Any home insulation experts?

I need to insulate my new shop ceiling. I currently have open scissor trusses that will eventually get some type of sheeting on the underside. If I could afford to spray enough foam to handle all of the insulating, then I probably would but that's not budget friendly, so I'm trying to come up with a decent and cost effective middle ground.

I'd like to use about 1 inch of spray foam on the underside of the roof (metal with a moisture/vapor barrier underneath) in order to totally seal it up and eliminate any drafts. Then I'd use a blown in cellulose type of insulation on top of my ceiling panels to handle the bulk of the insulating duties. Anybody see an issue with this approach? Will I run into weird moisture/ventilation issues having 2 layers of different insulation sandwiching an air cavity?

In reply to STM317:

Why blown-in vs. fiberglass batt? I'm no expert, but I've done both methods & found fiberglass to be way easier and less mess. I think it may have better R value per-inch too?

In reply to Pete Gossett:

Based on what I'm seeing locally, cellulose is cheaper here by $0.10 / sqft or more for an equivalent R value (I have a bit over 1500 sqft of ceiling to insulate so that price difference does add up), and it doesn't have the drop off in performance that fiberglass supposedly does in cold temps. I also have trusses 48" on center, which limits my options for fiberglass. And I'll be able to add depth to the cellulose at a later date pretty easily if the initial depth proves to be insufficient.

However, for my walls, I'll be framing them out more normally, and plan to use a faced fiberglass batt for the ease and cleanliness of it.

I believe you need 2" of foam to make a non vented assembly. (in Seattle area anyway Im 95% sure thats the code) otherwise no that sounds like a good plan.

Couple of thoughts...

• First off, the spray foam will completely seal the roof panels, but will probably not seal the eaves. It can be done, but is unusual.

• Is the metal roofing on plywood, or open to below?

• Is your shop a commercial building, or a residential one? Different codes. (It sounds like you are describing a commercial building).

• If you do seal the eaves, you will change the building from a ventilated roof system to an unventilated one. This takes different considerations for moisture vapor control.

• Ceiling panels- do you mean a suspended ceiling? Spraying cellulose on a commercial suspended ceiling would be a big mistake. It would leak through the ceiling tile joints, and be a total mess. Technically, laying fiberglass on a suspended ceiling is no longer permitted (but you may want to ignore that)

• The vapor barrier should be on the warm side of the insulation. Closed cell spray foam is its own barrier, so that is not an issue, but open cell spray foam would be a problem. If you later add cellulose below that, you will need another vapor barrier below the cellulose. That's another reason fiberglass may work better.

• What kind of shop? (It matters code wise)

In reply to SVreX:

I was hoping you'd chime in. I should've had a pic up earlier. Here's what I've got right now:

The building is a personal garage on my private property. The metal was laid directly on top of a vapor barrier membrane that is visible in the pic(looks like white bubble wrap) that is laid on top of the framing. No plywood or sheathing under the metal roof.

It currently has no venting (no ridge vent, gable vents, or soffit vents).

My plan, was to run Tyvek along the underside of the bottom chord of each truss, and eventually, to finish that with metal panels. Basically like this, except not a flat ceiling:

The Tyvek would keep the cellulose from resting directly on top of the metal ceiling, and keep it from falling through any gaps. It would also allow me to insulate the barn sooner, rather than waiting for my contractor to clear enough time to come back and do the ceiling.

I don't plan on running any HVAC, but I hope it will have a small gas heater before it gets too cold. It will probably be used to keep the temps between 40-50 degrees during the coldest weeks of the year.

I'm pretty set on the cellulose for the ceiling. It seems like it's just going to depend on whether it's a good idea to spend a little more and foam the underside of the roof first, and use less cellulose or skip the foam all together and use more cellulose.

You are trying really hard to come up with a new way to insulate a space that is not even going to be conditioned.

Tyvek is not a vapor barrier. Insulation does not stop airflow.

A proper insulation system is basically trying to accomplish 3 things- stop the heat flow, stop the air flow (drafts), and stop the water vapor from reaching the dew point in the wall.

A standard wall has 1) insulation to stop the heat flow, 2) air barrier to stop the air flow, and 3) vapor barrier to stop the moisture on the warm side of the wall.

Tyvek is an air barrier, not a vapor barrier. It is usually on the exterior of the thermal barrier (insulation). It prevents air from flowing into the building, but allows water vapor to dry to the outside. Taped drywall is also an air barrier.

Vapor barriers may be paper facing on insulation, plastic, vapor barrier paints, etc. They are applied to the warm side of the wall, so the moisture in the warm air does not enter the thermal mass (insulation) and find the dew point (the physical point in the wall that is a particular temperature which will enable water to condensate at a particular humidity). You don't want water condensating in your insulation. Vapor barriers SHOULD NOT be applied to the cool side of the wall- you want a wall to be able to dry to the outside.

So, here's what you've got...

• The metal ceiling is not anything. It doesn't stop water vapor or drafts. It is, however, an excellent surface for water to condensate on.

• The Tyvek is on the inside of the insulation. It draft seals, but does not stop water vapor.

• The cellulose has thermal transfer limiting abilities, but it is loosely installed on top of a slippery sloped surface (which might get wet). Over time, it could slide downhill a lot, leaving large gaps at the top.

• The attic air gap is useful to limit conductivity.

• The spray foam is useful as an air infiltration and as a vapor barrier (assuming it is closed cell), but it is on the wrong side of the cellulose thermal barrier. It will prevent the assembly from drying to the outside. In fact, if warm moist air gets into the unventilated attic, it may condensate on the spray foam, and drip back down into the cellulose.

• Then you have an additional plastic vapor barrier, again on the wrong side of the thermal barrier.

• Then you have no decking, which also typically serves as an air infiltration barrier.

• Then you have metal roofing- again, an excellent place for water to condensate.

• Then you are going to heat it with a portable gas heater, which creates excessive moisture.

So no, I can't say it will work. I am not saying it will fail (because the whole assembly is hard to predict). You are reinventing the wheel, and I question the overall assembly.

I recommend trying something more conventional. Kraft faced fiberglass batts with ventilation above.

The short answer to your question about the spray foam- I don't think it is an effective use of your money.

Where is this building located?

These guys are decidedly anti-foam but they might be willing to help you out with a strategy. https://www.foursevenfive.com

Question for OP: Is that double bubble insulation under the roofing panels? I ask because I had a metal roofed shop that used it under the roofing panels. I insulated the walls from the trusses down, and drywalled. It held both cool and heat perfectly well for a shop, I thought. I live in a relatively mild TN climate, and think the drywall helped a lot. YRMV.

I design this kind of thing for a living. SVrx is spot on with his assessment.

I'm located in Indiana. Basically right on the border between Climate Zones 4 and 5.

I appreciate the input so far. It's given me some stuff to think about. I'll continue researching. I do want to clarify a couple of small things that won't really change your answers, but could influence things a little bit.

• The condensation collecting on the metal would never directly contact any insulation. There are barriers in place to prevent that.

• The heater I'd plan on is not a portable unit. It would be a suspended gas heater that vents to the outside like this:

As such, any water issues would arise strictly from humidity in the air.

Again, thanks for the insight so far. Don't be surprised if I have some follow up questions after a bit more research.

In reply to STM317:

Im not sure I agree with you.

The metal ceiling is not separated from the cellulose insulation by a vapor barrier. Therefore, condensation on the metal can get into the insulation.

The spray foam at the roof is a vapor barrier on the cold side of the assembly in an unvented area, therefore condensation can form on it and drop back down to the cellulose.

If you drew a diagram of the cross section of the building showing the building thermal envelope, it would go up to the insulation. But a diagram of the building vapor envelope would go up to the spray foam. Those 2 diagrams would overlap each other by the full depth of the trusses, and the overlap would include all of the cellulose. If you don't keep the vapor out of the cellulose, you will have problems.

The vented Reznor is definitely better than unvented, but warm air is still moister than cold.

Fair enough. I knew the changes wouldn't alter your opinion much, I just wanted to be as clear as possible.

For the sake of discussion, lets substitute something other than Tyvek between the cellulose and the ceiling metal. A true vapor barrier, instead of just a waterproof material.

Also, in the coldest days of winter (the only times the heater would be running) I'd probably welcome a little humidity from the heater. It gets pretty dry here once temps dip much below freezing, and it's not uncommon for house furnaces to have built in humidifiers for times like that.

In reply to STM317:

Fair enough.

I didn't think my opinion would alter your's much.

If you are going to insulate between the lower cords of the trusses then you will need to vent the space above that is between the insulation and the roof. Pop in a couple gable end vents at a minimum. Adding eave venting with the gable end vents would be much better. Having eave and ridge vents would be the best just don't install both ridge and a gable end vents. They short each other out and actually reduce venting as compared to having one or he other in conjunction with eave vents. This spray on foam to the underside of the metal roof is a wast of time and $$$$$.

More thoughts added to my above comment. Put your vapor barrier on the warm side of the insulation across the bottom of the cords. Tie it in to the walls such that you are tight on the inside (you do have a vapor barrior on the walls?) if so tie in to that. The hardest part of a structure to insulate and seal is where the truss's or joists meet the wall. Just a PITA but done correctly this will keep the heat in and seal things up while allowing for venting at the eaves. For houses they make things like durovent or acuvent that is meant to go between the rafter/joist at the eave to keep the insulation away form the underside of the roof to allow venting of the space below the roof deck.

The only down side to everything I have just said is if you have any pipes up in the space below the roof that can not freeze. Like water lines. Then things get a bit more complicated.

SVreX wrote: In reply to STM317: Fair enough. I didn't think my opinion would alter your's much.

Implying that I'm stubborn huh? Well, I'll show you!

I truly do appreciate the advice. Not sure if it comes off that way or not, but I hope it does.

I have actually done this so let me give you some real world observations.

If it's a metal roof RUN SCREAMING FROM FOAM INSULATION!!

If you have a layer of tyvek already in place what is to hold the foam up? The tyvek definitely won't support it and with the tyvek in place it can't adhere itself to the metal.

Now think about the metal building type of metal roof. It doesn't have a lot of overlap and the guys who install it use the self drilling type of screws. How many times have you used those type of screws and found that after it drilled through the first layer it raised that layer up before completing the drilling through the second layer leaving you with a gap there? Tons right?

Do you really think those guys are being super careful to make sure that never happens?

Those screws have a rubbery washer on the head to kind of seal them so another fail point is that they don't screw it in tight enough or they screw it in too tight and bust it.

And then there's time. Father time, although I think it ought to be Mother time because time is an absolute MOTHER, causes those washers to crack and shrink which means what's sealed today isn't tomorrow.

So let's recap.
You have limited overlap.
Lots of areas with large cracks between the material. Many washers not sealing from the get go & Washers failing with time.

That means you get leaks. Lots more than with a traditional roof or a metal roof designed for a residential home.

So now lets look at the foam. While this is very simplistic there are 2 basic types of foam you will get installed in your building. Soft foam and hard foam.

Soft foam is the most prevalent by far. Soft foam will hold the moisture, hard foam won't.

So the soft foam gets wet, stays wet and if you have a wood decking it will rot it. Also it will let loose after time and suddenly you will have a large section falling down on your head.

The saving grace in a home is that traditional wood decking with asphalt shingles doesn't leak often or as much as a metal roof so it works in a home, but is a fail point in a metal building.

All of the above I've seen happen with my own eyes.

Back to cellulose vs. fiberglass. Cellulose is powdered paper and what happens when it gets wet? It clumps, stays wet a long time and causes rotting and stains. I love cellulose and have it in my house, but in this case fiberglass is the best because METAL BUILDING ROOFS LEAK! I go up every other year to caulk, tighten screws & replace screws on my metal building and I still get leaks.

Foam tends to be alright on the walls because there's not as much moister there &/or it can run downwards and the foam doesn't separate from the walls.

In reply to carguy123:

Sorry to hear all of the hassles you've dealt with! I'm not sure I'd run into many of your leaky issues though as I have a layer of plastic under the roof panels, so any condensation that forms under the roof panels, or leaks from worn out washers should be caught and diverted out of the building, rather than dropping into the building or onto the ceiling/insulation.

As SVReX and Dean have discussed, water vapor in the air would be my biggest concern, and it sounds like if I put a vapor barrier along the bottom chords, and have my insulation on that, it will probably be ok (gable vents may be required).

In reply to STM317:

You are using plastic in case the metal roof leaks? You mean like polyethylene? I'm not sure what that is doing. (If the screws leak, they penetrate the plastic too)

Condensation won't be forming on the bottom of your metal roofing, because you have a vapor barrier (plastic) below it.

The condensation will form on the bottom of the plastic, and it won't drain out of the building. It will drip into the cellulose.

Dual vapor barriers are not generally recommended- they do weird stuff with the assembly.

If you've got plastic under the roof panels then the plastic is what your foam will adhere to. Is it roll plastic or plastic panels? If it's roll plastic that just complicates the problems as it won't hold the weight of the foam.

In reply to SVreX:

It looks like it's "double bubble" insulation as mentioned up thread by wheelsmithy. As you suspected, it's primary function is reducing the likelihood of condensation on the underside of the metal roof. It should also reduce the chance that water drips back into the building too. All screws go directly into the wood framing. This doesn't completely eliminate the possibility of leaks, but it does mitigate it to an extent.

Your initial suggestion was to use Kraft faced fiberglass, which would also present the same issues with having 2 vapor barriers would it not? With the paper backing on the insulation being 1 vapor barrier, and the plastic on the underside of the roof being the second?

In reply to STM317:

Ok, new variable.

"Double bubble" is not really insulation. It's a radiant heat barrier.

You've got conduction happening in the cellulose, convection happening in the attic air space, and radiation happening from the hot metal roof down.

The radiant barrier isn't designed for water. It's designed to limit radiant heat, and you've got it properly located near the roof. This stuff works best when there is an air gap of at least 1" between the product and the metal (which you don't have), and this air gap should be ventilated. (Oh well..)

Don't confuse the products. Radiant barrier is not for water control, and any roof that allows water infiltration and then try's to fix the problem with plastic )or anything) is portly designed and/or executed.

Just use the kraft faced fiberglass and ventilate the attic, and you'll have a good assembly.