Expert: Richard Taylor, AIA - 10/27/2006

Question
Mr. Taylor,

I read with some interest one of your past tussles, presumably with another architect, over the use (or, misuse) of the labels "warm roof" and "cold roof" (11/12/04).   As a matter of speaking to the other fellow's point, if I didn't misunderstand him, I offer the following definitions given by Mr. Richard Seifert; an Energy and Housing Specialist with the Extension Service of the University of  Alaska, Fairbanks.

According to Mr. Seifert's instruction:

"Cold roofs have a ventilated cavity (attic) above the insulation. It generally consists of lumber rafters or trusses with a 4/12 to 6/12 slope and a flat ceiling. The roof is commonly insulated with 6 to 12 inches of fiberglass batts or blown-in fiberglass or cellulose. A 6-mil polyethylene air-vapor barrier is installed under the insulation. For the roof to be classified as cold, continuous 2-inch vent slots must be installed at the eaves for natural eave-to-eave ventilation. Also louvres are installed at the gables. In some situations it may be acceptable to use ridge vents."

"A hot roof may be a flat, cathedral, or shed roof with no natural ventilation in the roof cavity. In a commercial roof, rigid insulation may be placed on the top of the decking. In a residence, the rafters may be packed with insulation, leaving no air space for natural ventilation of the cavity. Even a gable roof maybe classified as a "hot" roof if it doesn't have adequate ventilation. (Axel Carlson, Roof Ice May Create Safety Hazards, March l988.)"

The entire fact sheet may be reviewed at the Northern Research and Technology in Housing website at:

http://www.north-rthn.org/FactSheets/Attics-roofs.html#cold

In light of this accepted (at least, among builders in this neck of the woods) set of definitions, I wonder if there is perhaps some validity to Jeff's comments regarding the (apparently) backward treatement of these definitions by AIA...

After all, these would seem to travel 180 deg. to the (apparent AIA) definition that you previously supplied: "A warm roof is the standard method of roof construction. It is essentially an air space over the insulation with soffit and ridge vents. A cold roof is used with vaulted ceilings. It has no air space next to the insulation."

Thanks,

Paul W.

Answer
Paul - thanks for your question!

I do appreciate your thorough research, and have great respect for Mr. Seifert's data.  I prefer, however, to default to the opinions and experience of the guys who are building these roof every day and have been doing so for decades.

"Hot" and "cold" roof design and construction can get complex and can made it difficult to assign proper terminology to each.  Regional variations also muddy the waters - I would expect things to be a little different in Alaska!  But let's look at the roofs in their simplest forms:

A "hot" roof simply means that heat (and humidity) from the conditioned space below can get into the attic/air space above the insulation (there must be some degree of attic/air space above the insulation).  This warmer air can cause snow/ice to melt on the roof surface above, which then refreezes on the unheated overhang, leading to ice-damming.  The air cavities take on heat; that's the origination of the term "hot roof" and the reason why the cold roof method was invented.

A "cold" roof is "sealed tight"; there's no attic/air space for heat to leak in to, so the building materials remain "cold".  Hence the term.

That's the basics.  A cold roof is further defined (when properly designed and built) by a second layer of plywood, over sleepers, creating an airspace that keeps the slightly-warmer building materials below from making any contact with snow or ice.  That may be the source of confusion - there IS an airspace in a proper cold roof, but it's not above the insulation; it's OUTSIDE of the primary roof.

I would refer you to page 288 of the JLC Field Guide to Residential Construction and page 192 of Rob Thallon's Graphic Guide to Frame Construction.  This is information straight from the field (my preferred source).

As for the AIA, I don't know what they think about it; I didn't provide their definition - I was asked to comment on it.

I've read Mr. Seifert's paper and find it interesting that way down in section 7, Cathedral Ceiling Framing, he shows a very nice drawing of what builders in the northeastern US would call a classic cold roof.  It looks very much like the diagram in the JLC Field Guide of a cold roof.  He even states "...This alternative assures a cold, ventilated roof...".  I don't know quite what to make of that!

Finally, let me add that I'm simply answering questions posed to me in this forum and am not trying to challenge anyone's definitions of design and construction strategies. What we in the East call hot may be called cold in the West - I don't know!  All that matters - whether you call it a hot or a cold roof - is that you use the proper design and construction for your climate and situation.

Hope this helps!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtastudio.com