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Figure 2-57: Skylight roof venting details (C) J Wiley, S BlissRoof Ventilation Details for Skylights

This article explains How to Correct Improper or Inadequate Attic or Under-Roof Ventilation at skylights.

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Roof Ventilation Details at Skylights to Provide an Air Path - Eaves to Ridge

Figure 2-57: Skylight roof venting details (C) J Wiley, S BlissThis article series describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings. It describes proper roof ventilation placement, amounts, and other details.

These recommendations are based on a survey of building science literature combined with 30 years of building inspections, on the observation of the locations of moisture, mold, ice dams, condensation stains, and other clues in buildings, and on the correlation of these clues with the roof venting conditions at those properties.

We have measured very large changes in airflow, temperature, and moisture before and after installing roof venting.

As described in Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) :

Localized hot spots such as skylights can also lead to ice dams below, due to blocked ventilation as well as melt water from skylight heat loss.

Notching the rafters on either side of the skylight will help maintain airflow above the skylight (Figure 2-59 at left).

 

If icing on your skylight window interior surface in winter is still a problem, add an interior storm window to reduce heat loss through the glass in cold weather.

Figure 2-5: Flashing at Skylights, details (C) J Wiley, S Bliss

As a backup to prevent leaks at skylights, during skylight installation and even though modern skylights are usually provided with a factory-built flashing and counterflashing, it is always a good idea to seal the skylight curb and surrounding roof area with a bituminous membrane (see Figure 2-5 at left).

Also see SKYLIGHT LEAK DIAGNOSIS & REPAIR and see Skylight Glass Breakage.

Also see Ice Dam Protection for Skylignts for cases where under-roof venting is not provided around a skylight.

Cathedral Ceiling Ventilation Alongside of Skylights

Figure 2-57: Cathedral ceiling venting details (C) J Wiley, S Bliss

Cathedral ceilings require the same continuous air barriers, and balanced soffit and ridge vents, as attics.

Both air sealing and ventilation are more critical, however, since any trapped moisture in the roof cavity will remain longer and potentially cause greater damage than in an open attic.

Also, since there is little or no communication from bay to bay, an effective ventilation system must reach every bay (Figure 2-57 at left).

- - Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .


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Comments

John A · Sept 24, 2022

Should I place a baffle under and above skylight after notching ?

Thanks!

InspectApedia-911 (mod) · Sept 24, 2022

@John A,

If you're trying to get airflow through that space you need an inch or more between the underside of the roof deck on the top of the insulation

if you have that you don't need to baffle. If you don't or you think the insulation tends to fluff up and clog the airflow then a baffle May help.

John A · Oct 11, 2022

@InspectApedia-911, I decided not to notch. I am going to drill holes in the top and bottom sill and install pvc pipes for airflow and then insulate the pipes. Have you ever seen anything like that? Would be other option be to not ventilate? But the only issue is the soffits are going to bring in air anyways to the bottom rafter bays. What would be best option if I am not going to notch? Thank you!!

InspectApedia-911 (mod) · Oct 12, 2022

@John A,

In my opinion if the air passage opening under a roof is small such as the diameter of a small diameter PVC pipe, you're simply not getting a meaningful air flow through the space.

In that case you may as well go to a hot roof design and not try to ventilate that space at all.

John · Oct 13, 2022

@InspectApedia-911,
I have already baffled the entire attic space with exception to the 4 skylights that take up two bays each. Would I cover the soffits under those skylight bays so no external air enters those 8 bays? If so would rigid foam board with great stuff spray do the trick to block external air flow from soffit? I am using rock wool insulation For the attic. Thanks so much for your time!!!

InspectApedia-911 (mod) · Oct 14, 2022

@John A,

Sure, solid foam and or spray foam insulation are what's most- commonly used in hot roof designs.

Keep in mind the general concern about any hot roof design is that if you have a leak into that space from above it may cause considerable damage before you discover it inside the building.

So be sure that your roof covering is in good condition and inspect it Periodically as well.

Anonymous · Sept 15, 2021

how to block cold air vent skylight

inspectapedia.com.moderator (mod) · Sept 16, 2021

@Anonymous,

I'll be glad to help out but first need to understand the question.

If a skylight vent is leaking air - if that's your case - then it is either not fully closed (inspect, clean, lubricate hinges and latches and cranks) -
or there are lost, missing, or damaged gaskets or sealed (identify by inspection).

I would not simply jam up something to try to block the draft, as you may be leaving water leaks that damage the building.

danjoefriedman (mod) · Feb 4, 2021

David

If you removed the Skylight you would cut out or remove Or at least make some air vent openings in the blocking between the rafters and splice or repair any
Rafters that were interrupted. That would give you a continuous are path from soffit Ridge.

David · Jan 31, 2021

I removed a skylight and roofed over the cavity.
What is the best means for allowing for air flow so that there wont be condensation.

Bob · Oct 28, 2017

Is this sufficient for large skylights too? They take up 4 bays

 

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