Another insulation question

[Porcupine]

My attached garage has maybe two to three feet from the ceiling joist to the top of the peak in the middle of the roof. In the "attic" the insulation is blocking the proper soffit vents so I need to fix that. The roof support system is all built out of 2x4 including the ceiling joists, so there is not much insulation up there, nor is there much space to add any. Am I able to nail 2" thick ridgid polystyrene underneath the ceiling joists followed by vapour barried and then drywall?

The current ceiling is drywalled, but I am taking it down.

Just trying to increase amount of insulation without having to resort to spray foam.

10 x 20 are garage dimensions.

Any guidance/advice would be helpful,

Thank you in advance.

 

[toyotadriver]

Since you live in Canada, I suspect you need a lot of insulation. I'm not certain that I fully understand your issues but think that you should be able to do what you want to. I don't think that you need a vapor barrier over foam sheets. The foam should be a vapor barrier enough.

What I would do is to put 2 inches of XPS foam sheets on the ceiling. Tape the seams. Then, put 1x4 furing strips over the insulation and screwed to the joists. Then, put 3/4 foam sheets between the furing strips. Cover everything with drywall. Alternatively you could use 2x4 furing strips and put 1.5 inches of foam sheets between the furing strips. Either way, you'd have either 2 3/4 inches or 3 1/2 inches of foam between you and the attic area.

 

[Cobra6]

You might want to look at ADO ProVent baffles, since you are working on existing construction, the plastic may be easier to install.

http://www.adoproducts.com/provent.html


I am going to use ProVent with Windblock, then add Attic Foil like this -

But ..........I am in Alabama and more worried about radiant heat.

 

[mobetta]

pics......

you ned to maintain a vent channel from soffit to ridge, if you indeed have soffit vents.

 

[IH82BL8]

If you're removing the existing drywall, I assume that means that the ceiling will be completely open with unlimited access at the beginning of the project? Is it the entire ceiling or just part of it?

 

[Cobra6]

provent picture

 

[Porcupine]

Update - I can confirm that all the soffit vents are blocked with batt insulation. After tearing down some of the drywall from one wall starting in one corner, I noticed the vapor was not taped at all, and that aside from overlaping maybe two inches from one piece to the next, the barrier stopped at one corner near the garage door. Needless to say, the insulation is dark where there was no vapour barrier. I wouldn't be surprised if any of the vapour barrier had no tape on the seams. It also appears the drywall was installed using both nails and drywall screws. The heads of the nails are all corroded.

I did notice the drywall "fire barrier" that extends up to top of the roof on the wall attached to the house.

I will post pictures in the next few days - I plan on taking more of the ceiling down tommorow, and maybe more of the wall as well.

IH82BL8, yes, I will have complete access to the ceiling once all the drywall has been taken down.

Cobra6, not sure I can get those ADO vents in Canada (the plastic ones). I would prefer those over the foam. I will check around and make sure. If I cannot, then I can make my own out of 2x2 and thin sheeting (will cost more and take more time), but they will be longer than the foam vents and more durable too. I will see when I have the entire ceiling down.

Pics to follow soon.