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6 inch wafer cuts along the hip to permit installing a hip vent - cited in detail at InspectApedia.comHip Roof & Cathedral Ceiling Ventilation
Roofs or ceilings where ventilation is difficult, not feasible, not wanted

Solutions for roofs or ceilings that are hard to vent: hip roofs, odd roof shapes, hot roof designs, aka dense-packed insulated sloped roofs.

Page top illustration: a wafer cut along the ridge suggested by DCI Products, cited in detail just below.

Data & design suggestions for hip roof vents, cathedral ceilings, hot roof designs or for providing roof and ceiling ventilation, and we describe roof & roof insulation or venting system inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation, mold, or structural damage problems in buildings covered by an un-vented or hot roof or hard-to-vent roof design.

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How to Vent to a Hip Roof

Un-vented hip roof area is moldy - how do we vent this area (C) InspectApedia.com DanielReader Question: how can we vent this closed-off hip roof portion of our home?

2019/08/07 Daniel said:

We had our roof replaced 2 years ago due to a wind storm. Asphalt shingle roof. Previous roof was original to house built in 1993. We have an angled cathedral ceiling in our great room. This past March we noticed moisture running down the center beam and then eventually moisture in the Sheetrock.

See pictures for extent of damage discovered.

How do we ventilate or properly insulate this section of our home? There are soffit vents but as you can see no outtake venting.

[Click to enlarge any image]

Reply:

Daniel

Thanks for asking a helpful question; indeed there are some roof shapes that are difficult or nearly impossible to vent effectively.

One of them, that I think is shown in your photo, is a roof area under a hip. In your photo the hip line or under-side of the hip rafter is that white diagonal across the image.

While some roofers install a "ridge vent" all along the hip, fed by continuous soffit intake venting, that's rare; I suspect the worry is leaks into the hip vent low on the roof near the eaves.

GAF makes a low profile Cobra Vent for hip roofs - a hip vent specifically for this purpose; I don't think GAF, a leading roofing manufacturer, would recomment venting along the hip unless they were confident of the leak resistance of their Cobra Hip Vent installation.

Other roof vent suppliers include a similar vent but made of mesh that can also be applied over a hip.

Watch out: partial vent systems such as spot vents or "mushroom vents" between hip rafters are suggested by some writers, even ones we like such as Fine Homebuilding.

But they're not the best approach: these will not vent the entire under-roof area and are in my experience considerably less effective - an incomplete solution. However, depending on the hip roof venting system you install, even installing a hip exhaust vent (such as GAF's Cobra Vent) won't vent the lower half of the hipped roof. An incomplete solution in my experience.

Watch out: other writers suggest using fans or power-venting the hip roof area;

This might work (somewhat) provided that the hipped roof is over an attic rather than enclosed as a cathedral ceiling AND provided there is good air flow in along the hip roof eaves or soffits and enough air movement that **all** of the under-roof sheathing is kept dry.

I've seen mushroom vented and power fan vented roofs whose decking looked great around the fan but that was mold-hell just a couple of hip jack-rafters away.

You'd install the vent along the hip by the following steps:

GAF's Cobra Vent for Hipped Roof hip lines - cited & discussed at InspectApedia.com1. Remove the existing hip cap shingles

2. Cut back roof sheathing 1 to 1 1/2" away from the hip rafter on both sides of the hip rafter, for the full length of the rafter. Don't cut all the way to the gable end or hip top or bottom. The object is to have a 1" to 1 3/4" open slit along the hip on each side.

Watch out: see GAF's warning about hip roof vent slot location given below; the company says never cut hip air slots below th mid point of the hip - and other restrictions that basically mean that you will not be able to vent the entire hipped roof underside.

DCI's wafer cut method suggests that that company vents the entire hipped area. Check with your vent supplier before cutting into the roof.

DCI Products illustrates a clever way to cut back the roof sheathing along the hip rafter so as to retain sufficient nailing connections between the roof sheathing and the roof structure, though for many hip roof designs, the nailing of the roof sheathing to the hip rafters themselves ought to be more than adequate.

3. Install the hip vent (by Cobra [GAF] DCI, or another supplier) - follow their instructions for cut dimensions, vent nailing, etc.

Inspect the roof sheathing and roof condition - if possible from below - to assure that you're not leaving rotted or insect-damaged sheathing in place. If you find those the repair job needs to expand to replace any bad roof sheathing.

The installation manual for GAF's Cobra Hip Vent as well as other hip vent products and instructions are found below on this page.

4. Install new hip cap shingles to cover the hip vent.

5. Vent the soffit or eave: Assure that there is complete, continuous, un-blocked intake venting along both eaves below the hip roof area; if your roof has NO overhang then you might still install intake venting at the lower roof sheathing edge - we discuss that intake vent in this article series.

6. Clean the under-roof of any mold: For your roof I'd do some surface-clean-up of that moldy roof sheathing, It's not necessary to remove all of the dark mold stains on plywood - physically clean the surface and seal it.

and while we don't normally bother, in this case as you're going to enclose the roof cavity and don't want to do this job over again, I'd spray sealant such as a FUNGICIDAL SEALANT PAINT (search InspectApedia.com for

FUNGICIDAL SPRAY & SEALANT USE GUIDE to see details) over the roof framing and sheathing where it's exposed.

7. Assure the air path: When re-insulating, be SURE to leave 2" or more of air movement space up under the roof sheathing for the whole roof area - from eaves or soffit intake out to the hip vent outlet. Some roof baffles give only an inch or less of space - in my experience that's often not adequate.

So don't pack a "vented" roof cavity with insulation.

8. Add insulation: To get the required R-value for the roof, depending on where you live, you may need to add additional solid foam insulation to the underside of the rafters before re-installing the drywall.

9. Re-install ceiling drywall. Take care to avoid air leaks into the roof space from the building interior.

Alternative to a Roof Hip Vent: go to a hot roof design? Probably not.

An alternative is to abandon trying to vent the hipped area, converting the roof to a HOT ROOF DESIGN (search InspectApedia.com for

HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS to read those details) but even in that approach I'd clean the cavity first.

Watch out: trying to go to a hot roof design (insulating with closed cell foam is probably best though I've also done open cell foam) you will find that few or no foam insulation contractors are going to be interested in bidding on a very small partial-roof foaming project - the overhead of coming to the site to inspect, measure, bid, and sell the job just isn't worth the trouble. It's a money loser.

Post some outdoor photos so we can better understand this roof and I may be able to offer further comments.

 

Hip Roof Ventilation Products & Installation Manuals

Metal roof hip cap and possilble vent product from Buy Metal Roofing Direct - cited & discussed at InspectApedia.com

Illustration above: hip roof vent for a metal roof, excerpted from the Tuff-Rib Installation Manual cited below under supplier Buy Metal Roofing.com - the venting properties of this metal roof hip cap are not quite clear in the manual cited below: check further with the manufacturer. Note also that this is an exposed-metal-fastener metal roof, not a standing seam metal roof product.

Below: a hip vent for a standing seam metal roof, provided by Union Corrugating, cited below.

Standing seam metal roof hip vent from Union Corrugating - cited & discussed at InspectApedia.com

Watch out: discuss with your roof vent manufacturer whether there is a need for baffles in the product to prevent leaks from wind-blown rain and snow.

Hipmaster hip roof vent from Boral Roofing - cite and discussed at InspectApedia.com

Illustration above: Boral Roofing's Hipmaster vent for hip roofs.

Valley vent from DCI Products can also vent hip roofs - cited & discussed at InspectApedia.com

Website excerpt:

ValleyVent is an under-plywood, top-of-rafter vent easily installed during the construction of an addition or a new home. The ValleyVent provides adequate ventilation in the spaces to either side of the vent placement.

It is a required vent product when your soffit to ridge air flow is blocked by a skylight or dormer. ValleyVent easily allows air movement in those trapped areas.

It eliminates the need to drill holes in the rafters to ventilate areas trapped by construction supports. Use it in areas otherwise unventilated and considered problem areas. It is very versatile and can be used on wall surfaces between siding and wall sheathing, allowing air to move between those areas.

ValleyVent can also help stop mold and mildew when installed properly.

Standing seam metal roof vents using Emseal tape - at InspectApedia.com ... Standing seam metal roof vents using Emseal tape - at InspectApedia.com

Above: Emseal tapes are used to avoid leaks at roof hip and ridge vent systems.

 

6 inch wafer cuts along the hip to permit installing a hip vent - cited in detail at InspectApedia.comIllustration: a wafer cut along the ridge suggested by DCI Products, cited in detail just below.

[Click to enlarge any image]

 

GAF's Cobra Vent for Hipped Roof hip lines - cited & discussed at InspectApedia.com

Above: Cobra Hip Vent, produced by GAF, is discussed in detail in the article above on this pageand cited just below.

Reader follow-up:

Hip roof example - not vented (C) InspectApedia.com Daniel Hip roof example - not vented (C) InspectApedia.com Daniel

Thank you for your reply. Yes we have an overhang on the soffits. Do you use baffles to provide the 2 inch air space? What about a vapor barrier on the sheeting side of the insulation?

Moderator reply:

I've used styrofoam baffles and found them marginal at best when run all the way from eaves to ridge under a cathedral ceiling- the air space doesn't extend the full width of the sheathing between the jack rafters and often it squashes down to an inch or less - there's too little air movement.

CATHEDRAL CEILING VENTILATION has more details.

Either leave a good 2" above your insulation or go to a more labor intensive furring strip (to get the air space) and solid foam (between rafters) or give up and go to a HOT ROOF

I like a vapor barrier on the warm side of the ceiling under the drywall, but studies have shown that most of the air and moisture leaks into the roof space are not THROUGH the drywall but rather at PENETRATIONS.

I see your soffit overhang and though I can't see them it may have perforated panels but your earlier photo shows blocking between the jack rafters - no airflow was apparent.

Reader follow-up:

Does going to the hot roof option eliminate the ventilation condensation problem assuming no moisture entry from the roof side? I assume would remove the soffit vents as well

Moderator reply:

HOT ROOF DESIGN PROBLEMS

is more complete than an off the cuff reply here; Basically that design can work but because of the risk and cost of leaks into a foamed, enclosed roof cavity (un-discovered, rot) I prefer that design under very reliable long-lived roof surfaces such as a metal roof.

Reader follow-up:

So if I read all the links correctly I am thinking about going this route.

Possible insulation and ventilation scheme for a hip roof (C) InspectApedia.com Daniel

Moderator reply

Two designs for high-R ceiling under a hip roof that wants to be ventilatd (C) InspectApedia.com Daniel and Daniel FHere is an alternative for homes in cold climates where a higher R-value cathedral (under hip roof or otherwise) ceiling is in order.

Some ROUGH calculations:
If your rafters are 2x10 figure that's a true dimension of 9 1/2".

9 1/2" rafter
less 2 1/2" furring (true dimensions rather than nominal) for the air space
less 1" for the solid foam baffle
= 6" to fill with fiberglass. (See INSULATION R-VALUES & PROPERTIES inspectapedia.com/insulation/Insulation-Values-Table.php)

That gives us about
R8 or less (solid foam board 1" thick)
R19 (fiberglass)
= R27 ceiling.

If you need a higher R add another 1" (R8) or 2" (R16) foam board to the rafter bottoms and then install the drywall ceiling over that (using long-enough screws and taking care to map the rafter bottom locations so as to be sure your screws connecty)

Watch out: for leaks into the hip roof near the eaves, especially in areas of heavy rain. See GAF's warnings and see the DCI's design for the hip vent.

Reader follow-up:

Thank you! Yes we live in a cold climate (Spokane Washington) and get a decent amount of snow, so will probably go with the higher R value.

Now just need to get the right hip ridge vent that will vent the entire hip and all rafter bays. Looks like DCI

Moderator reply:

I like foil faced HI-R foam board; you can add another R8 or even R16 - I figure the insulation is cheap compared with the cost of heating the home over the rest of the home's life.

Talk with DCI about GAF's worry about leaks into the lower portion of a hip vent. Let me know what they tell you.

To install the drywall we marked the location of the upper and lower end of every rafter BEFORE covering the rafter bottoms with the foam board. Pencil lines down the walls long enough to not get lost.

Then with the foam board in place we could use those lines (and a string line if needed) to shoot 3 1/2" drywall screws through the foam into the rafter bottoms without missing the rafters. (YOu'll know if you miss the rafter by how the screw behaves).

With foil faced foam board it's a near perfect vapor barrier (and slightly higher R value than kraft faced or unfaced board) so you don't need another VB under the drywall.

Watch out: with an asphalt shingle roof over ANY totally enclosed cathedral ceiling you need to inspect the roof annually to be SURE there are no lost, damaged, leaky shingles as water leaking into any enclosed building cavity can cause a lot of damage before you see clues from the building interior.

Henri DeMarne (JLC conference 1985) showed photos of rotted rafters in HOT ROOF designs that were under leaky roofs.

Yours is a vented roof - that reduces the problem but doesn't eliminate it as that foam board will trap any leaks in the upper roof cavity - you won't see leaks showing up as ceiling stains until things are pretty bad.

Design Suggestions for Unvented “Hot” Roof Designs Where Venting is Difficult

Severe ice dams, eaves to ground in Poughkeepsie NYIn cathedral ceiling configurations where it is difficult to provide ventilation, some builders have eliminated the vent space, relying instead on careful sealing of the ceiling plane to prevent moisture problems.

While experts concede that this should work in theory, most caution that it is difficult to build a truly airtight ceiling assembly.

Also, cathedral ceilings are slow to dry out if moisture problems do occur, whether from condensation or roofing leaks. If a hot roof is the only option for a section of roof, take the following precautions:



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