Rafter insulation Question...

[Scarpozzi]

I'm getting ready to start insulation in the ceiling as soon as I get a framing inspection out of the way. Because I live in the South, we don't have really cold Winters, except for a few weeks out of the year.

I've sourced probably 1000 sq feet of unfaced R30 batts... Because it's unfaced, I'm curious what you'd use as a vapor barrier and how you'd install it?

Would you just staple plastic to the sheetrock side of the rafters and add the Batts? I've read a lot of conflicting stuff online. I'm thinking I may double up on insulation and go for R60, but am trying to determine if that's necessary since I'm not that far North and have 8, 3'X5' windows in the room.

 

[highland145]

The vapor barrier goes on the sheet rock side but I don't know about using plastic. I'd go with a heavy paper just because....

And it's not the winters as much as the hot as crap summers. S.C. here.

https://insulationinstitute.org/im-a-homeowner/about-insulation/how-much-do-i-need/

and H&G------>

 

[ch33zw1z]

When I had my ceiling/roof/crawl space insulated the guy I hire didn't put any vapor barrier down and said it wasn't required. He's been doing it 30+ years, and recommended vapor barrier in the walls.

I had the blown-in used in the attic.

For the walls he did for us, I chose the plastic as the vapor barrier. He presented it as either get faced, or use the plastic as the vapor barrier.

 

[highland145]

When I had my ceiling/roof/crawl space insulated the guy I hire didn't put any vapor barrier down and said it wasn't required. He's been doing it 30+ years, and recommended vapor barrier in the walls.

I had the blown-in used in the attic.

For the walls he did for us, I chose the plastic as the vapor barrier. He presented it as either get faced, or use the plastic as the vapor barrier.

Wonder if that's because of some air flow between the attic and crawl space causing some moisture buildup?

 

[ch33zw1z]

Wonder if that's because of some air flow between the attic and crawl space?

Not sure, it was part of the whole "proper ventilation and insulation to reduce ice damns" work.

Our work reflected in our summer bills as well. Our house stayed cooler, didn't need to kick on AC's until 85+ and only if it gets really humid...east coast humidity sucks. you guys know lol.

So proper insulation isn't just for keeping heat in, for us it's mitigating house temp overall.

 

[highland145]

Looks like I'll just leave it to the pros. Oh, wait, my house was built in 1937 and there ain't no stinking insulation in the walls.

Another link.
http://www.energyvanguard.com/blog/...-You-Put-a-Plastic-Vapor-Barrier-in-Your-Wall

 

[Red Squirrel]

Yeah vapour barrier should go on the drywall side, hopefully the drywall is not already installed. If it is another option is to spray foam but maneuvering around in an attic to spray foam does not sound very fun.

 

[Scarpozzi]

For the blown-in insulation, my understanding is that a vapor barrier isn't required because cellulose sort of seals itself after it settles. If fiberglass batts get damp, they still function, but don't work as efficiently until they dry out. I've read though that certain ceiling paints are sufficient as a vapor barrier. I just wanted to see if we had an 'expert' rather than me relying on stuff from other forums because this one is waaaay better.

Anyhow, the rafters are 2x10, so I'm stuck with 9.5" thickness in the cathedral ceiling. I'm using faced insulation in the walls and cathedral ceiling. I suppose I could just calculate the faced insulation requirements and order another 700sq feet (8 bundles) of faced insulation for the flat ceiling and use my existing batts to double my R-value on the areas that are flat since I have the space by running them perpendicular to the rafters. At least on part of this job, it's going to make a lot of sense to pack as much insulation as I can the first round because accessibility will be an issue once the drywall goes up...(no access).

 

[NutBucket]

Don't you still need to allow for the underside of the roof to be vented? You could also add rigid foam on top of the plywood too.

 

[Scarpozzi]

Don't you still need to allow for the underside of the roof to be vented? You could also add rigid foam on top of the plywood too.

I've got a box of foam vents that allow for a half inch of ventilation above the insulation that I'll be using, except for where I'm doing sprayfoam. That portion will not be vented due to the limited depth of the ceiling cavity.

 

[NutBucket]

I seem to recall reading spray foam doesn't require venting because of how it traps air? Hopefully someone can confirm or deny...

 

[stormkroe]

As far as I know, if there is no air space (like in vaulted/cathedral, etc) there is no need for ventilation.

I do wonder about the need for vapor barrier in southern US though, because it IS supposed to be on the hot side. You can actually have moisture form up against it in your insulation if your house is air conditioned and the outside air is hot and humid (the opposite of canadia for most of the year, where vapor barrier is vital).
If you're going to do it, however, use 6+ mil plastic and make sure your electrical is sealed with taped polyhats. Craft paper isn't waterproof.

 

[Red Squirrel]

Oh wait is this like a house with a conditioned attic and the roof itself is being insulated? I think you mostly treat it the same you would a wall, but I think that may still require venting, basically you leave a small gap above the insulation, so like if it's 2x6 you would use 2x4 insulation. Or put in baffles that go all the way to the top. One would think that if it's just packed with insulation it does not need venting though but I heard it still does. Conditioned attics arn't seen a lot here as they are harder to insulate properly. Though I see these new houses going up with super high pitched roofs and can't help but think what a waste of space that is haha. Conditioned attic spaces always look so cozy if done right.

 

[ch33zw1z]

Ah, I guess I thought he meant joist, sorry guys!

Now that I know rafters specifically are the pieces from your joist ends to your Ridge vent /peak....disregard my previous post regarding insulation.

I was also recommended a possibility of using some kind of foam to insulate the rafters...but chose just to drop the insulation as we have a ridge vent that's needs proper ventilation

 

[Scarpozzi]

I seem to recall reading spray foam doesn't require venting because of how it traps air? Hopefully someone can confirm or deny...

The spray foam installer told me that it blocks like 96+% of the air so it's not needed. This is open-cell foam...which is less R value, but is $2/sq ft vs $5/sq ft for closed cell. I was told that due to the limited space and efficiency of the air being blocked, I won't get any real ROI if I go with the more expensive product. I'm already getting 5-6" of spray foam in my garage ceiling, so this is only adding another few hundred bucks on that project.

Back to my original question, I've decided that I'm just going to pickup another 1000 square feet of faced insulation and use the R30 unfaced on top of that running perpendicular to the joists to maximize my overhead insulation....it may cost me $800 more, but will help block more air in the attic spaces...it should be worth it considering how hot the summers are and how this will be heated/cooled by electric...

 

[Red Squirrel]

Hmm I thought open cell was higher R value, except it does not block air. Closed cell will block air. Normally when you spray foam it's closed cell, as the main point of it is to block air in all the nooks and crannies where a vapour barrier might be hard to install.

 

[Scarpozzi]

Hmm I thought open cell was higher R value, except it does not block air. Closed cell will block air. Normally when you spray foam it's closed cell, as the main point of it is to block air in all the nooks and crannies where a vapour barrier might be hard to install.

I just read another article about the differences in Open vs Closed cell and it makes me think I should really consider getting a second quote for closed cell... It's tough though because I don't know how many spray-foam contractors we have in this area. From what I read, closed cell is superior in every way, except for the fact that it doesn't expand.... I'm sure installers like open cell better because it's easier to get it to 'stick' to ceilings and inches of the stuff can be installed in one pass. I think closed cell has to be applied in multiple passes to achieve the R value you want.

I may talk to the city inspector when he comes out for my framing inspection and tell him my plans...I hope he's the insulation inspector too so I can get feedback before I put up the cash.