Attic Ventilation

[north star]

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I am seeking input on the ventilation rate(s) of some attic spaces over
some single story, military barracks and latrines........The plans show
vented soffitts, but no other means of exhaust air on the barracks,
and vented soffits with two gable vents on the latrines.

The barracks in question are 116 ft. L x 19 ft. W, with vented soffits on
both sides of the entire length, but no openings for the air to exit the attic.
The latrines are "L Shaped", with vented soffits on all sides and two gable
vents.

No calculations have been provided, and no mechanical means of ventilation
are planned for any of these structures.

I am seeking ventilation rates for these attic spaces, so that I can discuss
with the RDP's........The applicable Codes & Standards are: the 2015 I-Codes,
...the most current edition of ASHRAE 90.1.

Thanks for any input !

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[ICE]

Convection will not work if there is only soffit vents. Code requires 150th of the attic footprint. That drops to a 300th if there are upper and lower vents. I assume that if there are upper and lower they are close to balanced.

Okay so this is from my memory. Code changes....so does my memory.

Considering that this is a military installation, there’s probably 40 pages of specifications on the subject complete with testing criteria.

 

[tmurray]

Are the attics habitable? if not, ASHRAE 90.1 is not applicable. Then it is just RDP judgement.

For what it is worth, we mostly do vented attic spaces here. We usually have gable vents, soffit vents, and ridge vents.

 

[ADAguy]

So if heat rises and there are no ridge or end vents, then where is it to go? Temp in attic will rise to very high levels leading to early degradation of roof materials and exposed wiring. Purpose of vents is to maintain airflow, no?

 

[north star]

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Thanks for the input so far !

These single story barracks and latrines are existing
structures that will be ungoing a complete renovation and
upgrade ( Level 3 Alterations ) and will have new, low
sloped, metal roof assemblies and will not be habitable.
They DO have new thermal insulation planned for installation
in these spaces.

As mentioned earlier, on the current set of plans, the four
barracks and the two latrines have vented soffitts, and only the
latrines have two gable vents indicated.........No exit points are
indicated on the barracks, which is why I will have comments
back to & with the RDP's about this.

My intent is to have an idea or the actual calculations of the
"requirements" for ventilating unhabitable attic spaces.......These
particular RDP's have these barracks & latrines labeled as
"proto-typical".........Uuuuuuuuggggghhhh !

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[ADAguy]

Bet you won't get a LEED cert if you allow the heat to accummulate in the attic.

 

[Robert]

Use the calculations that ICE mentioned...they still apply (at least in CA). If trying to get the lower ratio (1:300) the balanced ratio has 40-50% venting in the upper portion of the attic, the remainder in the soffit. Ridge vents, eyebrow vents, gable vents can all be considered upper portion. If the new insulation is impermeable and applied directly against the underside of the roof sheathing, no ventilation is required.

 

[ADAguy]

Apparently no concern about condensation collecting between the underside of sheathing and the top of the impermeable barrier?
Does applied mean "adhered" to?

 

[Yikes]

The requirements for vented attics are in IBC 1203.2.

IBC 1203.3 lists the conditions under which unvented attics and unvented enclosed rafter assemblies are allowed. These requirements address some of ADAGuy's concerns about condensation.

The ICC also has the International Wildland-Urban Interface Code 505.10 which limits the amount of attic ventilation openings in regions of elevated fire danger.

If the military decides they also want the project to achieve LEED or other sustainability certifications, those would also need to be checked to determine the most stringent requirements.