Uninsulated fan ducts in an attic (C) Daniel FriedmanBath exhaust vent slope recommendations
Which way should bath exhaust fan duct slope to avoid building leaks?

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Bath vent fan slope: this article describes the recommended slope direction on bath exhaust fan ducts to avoid icing and related building leaks.

This article series explains how to install bathroom exhaust fans or vents, the vent ducting, the vent termination at the wall, soffit or roof, vent fan wiring, bath vent duct insulation, bath vent lengths, clearances, routing, and we answer just about any other bathroom ventilation design or installation question you may have.

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Proper Bath Vent Fan Duct Slope & Slope Direction

condensation in vent ducts in an attic (C) Carson Dunlop AssociatesBath vent fan duct slope: Bath fan vent ducting in most installations is sloped gently back towards the bathroom. Especially where flex-duct is used, it is important to to avoid a low spot that collects condensation moisture in the attic.

Not only does such accumulation risk leaks into the ceilings (mold hazards) but in freezing climates water accumulating in duct work can freeze, accumulate further, block the duct, and when temperatures rise, cause extensive leakage back into the building.

We like to slope the bath fan vent duct downwards towards its building exit in non-freezing climates or where insulation protects against freezing. This will avoid condensation accumulating inside the ductwork and dripping back into the building ceilings or insulation. We illustrate a down-sloped bath vent duct installation in this article.

Question: ice accumulation around the bath vent cover on the outside wall

I have a brand new house that was built last summer. I noticed that the bathroom vents right below the roof line on the wall. The problem is there's an ice accumulation around the vent cover on the outside wall.

This does not seem normal to me, or is it? I should specify that I haven't even started using this washroom yet, since my shower has not yet been installed. I went up in the attic and they used a flexible vent and there's insulation on top of it. Is heat loss normal when the fan is not operating? Could this be the cause of the ice buildup outside at the vent cover? - Marc


Marc, it sounds as if warm moist air is exiting at the vent and you're seeing ice accumulation as a result. I agree that we don't want house air venting itself through the bath vent duct when the bath vent fan is not even running = that's an unnecessary heat loss.

But if the ductwork is routed all sloping "up" from the vent fan location this might happen, in particular if the bath vent fan system does not include an automatic closing mechanism indoors or outside at the wall. Take a look at the vent on the exterior wall (if you can safely do so in this weather and at that height) - see if there is a closing mechanism?

Question: moisture condensation on & in the bath vent connector tube that terminates outside. Why?

(Apr 15, 2014) Bill said:

I vented my bathroom fan to the side of the house. Condensation is building on the inside and outside of the connector tube that terminates outside. Why?


Bill I've thought about this for a while without dreaming up a magically clever response. Condensation means temperature differences and the presence of moisture. Warmer, moisture laden air gives up moisture that is deposited on the cooler surface.

So if by "why" you don't ask for theory but for a diagnosis we'd need to look at the home to see where moisture is present, and where temperatures differ. Perhaps also look for missing insulation.

You can handle condensation on the fan duct surfaces by these approaches:

  • Reduce indoor moisture levels - which is nonsense if we're talking about a bath vent fan that is specifically intended to exhaust moisture to outside of the building

    Watch out: Keep in mind that moisture forming on the outside of the bath exhaust fan duct must be coming from moisture-laden air in the area where the duct is routed. If you are seeing lots of moisture on the exterior of a bath exhaust fan duct I'd be worried that shower moisture is entering the ceiling or other building cavity when it should not. Look for leaks at light fixtures, around the fan fixture, at electrical receptacles, etc. Then look for other moisture sources (such as roof leaks).
  • Insulate the exhaust fan duct - Insulation on the exterior of a bath exhaust fan duct prevents moisture from forming on the duct exterior, as you are preventing the temperature of the exterior from reaching the dew point.
  • Proper exhaust fan duct material & slope - moisture may form inside the fan duct because of the combination of high moisture-content air being vented to outside (say from shower steam entering the fan), along with cool duct surfaces (say from a duct running through a cool attic or wall space).

    In addition to insulating the fan duct to reduce the condensation rate on both interior and exterior surfaces (insulation should be placed only on the outside of such exhaust ducts), use of smooth surfaced solid metal avoids little depressions where condensate can accumulate (and leak into the building), and sloping the duct properly (back towards the bathroom as discussed in the article above), will avoid water and ice accumulation at the exhaust duct termination.

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BATHROOM VENT DUCT SLOPE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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