Roof Insullation Advice

[wedel1]

I'm preparing to to insulate the my 30x40 garage and figured I would start with the roof area first. My problem is I don't know exactly how to approach it even after doing research.
My garage is pole construction with roof trusses on each pole with 8' spacing. Its not a typical pole construction because its not a steel sided or steel roof building. The 2x6in purlins were put between the trusses on metal brackets and the roof sheeting is put across that topped with shingles. The sides are OSB with vinyl siding. I want to put kraft back fiberglass bats under the roof deck across the purlins and leave the rafters open.

I have 2 main concerns.

#1 I would like to go R19 in the ceiling but filling the space w R19 will put fiberglass insulation in contact with the sheeting which I understand to be unacceptable. I don't want the roof deck to rot, so I can do R13. I also thought about perforated radiant barrier there too, keep the insulation off of the sheeting . That leads me to #2...

#2 Ventilation. I don't want to shorten shingle life by having no air movement. The roof has a ridge vent. The eaves sort of have ventilation. There is a gap between the end board on the trusses and the osb sheeting where the vinyl siding is slid under that does have air movement from the outside. I had considered sealing it up with foam sheet but if it can be used for roof vent it would leave it. The next hinderance is the purlins that would block off ventilation. I would have to drill holes for air flow.

I have included photos to support my ramblings. Also I'm located in Central Illinois.

What suggestions or approaches do you have for me?

 

[hockey88fan]

Sorry I can't help, I have no idea, but am interested in seeing suggestions. There's got to be someone on here who has the exact same set up.

 

[50cal]

It looks like you have no overhang. My first crazy idea was 4 inch perforated tube like used for drainage the length of the building. holesaw both ends cover with a vent. now you need something on the same concept as a duravent to hold back the insulation. Thousands of buildings like this there has to be a way assuming you want to use blown in. My metal building is the same way we used the wide bats with vinyl backing.

 

[Strouty]

I think they make a corrugated styrofoam, it looks like the metal used in roofing or siding. I think if you used that first, then put the insulation against that you may not have the issues you are worried about. I am not a carpenter, but I would like to know the answer to your questions as well.

 

[rsa]

From my reading, these are my thoughts. I'm not a building professional.

Option 1: Vent above the roof deck. Have you roofed yet? If not, that would allow you to vent above the roof deck. Keeps the roof cold and addresses ice damming. See post 17 by Robert Riversong in this discussion. Edited slightly to apply to your roof:

...sheath the trusses with 3/8 CDX covered with #15 felt, install 2x4 flat sleepers over each truss and a thicker layer of sheathing of your choice with whatever peel-n-stick nonsense and roofing that floats your boat. Then vent the 1½" air gap bottom and top...

This creates a perfectly vented and insulated roof assembly with a complete secondary drainage plane below the roofing/sheathing - like a rainscreen siding approach. Both the secondary sheathing and the primary roof sheathing can dry to the vent cavity for long-term reliability and durability.

Option 2: Unvented Roof. You could also go with an expensive, unvented hot-roof. See: A Crash Course in Roof Venting.

Option 3: Venting is Overrated. If it were me, on a budget, I'd try to optimize the existing layout without venting-heroics. You appear to have great intake at the soffits and a ridge vent for exhaust. Accept what ventilation those will provide in your situation and concentrate on making the ceiling plane as airtight as possible, then go overboard insulating the attic floor. I think this is especially defensible because you'll have fewer or reduced sources of moisture in your garage than you would in your home.

• Ventilation: Ventilation is overrated. At least according to Martin Holliday of Green Building Advisor.

  ...attic ventilation is overrated. (If you have an air-sealed ceiling and plenty of insulation, attic ventilation is fairly unimportant.) But as long as you've gone to the trouble of installing soffit vents, you should certainly make sure that there are clear air channels from your soffit to your attic. That usually requires insulation dams above your perimeter top plates, and vent chutes to create an air space below your roof sheathing.

  The link also includes a rebuttal, but the rebuttal seems to justify Holliday's position if scrupulous attention is paid to sealing the ceiling and using adequate insulation.
  
  Good attic venting reads by a real Building Scientist, Joe Lstiburek: Understanding Attic Ventilation and Lstiburek’s Rules for Venting Roofs .
  
  Note: high-snow-load areas require venting regardless.
• Airtight: Drywall the ceiling and use the Airtight Drywall Approach and maker sure all penetrations were sealed.
• Insulation: Blown cellulose on the attic floor. The 2012 IECC recommends R-49 for Zone 5.
  
  The perimeter of the attic is a problem—you don't have enough depth for insulation. Joe Lstiburek's rule of thumb is "Thou shalt never have less R-value on the top of your top plate than in the wall. It would be nice to have even more, but not less." Here's where spray foam or rigid foam would be appropriate, even on a budget job. You can then "over insulate" the rest of the attic to achieve an average or R-49 (or whatever value you choose).
  
  My DIY choice would be to use a site-built rigid-foam vent-channel that keeps the cellulose off the ceiling, blocks the cellulose from getting into the eaves, and contributes to insulating the shallow area above the top plate. Easier would be to use Accuvents and fill them with closed cell spray foam, at least above the top plate.

Stewart
_____
I wish more building science articles dealt directly with accessory buildings

 

[wedel1]

Re: Roof Insulation Advice

I am familiar with the roof insulation articles posted, those were found in my research, but I couldn't see how those plans would apply to my situation. I was pointing out that I had little to no air intake for venting, a ridge vent, no overhang on the roof, and purlins that would be great for stapling fiberglass kraftback insulation to but would block any roof deck venting I could create. It's also already roofed.

I'm still thinking fiberglass bats between the purlins, probably r13 to leave a gap between the roof sheeting and the bats. I am considering foam sheets to put in there sheeting and the bats but that would just about double the material bill.

Anyone got any other ideas to talk me into?

 

[JimVonBaden]

This is the same issue I had. I got mostly the same responses.

Jim

 

[wedel1]

Well, as a positive, no one is trying to talk me out of doing it that way...

 

[stevefasano]

If it was my building..Nonvented with sprayed foam under the deck/ roof and batts for walls..

 

[wedel1]

Spray foam isn't out of the question but I was looking to keep it a DIY project. I really wouldn't mind buttering the whole thing with an inch of foam to seal it up but $$$ is an issue.

 

[buzz4041]

An inch of foam will give you about an R6 or so but will help for sure. They have DIY kits from many manufactures but is pricey. You could budget a kit every few months and continue till done. I put up 3/4" fiberboard on my whole house at bottom of rafters and would do 10 sheets every couple months. About 2 years later I had it all done.

 

[wedel1]

Let's bring this thread back from the dead....

After much consideration, I think I have an insulation solution for my roof.

First layer against the roof will be 1 1/2in foam board with w/foil cut to fit the space between the trusses and purlin. I would use adhesive to stick it to the roof deck.

Second Layer will be R13 fiberglass bats stapled to the purlins to fill the rest of the cavity.

Then I'll finish the with corrugated panels leaving the trusses exposed and mostly block the ridge vent.

Talk me into it or out of it....

 

[Shop Specialties]

Instead of going in between the trusses and purlins I would go over them creating a air gap between the roof and the foil backed rigid insulation. Air would flow up through the soffit vents and out the ridge vent. You do not want to go up against the roof because the heat or cold will transfer right through. I see where Johns Manville has a 3.5" foil backed rigid insulation with a R21 but I have no clue how much it costs.

 

[DanCo]

^^^user has been reported for double posting^^^

 

[TimRogers]

Instead of going in between the trusses and purlins I would go over them creating a air gap between the roof and the foil backed rigid insulation. Air would flow up through the soffit vents and out the ridge vent. You do not want to go up against the roof because the heat or cold will transfer right through. I see where Johns Manville has a 3.5" foil backed rigid insulation with a R21 but I have no clue how much it costs.

This is exactly what I want to do but I have only found the foil backed foam insulation in a 1/2" thickness at the big box stores here in metro Atlanta.
The Johns Manville website doesn't even seem to mention the stuff.

Any ideas on where to find it and the best methods of attachment to the trusses and sealing the edges together?

 

[Shop Specialties]

I had just done a Google search for foil backed rigid insulation and found Johns Manville. The big box stores here in Montana carry up to 2". I thought long screws with fender washers would hold it up fine but have still not figured out how I would seal the edges.

 

[TimRogers]

Thanks for the advice.

Looks like I'll be using foil tape to seal the edges.