Attic ventilation

[galute]

I'm finishing out the inside of my pole barn and before I get to the ceiling I need to make sure I have enough attic ventilation. The plan is to replace the ridge cap with a vented one. My question is the eve vents. What I have is basically the opening created by the ridges in the wall metal. Is this enough for the attic to properly vent?

Here's a pic of what I have. It's 50 feet of this down each side. Those holes are 1.5 square inches 9 inches on center. Is this enough?

 

[Krodad]

Not enough info on the ceiling… do you have trusses with a straight bottom chord? Are you doing a vaulted ceiling?



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[galute]

Straight bottom chord. Attic space will be about 4 feet high from top of insulation to bottom of ridge cap. Not sure what exactly the pitch is but it's not steep at all.

 

[jkwilson]

How will the building be used?

If you will just be heating when you are in there, I wouldn't worry so much about the vents.

 

[kbs2244]

From a quick look, I don't think you are going to have enough inlet.
The ridge vent will allow a lot of air out, but it has to be replaced.
I would try and match the total sq. in. at the eves and ridge.

 

[galute]

How will the building be used?

If you will just be heating when you are in there, I wouldn't worry so much about the vents.

It's going to be fairly well insulated and used for a workshop. It will be heated, not likely air conditioned but you never know.

 

[Krodad]

I have to be honest and say I'm not really sure what I'm looking at in the pic...

Is that an overlapped section of roof steel a couple feet into the building? If so, why? So the straight bottom chord of the trusses is actually below the level of the top girt, and the top of truss is flush with the top of the girt at that outside edge?

If it's what it looks like, I really don't see any options for you in getting more ventilation up there at the edges, unless you were to insulate the walls inboard a bit, and add some vented plugs in the siding itself, leaving a gap for that intake air to move up the wall and into the attic. Regardless, you're going to need to be careful about the insulation placement at the edges so you don't block any flow. Another option is to add some gable end vents in the endwalls as low as possible, but it's not as ideal as soffit vents.
In the end, if you added up all the gaps in the roof steel, you might be surprised at the total square inches of free area available.

 

[galute]

I have to be honest and say I'm not really sure what I'm looking at in the pic...

Is that an overlapped section of roof steel a couple feet into the building? If so, why? That's the roof metal of the lean to that runs down that side.So the straight bottom chord of the trusses is actually below the level of the top girt, yes, by about 8 inches and the top of truss is flush with the top of the girt at that outside edge? pretty close.

If it's what it looks like, I really don't see any options for you in getting more ventilation up there at the edges, unless you were to insulate the walls inboard a bit, and add some vented plugs in the siding itself, leaving a gap for that intake air to move up the wall and into the attic. Regardless, you're going to need to be careful about the insulation placement at the edges so you don't block any flow. Another option is to add some gable end vents in the endwalls as low as possible, but it's not as ideal as soffit vents.
In the end, if you added up all the gaps in the roof steel, you might be surprised at the total square inches of free area available.

Now that I think about it you make some good points. We are being careful to make sure the wall insulation does not touch the outside wall metal. There will be quite a bit of ventilation up the side walls like you mentioned. Also there is going to be some ventilation where the roof metal meets the end walls in the area where the trim covers the two. I'm adding another pic below that may give a better idea of what that area above looks like.

 

[jkwilson]

I'd be concerned that you are going to end up with condensation on that bare metal roof. It will rain inside a metal building full of humid air when the metal cools, and I think that will happen even in the attic.

Seems to me it would be a better choice to insulate the roof between the purlins and add a vapor barrier.

 

[galute]

I'd be concerned that you are going to end up with condensation on that bare metal roof. It will rain inside a metal building full of humid air when the metal cools, and I think that will happen even in the attic.

Seems to me it would be a better choice to insulate the roof between the purlins and add a vapor barrier.

That is a problem I am giving a lot of consideration and will be addressed when I get to the ceiling. Not sure yet how I'm going to deal with it. That's the reason I'm trying to make sure I have proper ventilation.

 

[Krodad]

Hmmm… is that sunlight I see around the vertical poles at the top?

I see light at the gable ends also. I think you'll be okay on the ventilation, but don-t take anyone's word for it until you do some rough calcs on the free area. Another good source for information would be thd supplier of the ridge vent itself.

I can't see where you're from on my phone app, but keep in mind that if you have snow, a ridge vent without a ceiling will result in snow inside the building.

Guess how I know?

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[galute]

Hmmm… is that sunlight I see around the vertical poles at the top?

I see light at the gable ends also. I think you'll be okay on the ventilation, but don-t take anyone's word for it until you do some rough calcs on the free area. Another good source for information would be thd supplier of the ridge vent itself.

I can't see where you're from on my phone app, but keep in mind that if you have snow, a ridge vent without a ceiling will result in snow inside the building.

Guess how I know?

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Yep, lots of light from a lot of places. All of them are under roof or covered in a way that allows air or light in but nothing else. The area around the top of the poles is the only place we need to put some kind of screen just to keep out insects. We don't get a lot of snow but it does happen. I have yet to see even one snow flake blow in anywhere. The ridge cap I am going to install is "supposed to prevent snow blowing in". At least that is what I was told by the dealer. I think with all of the places where light show and ventilation from the side walls factored in there will be plenty of ventilation. Sometimes I just need input from others to help me think things thru. Thanks guys.

Here is an old pic of what's on the other side of that wall. The 2x12" you see supporting the lean to are the same as the end's you see sticking thru the wall on the inside.

 

[galute]

I should probably add. When the ceiling is installed I plan on putting a whole house attic fan in the ceiling to ventilate and draw cooler air into the shop whenever I want. I will be able to change the air out in the attic pdq if I think there is a need to.

 

[timewarp]

I have a friend with a large pole building like you have and he put an attic fan connected to a humidistat, if it starts getting moist in the attic, the fan comes on for I believe 15 minutes. If you are going to add a fan then add a humidistat with a timer also.

 

[aar0s]

That is a problem I am giving a lot of consideration and will be addressed when I get to the ceiling. Not sure yet how I'm going to deal with it. That's the reason I'm trying to make sure I have proper ventilation.

To really fix it your going to have to put something between the purlins and the metal, like fan fold underlay or the rolled bubble type. Otherwise you will get condensation on the metal that will, over a few years, rot out the purlins. We are having this exact problem with a barn at work. Its about 20 years old and some of the purlins are completely rotted.

 

[galute]

To really fix it your going to have to put something between the purlins and the metal, like fan fold underlay or the rolled bubble type. Otherwise you will get condensation on the metal that will, over a few years, rot out the purlins. We are having this exact problem with a barn at work. Its about 20 years old and some of the purlins are completely rotted.

This building is just over 4 years old and I have never seen even one drop of condensation in it. Lucky for me in the summer when the metal gets hot the shop is shaded by 4 pm and the metal has plenty of time to cool down before the dew falls. Putting a vapor barrier between the purlings and the metal would require removing the roof tin. I'm not going to do that unless it becomes a problem. Right now I'm going to make sure I have enough ventilation and put a vapor barrier between the ceiling metal and insulation and hope for the best.