Shop Roof / Ceiling Installation?

[T411]

I have a shop that’s 40’x18’ 1-12 pitch slanted down long side to long side. The ceiling/roof is 12” wood I-joist 24” on center slanted long side to long side. The wood I-joists are about 21’ including the vented roof eaves. The roof is decked with OSB vapor barrier and corrugated metal. I already have the 2”x6” walls insulated with rockwool and I’d like to do the same with the ceiling/roof.

Now to my question:
DoI need to leave an air gap between the bottom of the OSB deck and the rockwool insulation? My city code says I can just fill the whole ceiling gap with insulation but the people at rockwool said I should leave a cap between the roof deck and rockwool insulation. I live in Albuquerque and the climate is high desert/fairly dry.

 

[jbailey927]

I would think if you have any sort of venting down at the eaves or at the crown you would want to put in something like this rafter vent:

ADO Products Durovent 23-1/2 in. x 46 in. Rafter Vent with Built-In Baffle 10/Ctn UDVB15234610 - The Home Depot

ADO Products - Durovent 23-1/2 in. x 46 in. Attic Ventilation System with Built-In Baffle 10/Carton - Install between rafters to provide an air channel through insulation to help keep air flowing freely.

www.homedepot.com

 

[racecougar]

From the photo, it looks like the blocking between I-joists is already going to prevent airflow. Venting certainly would be preferable, but it seems you're already going down the hot roof path.

 

[bluedog225]

I’ve got the same situation going on here in Central Texas. Also 1:12. The right answer for me would’ve been foam on top of that decking. Makes all the difference. Even 1 inch. I’ve failed at that and I’ve been struggling with this for a while.

I ended up putting 3 inch holes in all my blocking. The area of these 3” holes is greater than the area of the holes and the hardy board soffit vent. The mild slope of the roof puts it in the questionable territory of whether it will naturally ventilate sufficiently to remove moisture.

You can use those foam air channel deals. They’re fine. Or if you’re putting in batts, you can just put 10 inches of bats in a 12 inch space. No need for a very flammable air channel guide. Just leave 2 inches of space above your insulation. And drill appropriately sized holes in your blocks.

A couple of concepts if you want dig deeper. There’s some thing, I’ve forgotten the name, where a flat roof facing the dark sky at night will radiate more heat than you would expect. And result in a roof that is cooler than you would expect in the mornings. This will bring the temperature down below the dewpoint and you can get condensation under that deck. Nightime radiation or something like that.

There’s also something called the dry deck? I probably got that name wrong too. But it’s a relatively new deal where you foam it all up and seal it all up. The theory being that you’re going to exclude moisture completely from the bottom side of that deck.

I’m not comfortable having undetected moisture get through my roof and into the deck and structural supports of the ceiling without any way of knowing about it until it’s a catastrophic failure. Even though I’ve got double lock standing seam and rain guard on top. I’m just not going to do it.

At the end of the day, I’m probably going to put in quarter inch plywood sheets with 2 to 3 inch blocking to maintain the airspace. Then blow celluloses into the remaining 14 inch space.

It’s a huge pain in the *** and a lot of money for materials. But that’s where I am. Alternatively I might use our R49 fiberglass batts. That would be once and done and pretty easy. Though I really like the performance characteristics of mineral law, and cellular, over fiberglass.

Powered venting is another option.

bottom line is I haven’t found a great solution. But you don’t want to trap moisture up there.

 

[ericm]

From the photo, it looks like the blocking between I-joists is already going to prevent airflow. Venting certainly would be preferable, but it seems you're already going down the hot roof path.

The roofs of my new house are made like that. They cut three holes near the top of each blocking piece for air flow. It might take a 90 degree drill and be a bit of a pain but the OP can probably do it in place.

But venting would for sure also need soffit and ridge vents. BTW the fire resistant ones like Brandguard aren't all that expensive these days due to widespread adoption on the west coast.

 

[racecougar]

Agreed that the OP can/should open up holes to allow for venting; my point was that should he/she not do so, there isn't much point in leaving an air space. That said, as bluedog225 pointed out, a 1/12 slope may not vent well on its own. The whole situation isn't ideal.

 

[bluedog225]

Whatever you end up doing. Or you can do this ahead of time. Stick one of these up on the ceiling. You can get data about temperature and humidity which will help you make a decision.

Or stick one up there before you put up your installation or whatever solution you end up doing. And you can monitor it. It takes a ton of readings and you can download them via Bluetooth. From memory the battery lasts a year, but I haven’t checked mine recently. I need to go up there and download the last eight months of reading so I can see how it’s doing.

If you put this up there and go ahead and install your insulation solution, it would be good to know if that OSB is in the process of rotting so you can take it out and try something else.

Next, I’m going to build a little mock insulated compartment. To see how it does in more real world conditions before I slap all the insulation up there.

My build is proceeding very slowly since I’m doing most of the work myself. This fall I’d like to hire some folks and knock out some big parts.

 

[T411]

From the photo, it looks like the blocking between I-joists is already going to prevent airflow. Venting certainly would be preferable, but it seems you're already going down the hot roof path.

Thank you for that! I’ll have to make a change there!

 

[T411]

I’m not comfortable having undetected moisture get through my roof and into the deck and structural supports of the ceiling without any way of knowing about it until it’s a catastrophic failure. Even though I’ve got double lock standing seam and rain guard on top. I’m just not going to do it.

At the end of the day, I’m probably going to put in quarter inch plywood sheets with 2 to 3 inch blocking to maintain the airspace. Then blow celluloses into the remaining 14 inch space.

I agree, I don’t like the idea of not knowing about a problem until it’s too late. I will say that because I’m Albuquerque and the climate is high desert/fairly dry I don’t have as big of a worry as you being I Texas with the humidity.
Thanks for the response. I like your other idea!

 

[T411]

The roofs of my new house are made like that. They cut three holes near the top of each blocking piece for air flow. It might take a 90 degree drill and be a bit of a pain but the OP can probably do it in place.

But venting would for sure also need soffit and ridge vents. BTW the fire resistant ones like Brandguard aren't all that expensive these days due to widespread adoption on the west coast.

Thank you for that suggestion. I’ look into that!

 

[T411]

Whatever you end up doing. Or you can do this ahead of time. Stick one of these up on the ceiling. You can get data about temperature and humidity which will help you make a decision.

Or stick one up there before you put up your installation or whatever solution you end up doing. And you can monitor it. It takes a ton of readings and you can download them via Bluetooth. From memory the battery lasts a year, but I haven’t checked mine recently. I need to go up there and download the last eight months of reading so I can see how it’s doing.

If you put this up there and go ahead and install your insulation solution, it would be good to know if that OSB is in the process of rotting so you can take it out and try something else.

Next, I’m going to build a little mock insulated compartment. To see how it does in more real world conditions before I slap all the insulation up there.

My build is proceeding very slowly since I’m doing most of the work myself. This fall I’d like to hire some folks and knock out some big parts.

On order!

 

[T411]

Ok ladies and gentlemen, don’t laugh me out of the forum for my new question concerning the same topic. I appreciate all the responses from everyone that took the time.

I just got back from Lowes looking for these Baffle vents and I was surprised by how flimsy they are. Maybe there is no need for them to be anything other than what they are but made me look to what Bluedog225 suggested with quarter inch plywood sheets with 2 to 3 inch blocking to maintain the airspace… until I looked at the pice of 1/4” plywood… WTF!

As flimsy as the off the shelf Baffle Vents are and as expensive 1/4“ plywood is I feel like there has to be a better more cost effective alternative? so I looked at a couple of alternatives and wanted some feedback. I still like what Bluedog225 suggested so I found an alternative to plywood… Hardboard. It’s the same material that pegboard is made of except no holes. It’s less expensive than the off the shelf Baffle vents if I rip the 4’x8’ sheets down to size and definitely more rigid.

The other product that I thought could be used in the same manner as Bluedog225 suggestion is Kingspan Insulation Q250 Fanfold Siding Underlayment that comes in 4’x50’. It is Faced Polystyrene Board insulation, probably 1/4” thick and is more cost effective than the off the shelf baffle vents if I cut it to size with a razor knife… also more ridged.

Okay… should I expect to hear the sound of the gong?

 

[bluedog225]

Yeah. Ply is pricey. I’d use whatever sheet stock material is the cheapest and stiff enough to hold back the insulation. It’s not going to take much. Unless you are dense packing cellulose.

Also, blown cellulose will settle. I think it settles reliably. So you could in theory blow the insulation in the cavities and just rely on it settling 1-2 (?) inches. Logically, I know it would work but I’m still going to have a spacer because I’m not sure how to avoid dense packing accidentally.

The foam baffle vents are flimsy. But combined with the settling, maybe good enough.

My issue with the foam is it burns so easily and energetically.

I vaguely recall that there are some stiffer cardboard spacers.

 

[bluedog225]

Edit-also Craigslist plywood. You don’t need pretty. Just be careful with wood boring insects if they concern you.

 

[bluedog225]

@T411

One more thing, if you’re someplace hot, (albaquirky will do ) you can lay whatever would you buy off craigslist out in the sun wrapped in plastic. I’m up in the air whether it should be clear or black. I think that will kill anything in the wood like powder, post beetles, or longhorn beetles. Even though they’re not supposed to like plywood, I’ve seen them dig in. Or use timbor.

 

[C-S-H]

The other product that I thought could be used in the same manner as Bluedog225 suggestion is Kingspan Insulation Q250 Fanfold Siding Underlayment that comes in 4’x50’. It is Faced Polystyrene Board insulation, probably 1/4” thick and is more cost effective than the off the shelf baffle vents if I cut it to size with a razor knife… also more ridged.

You need to know the vapor permeability of that material. It needs to be vapor semi-permeable or you will create a condensating surface inside your ceiling assembly. If you add a vapor barrier then you must build-in condensation prevention into the assembly, such as having a certain minimum % of your total R-value in XPS board. For where you live I would want vapor semi-permeable inner and outer layers in the ceiling assembly with mineral wool batts inside. Ex.

Inside air
Latex paint
Drywall
Mineral wool
Plywood or similar
Vented cavity
Roof deck OSB
Underlayment
Metal Roofing
Outside air

 

[Muckin_Slusher]

I've been thinking about similar issue for years now wanting to finish out my warehouse roof.

I've got 16 inch I joist roof, 48 feet from eave to peak (has midspan support wall). I cut the holes in the blocking as big as allowed, and it's still not enough area to satisfy the shingle manufacturers recommendations.

I also will not "hot roof" it with spray foam because I want to know about any leaks that may happen.

My plan is to power vent each space between joists. Maybe with little computer style fans?

I'm also making sure that I'll have access to inspect, possibly with a sewer camera on a little wheeled trolley.

 

[danfromsyr]

Staple up the foil faced bubble wrap

Double Foil Floor Joist Rafter Insulation Reflective Bubble Roll 16X50 R8-24 - Amazon.com

Double Foil Floor Joist Rafter Insulation Reflective Bubble Roll 16X50 R8-24 - Amazon.com

www.amazon.com

iirc it can even be bought in a ready width roll

I used 1in foil faced polystyrene with strips on the sides of the rafters as blocking.

the hardboard will get moldy in my environment. maybe not in yours.

I stapled in stainless steel window screening at the bottom of the channels to keep out bugs & rodents.

 

[T411]

You can use those foam air channel deals. They’re fine. Or if you’re putting in batts, you can just put 10 inches of bats in a 12 inch space. No need for a very flammable air channel guide. Just leave 2 inches of space above your insulation. And drill appropriately sized holes in your blocks.

i just re-read your post. Are you saying that I don‘t necessarily have to use those foam air channels or even an alternative if I just leave an air gap above the Rockwool and roof decking? Along with shortening the blocking to allow airflow?

Staple up the foil faced bubble wrap

I also like this idea and seems cost effective but is this material in contradiction or contrast to what C-S-H said below?

You need to know the vapor permeability of that material. It needs to be vapor semi-permeable or you will create a condensating surface inside your ceiling assembly. If you add a vapor barrier then you must build-in condensation prevention into the assembly, such as having a certain minimum % of your total R-value in XPS board. For where you live I would want vapor semi-permeable inner and outer layers in the ceiling assembly with mineral wool batts inside.

Are you saying that if I use a material for the air channel it needs to be semi-permeable and the foil bubble wrap would not suffice?

 

[C-S-H]

Are you saying that if I use a material for the air channel it needs to be semi-permeable and the foil bubble wrap would not suffice?

Yes. Some of the thin foam boards are water vapor semi-permeable (1 to 10 perms), so you might look into them if you don't use plywood. Some leave the batt insulation open on the top, but I am not a fan of that in a cathedral ceiling assembly.

 

[T411]

Okay… do I even need baffles if I’m putting 10inch thick insulation in the 12 inch space between the joists? If I can keep the 2 inches of space clear from one vented eve to the other vented eve, above the insulation and roof deck, why do I need to install baffles? What are the baffles doing besides keeping airflow clear between the vented eves?

 

[ericm]

Keeping the holes or vents clear is all the baffles are for. If you can keep the holes clear without there's no need. 2" blocks nailed to each rafter at the roof side would keep the foam board from moving up and blocking the vents. Are you going to cut the foam exactly to the size of the space between boards, or cut them a little small and fill the gaps with spray foam? The latter might be easier and make for better sealing but you need to make sure the foam does not block the holes.

10" is a lot of foam!

I just ran a loadcalc on R38 (code minimum in my area) vs R50 for the traditional attic ceiling and it's like 3% difference in BTUs required at design heating temps. Of course your conditions may vary from Southern Oregon.

 

[bluedog225]

As long as the air reliably washes through it’s fine.

 

[T411]

Keeping the holes or vents clear is all the baffles are for. If you can keep the holes clear without there's no need. 2" blocks nailed to each rafter at the roof side would keep the foam board from moving up and blocking the vents. Are you going to cut the foam exactly to the size of the space between boards, or cut them a little small and fill the gaps with spray foam? The latter might be easier and make for better sealing but you need to make sure the foam does not block the holes.

10" is a lot of foam!

I just ran a loadcalc on R38 (code minimum in my area) vs R50 for the traditional attic ceiling and it's like 3% difference in BTUs required at design heating temps. Of course your conditions may vary from Southern Oregon.

No foam - I’m using fiberglass that’s 10 inches thick. I’m only getting a R-30 but it’s for a shop so I’m not worried about a higher R-value. I thought if I put the insulation support wire above and below the un-faced fiberglass insulation I can maintain the 2inch clearance for ventilation without baffles.

 

[racecougar]

Baffles would ensure that the air space is retained above your insulation, and would also provide less resistance to air flow than exposed fiberglass. With the low slope here, I'd give ventilation all the help it can get. I think you'll have a tough time ensuring that the top of the insulation is a uniform distance from the underside of the roof sheathing using just support wires.

 

[BobnCO]

“Okay… do I even need baffles if I’m putting 10inch thick insulation in the 12 inch space between the joists?”

Yes, people have done it this way for years, many permitted/ inspected projects. At most they use the baffles ONLY AT THE EVES, but if you install carefully it is optional. People often pitch the nth degree of ideal thermal design, but a 10” batt in a 12” space is SOP for most that don’t seek or demand perfection. (Insulation is not “moving around” up there either)

 

[bluedog225]

Baffles would ensure that the air space is retained above your insulation, and would also provide less resistance to air flow than exposed fiberglass. With the low slope here, I'd give ventilation all the help it can get. I think you'll have a tough time ensuring that the top of the insulation is a uniform distance from the underside of the roof sheathing using just support wires.

Agreed. My main worry inadvertent insulation blockage.

 

[ericm]

By "moving around" I really meant getting pushed up too far during installation. My brain wasn't getting the right words out.

In my house that's getting built now they put 10" of fiberglass batt in a 12" high bay just like the OP's doing. No baffles in the middle of the roof. They might have had them at the eaves but I don't remember.