Don't know if I'm having a brain fade here or not but need some help. Our home is 8 metres by 8 metres. We wish to install some ceiling insulation / batts in the roof.

1 pack contains 6 batts which will cover 2.4 square metres.

Now I would have thought our home was 8m x 8m = 64 square metres and that would mean we would need 28 packs. But that seems like too many?

Have I missed something?

No you are right.

Hmmm...........I make it 27 packs.

27 + 1 as a contingency seems reasonable.

I hate having unused building material lying around for years and years, Personally I would rather go back and buy one last batt if necessary.

If your measuring is accurate then 27packs should do with no waste.

A batt is 1m x0.4m

So you have exactly 20rows x8batts long.

And 2 spare.

Couldnt work out any better.

Of course the ceiling grid will be all over the place and its going to be difficult.

I need to do the same but i recently found out by extending some speaker wires that the grid is too weak under my weight for me to even attempt it. Be very cautious of going through it.

If you are doing a ceiling it is easier all round to buy the rolls of insulation at 2" ot 3" thick.

Each roll covers 2.4m2 and easier to roll out over those ceiling ties.

26 did the job. Got the 3" R21 Batts with the foil cover. (Does that mean ur house or the heat from above is cut by 21%?) They called it microfiber but looked like the standard yellow fibreglass Batts to me. Cant say they've made a noticeable difference however it's cooler wen u walk inside after the house has been locked up. The ambient temp outside has been so hot. I guess the air con must be working more efficiently. Amazes me how warm the block walls get even though they're not exposed to the sun? Anyway, cost me $200 and was a relatively easy job

do you have downlights in your ceiling, if so be careful.

26 did the job. Got the 3" R21 Batts with the foil cover. (Does that mean ur house or the heat from above is cut by 21%?) They called it microfiber but looked like the standard yellow fibreglass Batts to me. Cant say they've made a noticeable difference however it's cooler wen u walk inside after the house has been locked up. The ambient temp outside has been so hot. I guess the air con must be working more efficiently. Amazes me how warm the block walls get even though they're not exposed to the sun? Anyway, cost me $200 and was a relatively easy job

Unfortunately that’s not the case. The R-value is a value of resistance of an insulating or building material to heat flow.; and expressed as R-12;r-24 etc.

The higher the number the greater the resistance.; so no its got nothing to do with % reductions of heat.

The important thing is thickness of the materials, so 2” of R-24 may give the same heat insulation as 6” of R-12

This is how u- values was wall , ceiling and floor construction are calculated. What happens is they take the r-value for each component and multiply it by the thickness to get a number(over m2). So you have a number for blocks, bricks, cavity insulation, plaster, and even air gaps in the cavity.

The reciprocal of all the values together becomes the u-value which is the leading criteria for house construction..

In your case for example (and I am sure I have forgotten most of this!) you created 75mm R-21= 1.575 therefore a u value of 0.63 (1/1.575)

As I left the uk; the U value for a ceiling was at 0.25 but that’s for heat escaping out of the house not coming in!

Conclusion: I would say you need to either double that thickness OR find 2” with a higher R-value such as r-27, the one I will be using on my ceiling.

I apologise if I am slightly off with my explanation; its been 10 yrs since I had to sit down and design wall thickness based on u values; so please don’t hang me.

do you have downlights in your ceiling, if so be careful.

Yep 100mm away from all downlights is the spec. Starts fires quite easily.

26 did the job. Got the 3" R21 Batts with the foil cover. (Does that mean ur house or the heat from above is cut by 21%?) They called it microfiber but looked like the standard yellow fibreglass Batts to me. Cant say they've made a noticeable difference however it's cooler wen u walk inside after the house has been locked up. The ambient temp outside has been so hot. I guess the air con must be working more efficiently. Amazes me how warm the block walls get even though they're not exposed to the sun? Anyway, cost me $200 and was a relatively easy job

Unfortunately thats not the case. The R-value is a value of resistance of an insulating or building material to heat flow.; and expressed as R-12;r-24 etc.

The higher the number the greater the resistance.; so no its got nothing to do with % reductions of heat.

The important thing is thickness of the materials, so 2 of R-24 may give the same heat insulation as 6 of R-12

This is how u- values was wall , ceiling and floor construction are calculated. What happens is they take the r-value for each component and multiply it by the thickness to get a number(over m2). So you have a number for blocks, bricks, cavity insulation, plaster, and even air gaps in the cavity.

The reciprocal of all the values together becomes the u-value which is the leading criteria for house construction..

In your case for example (and I am sure I have forgotten most of this!) you created 75mm R-21= 1.575 therefore a u value of 0.63 (1/1.575)

As I left the uk; the U value for a ceiling was at 0.25 but thats for heat escaping out of the house not coming in!

Conclusion: I would say you need to either double that thickness OR find 2 with a higher R-value such as r-27, the one I will be using on my ceiling.

I apologise if I am slightly off with my explanation; its been 10 yrs since I had to sit down and design wall thickness based on u values; so please dont hang me.

20141106_100430.jpg

Thicker the better.