Insulation question

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Posted: 12/9/2016 1:12:09 PM EDT

So my step dad has a bunch of insulation that is 16 inches wide. I'm guessing that is wall insulation for a 16 inch stud. I need to insulate the roof of my garage and is 22 inches between the rafters. Is there a way to make this insulation work? He has quite a few rules of it and it would be nice to use that up instead of having to go buy new stuff. Never done this before. Thanks

Posted: 12/9/2016 1:23:55 PM EDT

[#1]

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?

Posted: 12/9/2016 1:28:56 PM EDT

[#2]

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.

Posted: 12/9/2016 2:20:37 PM EDT

[#3]

Quote History

Quoted:
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?

View Quote

It's the underside of a roof deck. In between the trusses. I will take a picture when I get home.

Posted: 12/9/2016 2:49:39 PM EDT

[#4]

Quote History

Quoted:
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.

View Quote

Copy. I could tape them together...

What about visquine being used to hold it all in and as a vapor barrier? Someone mentioned that I had to add visquine.

Posted: 12/9/2016 3:13:38 PM EDT

[#5]

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.

Posted: 12/9/2016 3:29:49 PM EDT

[#6]

Quote History

Quoted:
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.

Posted: 12/9/2016 3:34:54 PM EDT

[#7]

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.

Posted: 12/9/2016 5:58:20 PM EDT

[#8]

Could be an issue in your environment. Are you going to vent the underside of the roof deck?

Posted: 12/9/2016 6:24:29 PM EDT

[#9]

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....

Posted: 12/11/2016 4:59:28 PM EDT

[#10]

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.

Posted: 12/11/2016 5:07:34 PM EDT

[#11]

Quote History

Quoted:
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.

Posted: 12/11/2016 8:51:28 PM EDT

[#12]

Bare rafters

Posted: 12/11/2016 9:02:30 PM EDT

[#13]

just cut a bunch of pieces at 22 inches long stuff in the short(?) way...

Posted: 12/11/2016 9:36:24 PM EDT

[#14]

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.

Posted: 12/12/2016 7:39:56 PM EDT

[#15]

Quote History

Quoted:
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.

View Quote

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

Posted: 12/13/2016 1:06:38 AM EDT

[#16]

Quote History

Quoted:

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..

Posted: 12/13/2016 4:46:39 AM EDT

[#17]

Quote History

Quoted:

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.

Posted: 12/13/2016 9:31:24 AM EDT

[#18]

There is one vent in the garage. Its a metal one in the middle at the top of the peak.

Posted: 12/13/2016 9:31:50 AM EDT

[#19]

The heater will be a vented natural gas heater.

Posted: 12/13/2016 11:58:32 AM EDT

[#20]

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.

Posted: 12/13/2016 12:25:23 PM EDT

[#21]

This is getting more difficult than I wanted. We just put a new roof on it a few months ago. If I can't use this insulation or I am going to have to go buy new I'm going to say F it.

Posted: 12/13/2016 12:38:23 PM EDT

[#22]

It sounds like it would be easier at this point to install a ceiling in the garage. Maybe Ill go that route. It would keep the heat down lower..

Posted: 12/13/2016 1:50:50 PM EDT

[#23]

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

Posted: 12/13/2016 2:10:09 PM EDT

[#24]

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.

Posted: 12/19/2016 12:23:35 PM EDT

[#25]

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.

Posted: 12/19/2016 1:15:46 PM EDT

[#26]

Just make sure you have vents in the eaves to vent the attic. Should work fine.

Posted: 12/19/2016 8:52:15 PM EDT

[#27]

Quote History

Quoted:
Just make sure you have vents in the eaves to vent the attic. Should work fine.

View Quote

There is

Posted: 12/20/2016 9:35:48 AM EDT

[#28]

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

Posted: 12/20/2016 1:59:37 PM EDT

[#29]

Quote History

Quoted:
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.

Posted: 12/20/2016 2:14:32 PM EDT

[#30]

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.

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