Warning

 

Close

Confirm Action

Are you sure you wish to do this?

Confirm Cancel
Member Login

Site Notices
Posted: 12/9/2016 12:12:09 PM EST
Link Posted: 12/9/2016 12:23:55 PM EST
Are you insulating against the underside of the roof deck or are you installing a ceiling and insulating above that?

Can you post a pic of what you are trying to insulate?
Link Posted: 12/9/2016 12:28:56 PM EST
Why not? Use a sharp knife and cut it at 6" to make 22 inches wide, and let it be fluffy don't cram it in tight. Just snug.
Link Posted: 12/9/2016 1:20:37 PM EST
Link Posted: 12/9/2016 1:49:39 PM EST
Link Posted: 12/9/2016 2:13:38 PM EST
MT is vapor barrier territory.

If the 16 inch you have has any type of barrier (like asphalt on the insulation side off the paper) you would need to do a lot of slitting to make sure it does NOT trap anything if you add another barrier.
A double vapor barrier is a BAD idea.

Slitting strips to fill in on a 22 inch span is going to be a nightmare no matter how you try to do it.

Just add the cost of the correct width to the job and ignore the old 16 inch left overs.
Link Posted: 12/9/2016 2:29:49 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By brickeyee:
MT is vapor barrier territory.

If the 16 inch you have has any type of barrier (like asphalt on the insulation side off the paper) you would need to do a lot of slitting to make sure it does NOT trap anything if you add another barrier.
A double vapor barrier is a BAD idea.

Slitting strips to fill in on a 22 inch span is going to be a nightmare no matter how you try to do it.

Just add the cost of the correct width to the job and ignore the old 16 inch left overs.
View Quote


I agree, keeping it in place would be a pain in the ass.
Link Posted: 12/9/2016 2:34:54 PM EST
Best to set it aside for another use.

May be able to run it the other way. Cut out a strip of insulation at every joist and run across joists. Major PITA, but if labor is cheap and materials are free it's an option.
Link Posted: 12/9/2016 4:58:20 PM EST
Could be an issue in your environment. Are you going to vent the underside of the roof deck?
Link Posted: 12/9/2016 5:24:29 PM EST
Link Posted: 12/11/2016 3:59:28 PM EST
I did my shop with the 15.5 inch wide r11 rolls from menards. My ceiling is 24 on center. Seriously it takes like 15 seconds to cut insulation to fit in between. Lay the insulation out put a 2x4 lengthwise where you are going to cut, compress it down and use a very sharp (new blade) utility knife it will cut like butter. The correct width insulation for the same r value would have cost me double as the 15.5 width.

If it has the paper I would just pull it off completely as you can't really use it.

I have 2x4 ceiling rafters so I filled in between with a full width piece then cut another in half long ways to fill the gap. Then I did another layer on top but this time perpendicular to the rafters so it cross laps any gap. Now I have r22 there abouts in the ceiling.
Link Posted: 12/11/2016 4:07:34 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By TOTHEMAX:
Thanks for the info guys.

I called an insulation place and they said it would be around 2 possibly 3 rolls if I bought from them. Each roll covers 224 square foot and has a built in vapor barrier. Its 121/roll. Not sure what I really want to do here....
View Quote


Do you have a ceiling or osb or drywall or just bare rafter?

My post above I assumed there is already a ceiling just no insulation.

Bare rafters yes just staple the insulation with barrier facing down would be perfect.
Link Posted: 12/11/2016 7:51:28 PM EST
Link Posted: 12/11/2016 8:02:30 PM EST
just cut a bunch of pieces at 22 inches long stuff in the short(?) way...
Link Posted: 12/11/2016 8:36:24 PM EST
Doing what you want to do in your climate is almost guaranteed to get moisture condensing on the underside of the roof decking and causing it to rot. If you are going to insulate below the roof decking in your climate with the insulation touching the decking, then you can only use closed cell spray foam. If you use fiberglass in your climate then you have to vent the rafters.

Without knowing exactly what you are working with makes it impossible to say for certain. But, if you put the fiberglass up against the roof decking in your climate, you will rot your roof.
Link Posted: 12/12/2016 6:39:56 PM EST
Link Posted: 12/13/2016 12:06:38 AM EST
[Last Edit: 12/13/2016 12:08:41 AM EST by billhw1]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By TOTHEMAX:


Why would it be any different from putting up the correct width insulation with a vapor barrier?
View Quote


Closed cell foam, IIRC, is a vapor barrier at 2" thickness. Fiberglass batts are permeable. The warm moist air will rise and condense on the underside of the roof deck just like a cold beer on a hot summer day, sweats on the warm side.

Do you have soffit and ridge vents?

Do you heat the space with a non-vented LP heater?

Also, read the fourth paragraph- FSK . Foam needs a coating too ( legally ) if not covered by drywall etc..
Link Posted: 12/13/2016 3:46:39 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By billhw1:


Closed cell foam, IIRC, is a vapor barrier at 2" thickness. Fiberglass batts are permeable. The warm moist air will rise and condense on the underside of the roof deck just like a cold beer on a hot summer day, sweats on the warm side.

Do you have soffit and ridge vents?

Do you heat the space with a non-vented LP heater?

Also, read the fourth paragraph- FSK . Foam needs a coating too ( legally ) if not covered by drywall etc..
View Quote



This right here. You can't use thin batts against the roof decking in your climate and NOT get roof rot. If you can't do closed cell foam, then you need to vent your rafters. You live in a very cold climate and what you want to do will Not work.
Link Posted: 12/13/2016 8:31:24 AM EST
Link Posted: 12/13/2016 8:31:50 AM EST
Link Posted: 12/13/2016 10:58:32 AM EST
You need to fix your venting issues before you start insulating.

As I see it, you have three choices......install proper attic venting and then add a ceiling and insulate above it......spray foam the underside of the roof decking with at least two and ideally three inches of closed cell(only) spray foam....or build a cold roof on top of your existing roof with rigid foam and then re-roof it with a new roof. The options are listed in order of cost from low to high.
Link Posted: 12/13/2016 11:25:23 AM EST
Link Posted: 12/13/2016 11:38:23 AM EST
Link Posted: 12/13/2016 12:50:50 PM EST
You might still be able to use the insulation you have. Install it across the rafters, then install new 24 inch wide insulation between the rafters. Then install your ceiling. You might even consider using some foil faced polyiso rigid foam AS your ceiling. It would give you some extra R value (very important for your climate) and also make a nice reflective ceiling surface. Tape it with foil tape and there's your vapor barrier as well.

Insulating in your climate will never be cheap.


Here's a few links for you to check out

Cold roof

Attic ventilation

This article is related to what you are wanting to do with putting the insulation against the roof decking. Basically you're building a cathedral type ceiling. .

Cathedral ceiling




Link Posted: 12/13/2016 1:10:09 PM EST
Can you install a ridge vent in the area? If you can keep an airspace between the insulation and the roof deck and provide ventilation for the space you should be good. You can get cardboard spacers designed to maintain clearance.
Link Posted: 12/19/2016 11:23:35 AM EST
Link Posted: 12/19/2016 12:15:46 PM EST
Just make sure you have vents in the eaves to vent the attic. Should work fine.
Link Posted: 12/19/2016 7:52:15 PM EST
Link Posted: 12/20/2016 8:35:48 AM EST
I used those 4x8 sheets of rigid insulation on my garage ceiling. Cost was only a little more and they are easier to hang.
Link Posted: 12/20/2016 12:59:37 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By TOTHEMAX:
Ok so new game plan. We are going to sheetrock the ceiling. My Stepdad has a sheetrock lift so it shouldn't be too hard. I have a few light fixtures and the door hangers to work around but it shouldn't be too hard. Once its done I will put the insulation above it in between the rafters. Should work fine.
View Quote


That's a good idea. Less space to heat and a much better fire rating.
Link Posted: 12/20/2016 1:14:32 PM EST
[Last Edit: 12/20/2016 1:15:01 PM EST by eazeaz]

I used those 4x8 sheets of rigid insulation on my garage ceiling. Cost was only a little more and they are easier to hang.




I do not think this would pass firecode - anywhere.
Top Top