Hot roof designs, aka "dense-packed" insulated sloped roofs:

This article describes various cold-climate building solutions for unvented cathedral ceilings and similar under-roof spaces, offering advice on how to avoid condensation, leaks, attic mold, & structural damage when roof venting is not possible.

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

These suggestions are further explained in Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , chapter on BEST ROOFING PRACTICES and as discussed

at COOLING LOAD REDUCTION by ROOF VENTS

In 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.

Photo: open celled foam used to insulate an unvented cathedral ceiling roof as well as walls in a building addition by the author. Foam insulation minimizes air leaks and can provide a high R-value as well.

We avoided nearly all ceiling penetrations in this building, and not coincidentally, where there was one penetration for a plumbing vent we had trouble with sliding snow and a broken vent pipe.

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:

• Install a continuous air and vapor retarder,
  
  such as 6-mil poly, carefully sealed at all junctures. See the vapor barrier and air barrier articles listed
  
  at VAPOR BARRIERS & CONDENSATION in buildings.
• Do not use recessed lights 
  
  or other details that penetrate the ceiling plane.
• Carefully seal all penetrations 
  
  in the ceiling assembly, including top plates of partitions, with durable materials. See the air leak articles beginning
  
  at AIR BYPASS LEAKS

Photo above: installing spray foam insulation between I-trusses supporting a low-slope metal roof for a hot roof design in a New York building constructed by the author (DF). Foam insulation options are at FOAM SPRAY INSULATION TYPES.

• Use a non fibrous insulation,
  
  such as plastic foam, and install it without voids where moisture could collect. Insulation choices are listed
  
  at INSULATION INSPECTION & IMPROVEMENT.
  
  While fiberglass insulation is an excellent and effective product for insulating most building cavities, in areas where there is extra risk of trapping moisture (and thus rot or mold infections), such as crawl spaces and cathedral ceilings where roof venting may be absent or minimal, we prefer to use closed-cell foam insulation products or spray-in icynene foam insulation: these products can seal the cavity against drafts and they do not as readily pick up moisture nor do they readily form hidden mold reservoirs.
  
  See MOLD in FIBERGLASS INSULATION
  
  and MOLD RESISTANT CONSTRUCTION for details.
• In regions prone to ice dams, use enough insulation to maintain a cold roof
  
  preferably R-38 or greater.
  
  See ROOF ICE DAM LEAKS and Ice Dams: Comparing Two Houses
• Ice and Water Shield:
  
  On roofs that are too difficult to vent, a second-best solution is to remove the shingles (or slates) from the lower 3 feet of those slopes where leaks and ice dams have been recurrent, install a waterproof but nail-able membrane such as WR Grace's Ice and Water Shield (other product names from other manufacturers) which will prevent any ice dam backup leaks from entering the building.
  
  This is basically a sticky membrane that is applied to the roof decking and through which shingle or slate nails can be nailed back onto the roof; the membrane seals around the nails so that those penetrations do not form leaks during a water or ice backup.
• Use of roof de-icing cables or heat tapes to avoid ice dam leaks 
  
  is described
  at HEAT TAPES & CABLES for ROOF ICE DAMS.
  
  While we prefer to avoid ice dam leaks by good building design and good under-roof ventilation, where conditions require stopping ice dam leaks on an existing structure, proper installation of heating cables may be the fastest and cheapest solution.
• Eliminate all sources of excess moisture 
  
  in the home (wet basements, uncovered crawlspaces, unvented bathrooms).
  
  See ATTIC CONDENSATION CAUSE & CURE.

- Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) . Expanded and illustrated by Daniel Friedman.

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Reader question: what is the best way to insulate my complicated attic?

Hello. I installed spray foam in my house a year ago. The foam never stopped outgassing and im in the process of getting it removed from my attic. I have lots of HVAC equipment up there including ERVS, air handlers, and a whole house dehumidifier.

I currently only have r21 on my attic floor (rock wool) with plywood over it. I dont want to put foam back, its too late to do rigid exterior foam, and my roof life is complicated and does no lend will to a clear path to soffit and ridge vents.

I am leaning towards venting the attic but need to add insulation to attic floor and am concerned about condensation/ mold related to HVAC equipment. Do you have any suggestions how to best do this? I am in New York. Thank you 2024-08-28 by Brian

Reply by moderator:

@Brian,

I will give this some thought and reply further but my first reaction is that I can't know enough about your building from just this text to be confident about giving the right advice.

I don't want to expose the HVAC equipment to freezing conditions, and if the same time I'm not sure you're going to be up for adding insulation atop the existing floor.

I would take care of about using solid foam board on the floors as you could create a moisture trap.

Reader follow-up:

@InspectApedia Publisher thank you so much. I am really torn about what to do. Many houses in New York have HVAC in vented attics and seem to get by with higher energy bills. I am more concerned with condensation/mold in HVAC from doing that and do need to add insulation to attic floor possibly pulling up plywood. I have heard of newer methods using batts up again the stud bays without baffles and taping a smart air vapor barrier like siga or intelligence over it but not sure how tested this method is. Would be my best bet probably.

I also have an attic wall that shares a wall with a cathedral bedroom. Only thing separating it is sheetrock. Is foam board with taped seems ok to put over this wall? no bays for Batt insulation. If I do find a way to vent the attic id need that wall properly insulated.

Reply by moderator:

@Brian,

When you talk about batts up against a stud bays I get confused because I wouldn't expect to find studs in the attic.

Just keep the basics in mind, such as taking care not to have air leaks into insulated cavities where you trap moisture.

About attic HVAC, people count on insulation on the air handler and insulated duct work.

When there's a problem in the ductwork it's very often that it's inadequately insulated, leaky, or that condensate is allowing off the coil and downstream into the duct system.

Reader follow-up:

@InspectApedia Publisher, oh sorry I mean rafter bays. Thanks so much.

Reply by moderator:

@Brian,

You can add insulation between your rafters but if you want a ventilated attic you need to be sure to leave a couple of inches between the top of the insulation and the underside of the roof deck, combined with continuous soffit intake venting and ridge exit venting...

An alternative is to go to a hot roof design.

If you're interested in that we have pages of this website that give details.

Reader follow-up:

@InspectApedia Publisher, can a hot roof design be done without foam board outside or spray foam? ill search hot roof... thank you

Reply by moderator:

@Brian,

There are multiple designs. The concepts that seem best to fit your situation are explained above on this page.

...

Continue reading at HOT ROOF DESIGN vs LEAK RISKS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CATHEDRAL CEILING VENTILATION
• CAPE COD ROOF & KNEE WALL INSULATION & VENTILATION
• CATHEDRAL CEILING INSULATION
• HOT ROOF DESIGN PROBLEMS
• HOT UN-VENTED ROOF DESIGN SOLUTIONS
• HOT ROOFS in HOT HUMID CLIMATES
• HOT ROOFS vs VENTING in COLD CLIMATES
• HOT ROOF DESIGN vs LEAK RISKS
• HARD to VENT ROOF SOLUTIONS for HIPS & CATHEDRAL CEILINGS
• ICE DAM LEAKS
• PROBLEMS with PARTIAL ROOF VENTILATION
• ROOF FRAMING TIES & BEAMS proper framing of a cathedral ceiling
• ROOF ICE DAM CURE: COMPARING TWO HOUSES
• ROOF NOISE TRANSMISSION
• ROOF VENTILATION SPECIFICATIONS pros and cons of hot roof designs

Suggested citation for this web page

HOT ROOFS vs VENTING in COLD CLIMATES at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING VENTILATION

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