Insulating my shop

[HawkinsHollow]

I need to insulate my shop this winter so I can get a mini-split in there by next summer. I am going to use batt insulation between the rafters. How important are those styrofoam channels people use between the roof decking and the insulation? I understand they allow better airflow from the soffit to the ridge. Just trying to decide if I will go that route. Roof structure is 4:12 trusses on 2 foot centers, with foil backed osb decking under 5v metal roofing.

 

[EddieWalker]

Heat rises. The air coming in from the soffits, the lowest point on the roof, is the cool air. It heats up in the attic space, which creates air flow to the highest point of the roof. The air flow dries out the attic. Dew forms just about every morning inside an attic, almost all year long. The air flow is needed to remove that moisture.

The Styrofoam channels, or baffles, keep the insulation off of the soffit vents. This is an issue with blown in insulation, but rarely a problem with rolled bats. If you are careful, and you do not block the soffit vents, then there isn't any need for the baffles.

Fiberglass bats will never block air or condensation from forming under the metal, not matter how tight you install it. The amount of air and the amount of humidity in your area will decide on how important the air flow is. I've seen houses with almost no airflow that where decades old that didn't have any issues at all. I've also been in newer homes that didn't have enough soffit vents that had horrible mold issues.

I have a job booked for later this month to install as many soffit vents as I can on a ten year old house that has mold issues. I'm also installing Styrofoam baffles because the blown in insulation is blocking some of the vents. It's easy to do, and a long term solution to spend a buck on each baffle then to try to clear the insulation out of the way and hope it stays out of the way.

 

[KennyG]

Not sure what size building we are looking at but have you considered installing a ceiling? A metal ceiling attached to the bottom of the trusses with a vapor barrier will let you blow in as much insulation as you want and leave the attic space open and ventilated. Not sure how the costs will compare but I think you will have a better conditioned space with a ceiling.

 

[HawkinsHollow]

Gotcha Eddie, thanks for that. But I am talking about what they call rafter vents. They are 22x48" long and they get stapled under the decking to allow that air to move all the way from the soffit to the ridge vent. They look like this

It is $20 for a pack of 10, so it is not gonna break the bank. I need about 80 of them.
Just trying to see if people think it is necessary.

@KennyG This is a 25x28 garage built with trusses. I do not want to put a ceiling on it I would rather keep it open.

 

[EddieWalker]

I'm talking about the same thing. They work great to keep insulation out of the soffit vent so the cooler air can enter the attic space. I've never stapled them in, they are sized just right so they squeeze in between the rafters and stay tight.

If you are going to staple fiberglass bats to your rafters, there will still be plenty of air that gets in through the ridges on the R panels on the metal roofing.

Or do you have a standing seam roof with wood decking? In that case, the condensation that you want to dry out from the air flow will be under the metal, and on top of the wood decking. There should be a layer of something waterproof on top of the decking so the condensation wont destroy the wood decking.

Either way, the baffles really wont offer very much to improving air flow, and I wouldn't install them on my buildings.

 

[BlueKnife]

How important are those styrofoam channels people use between the roof decking and the insulation?

Their purpose is to allow airflow when there isn't enough due to the insulation pushing up against the bottom of the roof. So it depends how your roof and walls were designed and how much room there is between the top of the wall and the bottom of the roof deck. Airflow, ideally from eaves to ridge, above the insulation is absolutely critical to prevent condensation issues.

 

[BlueKnife]

I need to insulate my shop this winter so I can get a mini-split in there by next summer. I am going to use batt insulation between the rafters. How important are those styrofoam channels people use between the roof decking and the insulation? I understand they allow better airflow from the soffit to the ridge. Just trying to decide if I will go that route. Roof structure is 4:12 trusses on 2 foot centers, with foil backed osb decking under 5v metal roofing.

You mention rafters and then later in your post, trusses. Those are not the same thing. Do you know which one you have?

 

[1 Old Man]

Gotcha Eddie, thanks for that. But I am talking about what they call rafter vents. They are 22x48" long and they get stapled under the decking to allow that air to move all the way from the soffit to the ridge vent. They look like this
View attachment 842388
It is $20 for a pack of 10, so it is not gonna break the bank. I need about 80 of them.
Just trying to see if people think it is necessary.

@KennyG This is a 25x28 garage built with trusses. I do not want to put a ceiling on it I would rather keep it open.

Hawk, go on and staple them. It's a cheap insurance there will be air flow. Fiberglass batts do not perform well when packed tight. I used that exact system on a rafted 22' vaulted ceiling w/ 5/8" drywall. Customer has had 0 complaints years later.
Good luck.

 

[1 Old Man]

You mention rafters and then later in your post, trusses. Those are not the same thing. Do you know which one you have?

He may not know. Trusses are pre-fab and can span large areas without other support (walls or posts). Rafter/ joist are usually cut and framed on site.

 

[HawkinsHollow]

You mention rafters and then later in your post, trusses. Those are not the same thing. Do you know which one you have?

I have trusses. These foam inserts are called rafter vents, that is the only time I mentioned rafters.