Calculating square metres

[Kenny202]

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?

[Mr Somtam]

No you are right.

[KarenBravo]

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

[canman]

27 + 1 as a contingency seems reasonable.

[cooked]

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.

[eyecatcher]

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.

[eyecatcher]

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.

[Kenny202]

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

Edited May 27, 2015 by Kenny202

[splitlid]

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

[eyecatcher]

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.

[krisb]

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

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

[krisb]

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.