U-value of a cavity wall question

I am currently building a house. Here are the specs on the cavity wall

• External sand and cement plaster(not sure of the thickness)
• external standard concrete block wall - 100mm
• cavity pumped with insulation - 100mm - u-value = 0.15
• internal standard concrete block wall - 100mm
• Internal insulated plaster board
• Skim coat

I was wondering what u-value of insulated plaster board I need to achieve the BER rating of A1 (0.12), A2 (0.16) A3(0.20) for this wall. My engineer has done the specs for me but he is away for a week so i'm looking for guidelines so I can price this insulation at suppliers

Apparently the price of insulation is increasing this week so I wanted to lay down a price with a supplier

bmoriarty

check your u-value calculation again as 100mm pumped cavity with EPS/wool will only give you a uvale of 0.27 or 0.29. the currnent regs are 0.27 or better.

bmoriarty

bmoriarty wrote: »

check your u-value calculation again as 100mm pumped cavity with EPS/wool will only give you a uvale of 0.27 or 0.29. the currnent regs are 0.27 or better.

Sorry from what I can see here http://www.ecobead.ie/u-values.html the u value i will achieve is 0.29 with 100mm pumped cavity insulation. I can decrease the u value with an insulated board so im wondering what thickness i need.

bmoriarty

Hi,

you could you a 100mm EPS backed board, i would have to do the calcs but it would be around 0.18

or

a 75mm phenolic insulation backed board, this would be around 0.15

bmoriarty

bmoriarty wrote: »

Hi,

you could you a 100mm EPS backed board, i would have to do the calcs but it would be around 0.18

or

a 75mm phenolic insulation backed board, this would be around 0.15

Just looking for a rough idea. Thank you. Are these boards supplied in 8ft X 4ft sheets. Any idea how much they cost approx?

bmoriarty

Deleted User wrote: »

Just looking for a rough idea. Thank you. Are these boards supplied in 8ft X 4ft sheets. Any idea how much they cost approx?

Yes to 8 x 4

Costs vary from 27 to 35 euro per slab depending on insulation type/thickness

galwaytt

Deleted User wrote: »

[*]cavity pumped with insulation - 100mm - u-value = 0.15

I was wondering what u-value of insulated plaster board I need to achieve the BER rating of A1 (0.12), A2 (0.16) A3(0.20) for this wall. My engineer has done the specs for me but he is away for a week so i'm looking for guidelines so I can price this insulation at suppliers

Apparently the price of insulation is increasing this week so I wanted to lay down a price with a supplier

As other's have pointed out: 100mm pumped cavity insulation is nowhere near 0.15. Even 100mm of top quality 300kpa XPS has a value of 0.29. Loose fill bead will be lower.

With regard to your BER of A1/A2/ A3 - you achieving a given u-value won't get you a particular A-rating anyway. I think you'll find that, approx, below u-values of 0.18 or so, airtightness becomes a much bigger issue, and has a greater influence. Ditto for floor build up.

fclauson

also you should look at where the dew point falls with you wall

if you are doing (outside to in)

Block
100 bead
block
insulation behind slab
slab

then you need to look carfully where the dew point falls

fclauson

fclauson wrote: »

also you should look at where the dew point falls with you wall

if you are doing (outside to in)

Block
100 bead
block
insulation behind slab
slab

then you need to look carfully where the dew point falls

What do you mean by this?

sydthebeat

Deleted User wrote: »

What do you mean by this?

if you use an insulated plaster board then you are making teh inner block leaf colder.... when warm air meets a cold surface the water vapour (humidity) in the air condenses... if this condensation point occurs between the insulated board and the inner leaf then you will get problems with mould and rot, which can lead to health problems.

There is a 'rule of thumb' that states the internal insulation should not exceed 1/2 of what is in the cavity.

personally speaking, if the house is going to be constantly occupied, you would be much much better off building a 150mm cavity and pumping it, and leaving the inner block to act as a heat store internally.

sydthebeat

sydthebeat wrote: »

There is a 'rule of thumb' that states the internal insulation should not exceed 1/2 of what is in the cavity.

Do you mean 1/2 the thickness?

sydthebeat

Deleted User wrote: »

Do you mean 1/2 the thickness?

yes

for example if you have 100mm EPS in your cavity, then dont exceed 50mm EPS internally slabbed.