Bathroom Exhaust Duct Material

[psulee]

I am working on a dormitory project where the mechanical system design has changed at the last minute causing the engineer to make changes to the exhaust system that were not originally planned. In this project, the engineer eliminated the metal bathroom exhaust ducts in lieu of a 2-hr rated shaft lined with gypsum wall board. The shaft will have continuous flow of air from toilet rooms (15 cfm) and showers (20 cfm). My concern is that during the high use times, the shaft walls could be suject to condensation or moisture from water droplets in the air. The code indicates that the shaft liner may be wallboard if the temperature of the interior surface is maintained above the air dewpoint. I was planning to add some sensors into my bathroom at home to trend the dewpoint in the space, but I wanted to see if anyone has an idea of the dewpoint in a single closed shower. Thank you.

 

[gxmplx]

Hi,

My humble guess is the shower's dew point is it's ambient temperature because it should be over saturated with water.

This temperature should (normally) be higher than that of the walls of the duct because the shower gets heat from hot water.

So unless you heat up the air in the duct I don't see how you could avoid condensation.

GXMPLX

 

[Wareagl487]

psulee -

Be careful treading down this path. Your concerns are well taken. I have seen gyp shafts after a few years of use that have evidence of mold. Not to a GREAT degree - but there none the less.

Of concern also to me are two things:

1 - By it's nature it is a "Shaft Enclosure" as defined by code. What are you doing to address the the rating of the shaft? Fire Dampers? Sub-Ducts?

2 - Gypsum shafts Can Not be built air tight. Drywall guys just can't make it mimic sheetmetal ductwork. I would put some liberal safety factor in the selection of my fan - speaking from experience here.

I hope they will be using Green Board for their shaft construction at the minimum.

Andy W.

 

[willard3]

Do not build a shower duct of anything but aluminum ductwork or the Owner will regret it.

If the duct passes through an air-conditioned space and is uninsulated, the Owner will regret it even more.

 

[RossABQ]

15 cfm from toilet rooms? Total? Sounds awful low... so does 20 cfm from a shower room. How many square feet are these spaces?

 

[psulee]

Thanks for the replies. My concerns seem to be echoed and I'm going to continue to pressure the design team to ensure that the system is designed for the long term operation. Regarding the airflow rates, I absolutely agree that they are low. The shower rooms are single stall spaces with a small changing area (approx 70 SF). The toilet rooms only contain a single bowl. The sink is in the common space. However, with that said, I believe the flow rates are still low. One of the big lessons learned on this project is that the design-build process can and likely will lead to undesirable system designs to save installation cost. This can be somewhat mitigated with a good D-B contractor and guideline spec, but it has the potential to be problematic. Without a code driven requirement, implementing changes from the owners perspective that result in additional cost are difficult - even if it is the right thing to do.

 

[paultg]

I believe the IMC allows those lower exhaust rates if the fans operate on a continuous basis; in residential applications.

What is the building classification of the dorm? If not residential, I believe they code requirement is 75 CFM/water closet; but I am more familiar with MA code, not IMC.

On that note, I am involved in my first project using the 2006 IBC and IMC. The point Wareagl487 makes about shaft ratings is a good one.

Depending on how tall the building is, a shaft enclosure may be required, and the 2006 IBC not only requires fire dampers, it requires smoke dampers as well. The sub-duct arrangement can be used to eliminate the fire damper, but the exhaust fans must be powered on generator to eliminate the smoke damper requirement.

Paul G.

 

[psulee]

The engineer has indicated that he is planning to use the subduct method, and my concerns about moisture in a standard gypsum shaft remain outstanding. In reviewing the application, I missed the smoke damper component and I appreciate you bringing it to my attention. Since it is residential, it appears the exclusion is only applicable is the exhaust system is powered on emergency power. I do not believe that it is, but I will have to check the documents tomorrow. Thanks again.

 

[paultg]

psulee,
Let me know how it works out. I don't have the codes in front of me; but if I recall correctly the IBC is specific on how many floors the duct penetrates (it would be worth reading the commentary). On the project I am involved in, it was determined that since the building is 3 stories and has an attic, the duct penetrates more than 1 story, so shaft enclosures are requires; therefore none of the exceptions under fire barries, etc. are applicable.

Also, I have used the sub-ducts before; however I use them with a sheetmetal duct enclosed in a shaft. I'm not sure, but the exception and sub-duct description in the code might require the sub-duct be used with sheetmetal; not a gyp-enclousre. The codes section is "ducts and air-transfer openings" in shaft enclosures. I'm not sure if a single sub-duct penetrating the gyp will meet the fire rating, I think the ductwork in the shaft is required.

Paul G.