Celotex or Kingspan which have about 4x the insulating property of glass fibre.

However, it costs £15-18 for a 2440x1220 sheet........

.andy

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[The Natural Philosopher]

Ah, but never underestimate the insulatng properties of a loft full of junk :-)

Indeed.

[mike ring]

Andy Hall wrote in news: [e-mail]:

No, I can't, you do them

Ta

Thanks Andy, you've confirmed my gut feeling about losses, and also that one episode of tripping over a pipe and putting a foot through the ceiling will take a hell of a time to balance against the extra 8" insulation.

mike r

[G&M]

Are you sure on this. It's just I heard the ODPM people who set the building regs were talking about as much as 450mm in the next part L.

Rockwool is on 3 for the price of 2 at Wickes and so matches the B&Q prices well.

[Andy Hall]

They talk about all sorts of things. This one is pure pandering to be seen to be doing something towards the Kyoto protocol. It has no basis in economics or anything else when put into the context of where domestic energy should be being saved.

So people in an average house can spend about £100-200 and save about £10 a year.

It's a complete nonsense.

.andy

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[IMM]

How mixed up you are. Overall the walls loose more heat than the upper ceilingof a house. That is misleading. Then look at the individual rooms below the loft. They will have a large area which is loft ceiling. Then it makes sense to heavily insulate, as these rooms will be warm in winter and cool in summer. In August the coolest place in my house was the main bedroom. No heat came down from the loft above as I have 350mm of insulation. A light wind through the two windows and it was fine in the upper rooms. In winter they are very warm.

And you said a uni made you think?

[IMM]

Put counter joists in to make it deeper for more insulation and board over.

[Andy Hall]

Why? In comparison to everything else, it's little more than a drop in the bucket.

.andy

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[Andy Hall]

Obviously. The area is generally larger and the U value greater or comparable to 100mm of loft insulation

They will have a large area of walls and windows as well.

That doesn't follow at all.

That doesn't follow either.

Provide the dimensions of the room including the windows and the construction type of the walls and windows and we can do the sums.

I would be very surprised if the difference in heat gain that you experience through having 350mm of insulation vs. 100 or 150mm is significant in comparison to gains through other surfaces.

Yes it does, as well as the ability to spot bullshit when I see it.

.andy

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[Christian McArdle]

Consider using Kingspan or Celotex. Although more expensive that rockwool,

100mm will give a lot more insulation that rockwool. It is also longer lasting and less prone to sagging and soaking, as it comes in solid blocks looking a bit like expanded polystyrene. You can board straight over the top.

Christian.

[IMM]

Not so. read other post.

[IMM]

Best put Rockwool up to joist height then lay Kingspan over then loft boards.

[IMM]

Bt what you have failed to see, is that these rooms have a large percentage of ceiling area. A ceiling that will be very cold above in winter and very hot in summer. It is worth packing in as much insulation as possible just to improve the comfort levels of these rooms alone.

See above twice.

< snip. He can't follow something simple, not worth going on >

[Christian McArdle]

Which doesn't give the OP his required boarded area. If he was going to cover the boards, he might as well use rockwool, as it is better pricewise for the same u-Value.

Christian.