compressing insulation

[krisway]

Another insulation question. There is a section of my attic sheeted over (on top of the rafters) for storage. Once I add the ceiling to the rafters, it will give me only a 4" gap to stuff insulation at these sections. Compressing the 6" batts to fit will loose some of it's R value, so my question is: should I use styrofoam in these sections, compressed 6" batts, or use 4" batts? Looking for the best solution with regards to R value.

Also, do I need to add those rafter cutes? The ones that allows better air circulation from the soffits? Or are these only necessary with blown in insulation? With the batts, I can lay them as to not block the soffit. Thoughts?

 

[Herb67SS]

If you compress6inch insulation to 4inch, you end up with 4 inch insulation or about R11. Why not just use 3 1/2inch in those ares. the difference is not worth the expense. As for baffles, the blck plastic ones at HD insure the soffit stays clear no matter what. Hopefully your soffit is vented or perforated. Ventilation from there up the underside of the roof sheathing prevents condensation and mold and helps cool in the summer. Ideally, you want well vented soffit and a continuous ridge vent for adequate ventilation. BTW, with that configuration, you DON'T want gable vents too. That defeats the system.

Just my opinion.

 

[joe_padavano]

If you compress6inch insulation to 4inch, you end up with 4 inch insulation or about R11.

Actually, it's even worse than that. The insulating properties don't come from the fiberglass, they come from the air pockets inside the fiberglass. Compressing 6" batts to 4" actually results in smaller air pockets than you have in 4" batts, so the insulating properties of the compressed 6" are even poorer than with the 4".

 

[Kevin C]

It depends on the density of of the insulation you are starting with. High density insulation may be worse than the next size down when compressed. In many cases standard density compressed works better than the material designed for the sized cavity.

Owens Corning had a spread sheet that shows the effect of compression.

http://webcache.googleusercontent.com/search?q=cache:IiNnEiXC1ZkJ:www.owenscorning.com/around/insulation/CompressionChart.xls+&cd=1&hl=en&ct=clnk&gl=us

If your using 6 1/4 R19 and compressing it to 3 1/2 the compressed section will be R-13
For R21 5 1/4 compressed to 3 1/2 you will get R-15
for R-22 6 3/4 compressed to 31/2 that section will be R-14.

All of those compressed figures are equal to or better than standard 3 1/2 insulation (R-13 to to R-11). There is a high density 3 1/2" insulation that is rated at R-15.

You could build your own vents out of foam board and strapping. Kind of a pain to do but it gives you the most open area and for that short section you can use a high r value per inch foam. For a garage the payback may not be there. I went pre-formed vents all the way up. Those are connected to a ridge vent.

 

[DekeT]

This topic has been beat to death on this forum.

http://www.garagejournal.com/forum/archive/index.php/t-88439.html

http://www.garagejournal.com/forum/archive/index.php/t-162146.html

I recommend reading those threads before you go any farther. Many posts perpetuate the myth of compressed fiberglass insulation.