Attic insulation advice please

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Sim Two

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I’m looking to top-up the insulation in my attic with Rockwool Rollbatt (150mm thickness).

Has anyone used this product and, if so, how much did you pay for it, where did you get it from, was it easy to use during the install and have you noticed a difference in the heat around the house?

I have fibreglass insulation in the attic already between the joists and have floored some of the attic with tongue-in-groove boards. Should I simply cover these boards with the Rockwool and should I remove the boards first.

I also have two skylights which I am looking to insulate. What product is suitable for this – would Therma Wrap be OK?
 
Simple Query. Forget rockwoll, get 150mm more fibreglass and its cheaper, lay on top of your current insulation in the opposite direction.

Skylights: Best would be to get some rigid board insulation 8x4 sheets and to side to insulate the skylight, say the tube per say from the room through the ceiling to the light in made a stud. Cut and fill between the studs with insulation such as rafterloc for ease of installation and then if you really wanted board over the stude again with more.

Photos would help.
 
Floor. Two ways:

Take up the floor and replace the fibre glass with 100mm rigid board as the skylight. 100mm of Polyiso i.e K7 = 300mm Fibreglass. Lift the floor put the 100mm of polyiso over the joists and floor ensuring your rescrew the floor to the joists. B&Q do a t&G board with the insulation bonded to it but the cost is stupid vs getting an 8x4 and flooring over it. Hope this helps.
 
i have 20 mm kingspan i think stuck to my ceilling and nothing else. is this enough????
 
Crodg2 said:
i have 20 mm kingspan i think stuck to my ceilling and nothing else. is this enough????
Click to expand...

20mm Kingspan, assuming it is polyiso, which has a u-value of 0.023W/m²K, then to meet the current building regulations of 0.16W/m²K, you will require a minimum total thickness of polyiso of 133mm. This is assuming normal contruction details of your roof contruction and I am assuming a typical R value for this as 5.8m²K/W.

Fibreglass insulation will have a typical U value of 0.045W/m²K and therefore will require a minimum thickness of 261mm to achieve the same requirement. The upside of fibreglass is cost. The downside is it's a health hazard and it will lose some of it's U value each year. Notice when it's say 5 years old, it is getting very flat and not as fluffy as when new. The more air within it, the better the U value. So fibreglass requires topping up every so often, whilst material such as polyiso do not.

There are other materials with in-between U values and varying costs. Phenolic foam can give a better U value of 0.020W/m²K so a thinner material can achieve the same results, but phenolic is expensive.
 
For assistance, if you take a typical construction required R value for a pitched roof of 5.803W/m²K, (I can post the calculation, but it's a bit long winded), then you can calculate any thickness requirement for any material, whether it be rockwool, polyiso, fibreglass, etc. as long as you have the material U value.

Just do the following calculation:

Thickness = R x U of material being used
=> Thickness = 5.803 x say 0.023 for polyiso
=> Thickness = 0.133
=> Thickness = 133mm

Pitched roof U value requirement = 0.16W/m²K
150mm thick Polyiso will exceed requirement to 0.14W/m²K

Assumptions:

Existing Roof R Value = 0.447 (without any insulation)
Required R Value = 6.250
=> Required Insulation R Value = 5.803
 
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