Proper ventilation of attic 3

[PostFrameSE]

This kind of goes along with my topic last week regarding the temperature affects on wood in a poorly ventilated attic. How should an attic be properly ventilated? The IBC reads that we should have net free ventilation area of 1/150 or 1/300 (depending on vapor barrier or not) of the attic area with 50% of openings near the peak (paraphrased.) The IMC suggests a mechanical ventilation rate of .02CFM per square foot of attic space. That tells me that a bathroom fan could meet the code for ventilating an 80' x 90' attic space!! Yeah right!

Does anybody know the basis for those figures in the IMC? Are there typo's? I know that the code specifies minimums, so what is a realistic ventilation rate to keep an attic well ventilated if I'm forced to do it mechanically?

Lastly, this building has a hip roof and only 10' of horizontal ridge so my ridge vents are very limited. I'm using a steel roof and don't want other vents to flash around, so is it crazy to think that I could draw in fresh air from the outside (through a duct at the overhang) and exhaust it via a duct into the center of the attic space........and by pressurizing the attic space actually turn my ridge AND my eaves into air OUTLETS rather than having my eaves be air INLETS?

 

[SlideRuleEra]

PostFrameSE - IMHO, trying to pressurize the attic and force the hot air out at the eaves is a bad idea for the following reasons:

1. I'm sure you know that natural convection will make the hottest air tend to rise. Forced convection in the opposite direction is certainly possible but neither efficient nor prudent. Any problems (air leaks, equipment failure, etc.) will make the system inopperable.

2. Making an attic "airtight" (except for designated inlets and vents) is not realistic using typical building materials and construction techniques.

3. There will almost certainly be (bad) unintended consequenses. Things that come to mind would be noise from the necessary powerful blowers and dust / dirt intrusion into the living spaces from forcing air toward the ceiling.

Power venting an attic (working with natural convection) does not take much power. Attic temperatures are very stratified since there is typically not much air circulation. While temperatures at the peak of the roof can be very high, the temperature falls rapidly as you approach the ceiling. For the triangular crossection of a gabled roof this means that the quantity of very hot air is realatively small.

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[PostFrameSE]

SlideRuleEra,

I appreciate your post. I agree with you on all accounts. I still need to meet the code however. I suppose on this hip roof building, as you said, natural convection will force all hot air to the 10' length of ridge. Do I just seal off the ridge (as well as is possible with construction materials) and force an exhaust vent through the steel roofing near the peak that meets or exceeds the mechanical ventilation requirements?

 

[SlideRuleEra]

How about an oversized ridge vent? Perhaps you can design a low-profile, elongated copola that covers the 10' ridge.

Since you are working with metal roofing, maybe the client would be willing to have light colored roofing (white is the best). This can help quite a bit to keep attic temperatures down.

Another option is radiant barrier insulation (it is glorified aluminium foil). This is best installed on the underside of roof joist, typically just using staples - it works in conjuction with ridge venting. I have specified this (and used it on my own home). It helps a lot. Here is a link for information:

http://www.ornl.gov/sci/roofs+walls/radiant/rb_02.html

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[smb4050]

I need to wade into this post, and comment on the post from last week.
First, if I recall correctly the post mentioned last week your client stated the attic temperature was in the 150 or 160 degree range. Based on my past experience, I seriously question if that high of temperature is accurate or if it is there is missing information. The surface of a BUR (built up asphalt) roof exposed to sunlight rarely gets that hot of temperature. In addition the heat lost, or heat gain depending on your perspective, into the building space would be huge and thus the high temperature doesn’t seem feasible. I would add that if this is accurate, and I were you, I'd to sit down with my client and say him/her we need to investigate this problem.

Now onto the new post. A rule-of-thumb for ventilation of enclosed space is 6 air exchanges per hour so take you attic volume and multiply times 6. That would be for power ventilation system, and make sure you've made provisions for make-up air. Often times that is ignored and the system never works properly. I've done a lot of roofing and I agree with your concerns about flashing vents into a metal roof but I'd put the vents in. I'd make sure to use good basic roof design techniques, and at some point the installer needs to be responsible for installing a water-tight system. It’s a trade-off; use another roofing material, pay money to have a qualified M.E. design a system that will be able to overcome physics, or install the vents.

What you are trying to do is feasible but not economically feasible based on what I've read. Read SlideRuleEra's 1st post again.

I don't want to sound preachy, but I sense a square peg is trying to be fit into a round hole.

 

[msquared48]

How about a dutch hip instead of a full hip? That way you could install gable end vents with forced air fans if needed.

Mike McCann
MMC Engineering

 

[PostFrameSE]

That's a good idea Mike, but this is a commercial building with draftstops in it. I wouldn't be able to get flow from one end to the other.

Perhaps you or somebody else has already dealt with that issue and have a way around it that would still meet code. I haven't come up with one yet.

Thanks.

 

[msquared48]

If you have only one draft stops, the flow could be done with eave vents and the dutch gable end fans. If there is more than one draft stop, I would suggest using the eave vents and the Cupola idea for the interior portions of the structure roof.

Mike McCann
MMC Engineering

 

[hokie66]

With a steel roof, the ridges and hips will not typically be sealed, just flashed to prevent water ingress. A lot of hot air can escape through these flashings.

 

[jheidt2543]

Sorry that this is a little late, but I'm working on a project with a similar concern. SMB4050, the six air changes/hour you mention, is that "normally" 24/7?

In my case, a 3:12 pitched roof, the .02cfm/sf = 125cfm and six air changes/hr = 2540cfm; a big difference.

Also, what about the use of a thermostat and/or a humidistat to control the fan?