# R-30 insulation in a ceiling framed with 2x8's



## rizbang (May 7, 2017)

Hey I need to put r-30 insulation in a ceiling framed with 2x8's regular fiberglass insulation requires 9" of space (2x10 framing). I have looked into spray foam which is pricey for 330sq ft it is $3,000 and rigid board at 4.5in (r-30 value) is $1,500 for the space. I will getting inspected so does anyone know of a cheaper product that will give me r-30 in 7.5 inches of space or is it ok to just jam the regular r-30 in that space? Thanks


----------



## Yodaman (Mar 9, 2015)

You can do it with Roxul, 

https://www.lowes.com/pd/Roxul-Wood...th-Sound-Barrier-15-25-in-W-x-47-in-L/3610406


----------



## Bud9051 (Nov 11, 2015)

Hi rizbang,
What do you have for ventilation issues in that ceiling? Most ceiling that are open to an attic will allow just adding more insulation over the top. If your ceiling cavities are enclosed is this a flat roof or are these sloped areas?

Bud


----------



## rizbang (May 7, 2017)

It's a flat roof with no extra space. I have 7.5 inches to get r-30. Looks like the roxul might work thanks.


----------



## Yodaman (Mar 9, 2015)

Need a air space for ventilation? What climate zone?


----------



## rizbang (May 7, 2017)

From my understanding I thought you need an inch of air space at least thats what the Owens Corning required. Doesn't look like the Roxul needs that or does it? Im in So Cal.


----------



## Yodaman (Mar 9, 2015)

I am in north country so flat roofs are scarce here. The Roxul will handle moisture better, but if you get condensation on the roof deck, that is where the ventilation comes in. You need it to prevent mold.
Bud or someone familiar with flat roof ventilation and insulating should be able to help.

You should add "So Cal" to your personal settings, so anyone trying to help will immediately know the location.


----------



## Bud9051 (Nov 11, 2015)

I'm also in a northern climate and rarely deal with a flat roof assembly. But there are basic principles that apply.
1. The roof assembly will not be able to dry upward.
2. Any moisture vapor that enters the roof assembly will need to either exit by way or ventilation or down through the ceiling.

If your heating days are minimal to zero then there should be very little moisture moving up through the ceiling, but what does go that direction may find a cooler surface, the bottom of the roof, and result in some condensation. Impossible for me to speculate as to this potential and thus your controlling reference should be local code requirements. from some reading S Cal has some newer codes which may affect this decision. Read some, clear as mud, but Ca can be a stickler. 

Some of the reading talked about end to end cavity ventilation. If there is no common attic then each cavity is isolated from all others. 

Sorry I can't help more,
Bud


----------

