Insulation roll or board?

[Sprogginz]

There is already about 100mm (not enough) insulation roll between the 400mm joists.

I want to use the loft for storage. As far as I can tell there are 2 options for my budget.

1. 170mm-200mm insulation roll with loft legs and chipboard on top.

2. 50mm insulation board over joists with chipboard on top.

Obviously the boards are thinner than rolls but according to websites they are better at insulating.

Does anyone know if that would be true under these conditions?

Using the boards (e.g. Celotex) workout to be a little more expensive when calculating the total cost of each project but if they are better then I'm prepared to pay that extra especially because it will be easier to install as well!

Thanks and hope you can help.

 

[JohnD]

I don't like the legs, I don't believe they give enough support to your boards (it is too localised) and they have negligible stability against a sideways force, e.g. if you were to fall or push something.

As the mineral wool is so cheap, use it to the full depth of the joists, and to the edges of your loft where there is insufficient space to stand. Do not block the ventilation gap in the eaves.

Look for rolls treated with Ecose which prevents it shedding irritant dust and fibres (it is brown). It will be marked on the wrapper. I will never use yellow glassfibre again.

The Ecose stuff is made by Knauf, but it is also sold as an own brand by Wickes and DIY sheds (check the wrapper in case they have got other stock)

 

[Sprogginz]

Thanks.

I'm not really comfortable using these legs either but if I choose not to then I would need to use insulation boards instead if I'm going to use the loft as storage place wouldn't I?

The recommended thickness of the insulation (roll type) is 270mm which of course is well above the joists and you're not supposed to compact it down as it would lose its effectiveness.

If the boards are more effective then I'll go with them but not sure if they are unless someone knows.

Thanks again.

 

[JohnD]

I believe (and it is possible to calculate it to check) that 50mm of Kingspan has the same insulation as 100mm of mineral wool. The savings diminish as the thickness increases (e.g. if the first layer saves 50% of heat loss, the second layer saves 50% of what's left, the third layer saves 50% of what's left, so they save, say, (50+25+12.5)= 87.5% of the original heat loss, and another layer would save you only 6.25%, so probably not worth the cost and effort).

There is also a "surface" effect so the boards you use will add a bit.

There is probably an insulation calculating tool on the 'net somewhere, I once did calculation exercises on a technology course, but I'm sure no-one does it in real life.

Sometimes you can get damaged or seconds boards, or site-soiled, on ebay at an attractive price, but make sure delivery is included.

I can just manage to get a 2400x600 board through the loft hatch, not bigger.

An alternative method is to "counterbatten" across the joists with additional timbers, and lay rolls between them. This spreads the load and makes the deck more rigid when screwed together, but does not increase its total strength, and the extra woodwork adds weight. I have done it in the past but would use rigid foam if doing it again.

 

[Sprogginz]

Thanks again, most helpful.