Attic insulation top up recommendation

cloonymee

Hi there, I've a 2 storey house built 2005 with a 100m2 attic. It currently has approx 150mm of blown in insulation. I'm thinking of adding a top up layer for the Winter but I'm wondering what would be the max thickness of fiberglass I should go for ? i.e. is there a point(say 100mm or 150mm etc) beyond which there wouldn't be much difference made to the house? thanks,

moonshadow

Few lines below yours..

http://www.boards.ie/vbulletin/showthread.php?t=2057330998

cloonymee

thanks for the information. Think 150mm is enough for my purposes so!

dathi

cloonymee wrote: »

thanks for the information. Think 150mm is enough for my purposes so!

no! put the extra 150mm in and here,s why.
you currently have 150mm insulation and attic is 100m2 this gives you a u value of 0.35 w/m2/*k . If you put in an extra 150 mm of insulation your u value will be 0.15 w/m2/*k to most people these figures mean absolutely nothing so here goes to find the heat loss you have to multiply out the w/m2/*k and then multiply it by the number of hours to change it to kwh. To find *k this is the difference between the inside temperature and the outside temperature most people heat there homes to 20* and the mean winter temperature is between 4* and 7* so taking the best mean 20* -7* =13* so k=13*
therefore
150mm insulation =0.35 x100m2 x13* =455w
300mm insulation = 0.15x 100m2x13* =195w
now you have to multiply these answers by the amount of time that the mean temp is at 7* to change the watts into kilowatt hours (kwh) the mean temp is for 7 months so thats 7x 4 x 7 x 24 =4704 hours that your house is loosing heat through the insulation. so
150mm insulation= 455w /1000 (to change it to kw) =0.455 kw x 4704h =2140 kwh
300mm insulation= 195w/1000 =0.195 kw x 4704h =917.2 kw
so for 150mm insulation you loose 2140kwh of heat through the insulation in a heating season and for 300mm of insulation you loose 917.2 kwh

Calibos

http://www.seai.ie/Publications/Statistics_Publications/Fuel_Cost_Comparison/Domestic_Fuel_Cost_Archives.pdf

Not sure if I'm using the correct figure but used the latest figure for gas bottom right hand corner of 0.076 per kWh for natural gas.

The losses for the 7 month?? Season work out at €162 for 150mm and €70 for 300mm. ie. the extra 150mm saves €92 in 7 months or €13 per month.

How much would it cost to purchase the 100 square metres of extra 150mm loft roll and the wooden battens needed to raise flooring if required and how much would you need to pay someone if not fitting yourself.

Payback could still be years and years??

cloonymee

Hi Calibos thanks for your reply. Yip I was intending to add the extra 150mm. It would only cost me about €250-270 as I would be doing it myself and don't need to raise any significant amount of flooring. Thanks for the comprehensive reply.

dathi

cloonymee wrote: »

Hi Calibos thanks for your reply. Yip I was intending to add the extra 150mm. It would only cost me about €250-270 as I would be doing it myself and don't need to raise any significant amount of flooring. Thanks for the comprehensive reply.

270/94 =2.87 years lets call it 3