Insulation Compression

[uminded]

Hello all!

I just built a garage 20' x 24' x 8', 2x4 walls. I am looking around for fiberglass insulation and have found that R-12 (4") is only $2.75 less than R-20 (6") in the same quantity.

In my basement I have my vapor barrier in a pillow like configuration so while its 2x4 walls I could put the 6" R20 insulation in without compressing it. The difference between the R-12 and R-20 is noticeable.

My question is would compressed R-20 still have a net average rating higher than R-12 once installed? Even if it is only R-15 once compressed its better than R-12. I also get a discount in volume so I am putting two layers of R-20 in the ceiling.

 

[Dennis93]

Yes, the net would. Usually the price difference would be larger, but if I was in your situation I would probably pay the extra for the R20. Fiberglass actually uses the air pocket inside to insulate, which is why they say not to compress it.

 

[uminded]

I purchase though a corporate account which is in the process of building 6" walled condos so its less than $100 difference to do my entire garage in R-20.

Can anybody think of a good reason why NOT to go with the R-20?

 

[sands35]

******** PITA to put up sheet rock when you have to compress the 6" down to 3.5" over a 4x8 sheet..... If you can even do it.

Just get the r13. R20 won't insulate any better when compressed.

 

[jack stand]

The down fall of FG insulation is any air movement in the wall cavity rapidly decreases it insulative qualities. This makes me wonder if a compressed FG would reduce the airflow in the wall and provide a better "r" value. I think I would go for the r-20 myself.

 

[rsa]

My question is would compressed R-20 still have a net average rating higher than R-12 once installed? Even if it is only R-15 once compressed its better than R-12.

Yes. R-12 designed for a 3.5" thick cavity tests at R-12. Batts designed for a 5.25" thick cavity tests about R-15, a 25% improvement.

Reference.

 

[rlitman]

Here's the thing. As you fluff up insulation, the R value increases - to a degree. Beyond that point, airflow between the fibers causes R value to decrease rapidly. However, in the other direction, more highly compressed (higher density) fiberglass will have a higher R value per inch of thickness.

As sold, fiberglass is fluffed up about as much as it can be. So, as you compress it, the R value will decrease, but the R per inch of thickness will INCREASE. What this means, is that a batt made to fit a 6" cavity will give a high R value when compressed into a 4" cavity than a 4" batt.

So, is it worth your while. Perhaps. You won't get R20 in a 4" space, but you will get more than R12. However, if you end up fighting each batt to get it compressed under the sheetrock, it may not be worth your while. Also, if you're absolutely worried about the maximum R per inch that is still affordable, you may as well just blow in cellulose instead, or spray on a thin layer of closed cell foam, and cover that in the R12.

 

[jack stand]

Here's the thing. As you fluff up insulation, the R value increases - to a degree. Beyond that point, airflow between the fibers causes R value to decrease rapidly. However, in the other direction, more highly compressed (higher density) fiberglass will have a higher R value per inch of thickness.

As sold, fiberglass is fluffed up about as much as it can be. So, as you compress it, the R value will decrease, but the R per inch of thickness will INCREASE. What this means, is that a batt made to fit a 6" cavity will give a high R value when compressed into a 4" cavity than a 4" batt.

So, is it worth your while. Perhaps. You won't get R20 in a 4" space, but you will get more than R12. However, if you end up fighting each batt to get it compressed under the sheetrock, it may not be worth your while. Also, if you're absolutely worried about the maximum R per inch that is still affordable, you may as well just blow in cellulose instead, or spray on a thin layer of closed cell foam, and cover that in the R12.

Thanks for your answer. You confirmed what I thought made sence.