3 layers of glass wool -> 1 layer + walkable insulation?

[LabudMoj]

I have the following "problem":

I have 3 layers of glass wool insulation on the loft:
1x15cm (the original - probably 40 years old)
2x10 cm (put in 2007.)

The joists are 15cm tall, so to avoid layering several levels of chipboard/insulation/chipboard, or adding another level of joists, I was thinking of leaving only 15cm of glass wool (probably just removing the original and compressing the new 2x10cm to 15cm) and adding a layer of "styroloft" (insulation on which you can walk on) - but I cannot find any reviews on the quality of this solution. BTW, my joists are approx. 16" apart, but it varies by up to 1" from pair to pair.

If anyone knows whether this would make a bad walking surface, or if it's a terrible insulator - please let me know! Otherwise I am ordering some 8cm (to get the R factor of 6.1) thick boards and getting on with the job!

Cheers,

Labud

 

[mointainwalker]

If it's physically possible just compress all the existing insulation, you will end up with the same insulation value as removing some and compressing what's left.

This will save you a filthy job. Fibre-wool insulation doesn't deteriorate with age, just loses some efficiency as it collapses on itself which is what you intend to do anyway.

As for the boards, well i haven't heard of them and they seem expensive to me looking at the site but if they are worth it to you...........

I would also take their insulation claims with a pinch of salt as there are many inaccuracies e.g. they claim that 18 mm of fibre-wool has an R value of 0.8 whereas it is around 0.5 ( think they do this to make their own product look more impressive ) and they confuse U and R values.

I would also be careful of the assumption you can walk freely on it. There are various products and I didn't read everything, but did see that they give deflection figures based on 150 kg/m2. That is a figure appropriate to storage, whereas an adult of 75 kg standing normally might exert 400 kg/m2. They are also careful in their description of what kind of foot-traffic is acceptable and recommend their own chipboard for normal traffic.

 

[LabudMoj]

If it's physically possible just compress all the existing insulation, you will end up with the same insulation value as removing some and compressing what's left.

Hmm, not so sure about that, see this:
http://wiki.diyfaq.org.uk/index.php?title=Insulation#Forms_of_Insulation
http://www.diybanter.com/uk-diy/295798-compressing-insulation-2.html

With 15cm joists and fibreglass I'll end up with the R number of 4.3 (at most), which is by around 2 short to where it should be - recommended 6.1, but more than 7 not uncommon.

This will save you a filthy job. Fibre-wool insulation doesn't deteriorate with age, just loses some efficiency as it collapses on itself which is what you intend to do anyway.

I'm not afraid of a filthy job - I'll have to turn the loft upside down anyway due to electrical installations and removal of the cold water cistern. So while we are doing that, I can as well redesign the insulation.

As for the boards, well i haven't heard of them and they seem expensive to me looking at the site but if they are worth it to you...........

I would also take their insulation claims with a pinch of salt as there are many inaccuracies e.g. they claim that 18 mm of fibre-wool has an R value of 0.8 whereas it is around 0.5 ( think they do this to make their own product look more impressive ) and they confuse U and R values.

I believe that the number they quote - R of 2.7 for 100mm of fibreglass - matches 0.5 for 18mm, so I am not sure where did you get these numbers from.

I would also be careful of the assumption you can walk freely on it. There are various products and I didn't read everything, but did see that they give deflection figures based on 150 kg/m2. That is a figure appropriate to storage, whereas an adult of 75 kg standing normally might exert 400 kg/m2. They are also careful in their description of what kind of foot-traffic is acceptable and recommend their own chipboard for normal traffic.

Yes, I am mostly concerned about this property of the board - will it deflect too much and how soft the surface is. Need more research...

Anyway, if this turns out not to be the way to go, I guess I have to find something with the R number of 6-7 for 15 cm - is there any insulation material with such property?

 

[mointainwalker]

where did you get these numbers from.

Their site


PIR board has an R value of 6 for 150 mm

If it's physically possible just compress all the existing insulation, you will end up with the same insulation value as removing some and compressing what's left.

Hmm, not so sure about that, see this:

This means that if 6" of insulation is compressed to 4", it has the insulation value of 4" of material. In practice the change in size of the trapped air bubbles means that the value is affected slightly less than this

Don't understand. both the Wiki and the other post confirm that compressed insulation insulates ( approximately ) to the value of the compressed level, so what are you not so sure about ?

 

[LabudMoj]

Their site

PIR board has an R value of 6 for 150 mm

I don't think that's on styroloft's site. Also, I don't think their boards are PIR, and afaik PIR boards are commonly of R value higher than 6.

If it's physically possible just compress all the existing insulation, you will end up with the same insulation value as removing some and compressing what's left.

Don't understand. both the Wiki and the other post confirm that compressed insulation insulates ( approximately ) to the value of the compressed level, so what are you not so sure about ?

Sorry, I misread your comment - I thought you were implying I would end up with the same insulation as the uncompressed amount. From what I read it is the height that matters, and if it is compressed it is effectively even a bit lower due to less air between the fibres. In any case, this is not good for me as I would get the R value equivalent to 15cm (or less) of fibreglass, which is not enough, so I would need something on top of that.