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If the attic floor is well insulated, help me to understand why there's an additional advantage in terms of reducing loss of heat exchange in the main room area. Why would my unit likely run less frequently in such a scenario (assuming the thermostat was in the same room)

Assume in my case a room similar in style to this with vaulted ceilings.

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    You really in my opinion, don't have much attic floor since most of it is taken up with living space. To insulate under the living space would be counter productive – Jack Aug 15 '16 at 0:34
  • Perhaps the diagram is a little misleading. The ceiling joist is still long, but slightly less long than the floor beneath the room. I'm not understanding the insulation beneath the living space comment. Everything I'm speaking of involves insulation above the living space. – saigafreak Aug 15 '16 at 0:42
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    @saigafreak, in your post you say "...the attic floor is well insulated", this leads one to believe that the floor/ceiling joist spaces beneath the attic room are insulated. I think that was the point of Jack's comment. When planning insulation strategies one would normally envelope the living spaces as a whole; not between them, as that would be counterproductive to efficient and uniform heat distribution. – Jimmy Fix-it Aug 15 '16 at 1:11
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    I think you should research carefully about insulating the actual roof rafter bays. You need to understand the disadvantages rather than any advantages. Depending on your location it can cause all sorts of problems; like ice dams with the resultant water intrusion, mold/mildew, etc. – Jimmy Fix-it Aug 15 '16 at 1:15
  • The entire living space is insulated. I'm probably overloading the term "attic", as there's the converted attic room, and the space above that converted room. The latter is a sauna and I was hoping to find out if there's a good way to regulate the temperature, but more importantly why does that impact the efficiency of the unit? One other note is that much of my ductwork is located in this unconditioned area above the converted room, so perhaps there's additional benefit to insulating or using a radiant barrier? – saigafreak Aug 15 '16 at 1:20
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First, since you have ducts, and an attic-floor-to-rafters room(!) in your attic, it's practically impossible to meet the conditions needed for a vented attic to work in your case.

Since I see you are in the eastern 3/4ths of NC, where ground snow loads can be guaranteed to be below 50lbs/ft^2 and in fact don't exceed half of that as per IRC Figure R301.2(5), a compact over-roof is how I'd solve your problem, instead.

Why a compact roof?

There are two conditions needed to make a vented roof work. The first is that there must be a barrier plane at the ceiling capable of keeping heat and hot (or moist) air from entering the attic and warming the roof deck (or condensing out on the bottom of the cold roof deck -- hygric buoyancy is a thing). This barrier must be as good thermally as the wall is directly over the exterior wall's top plate (i.e. R-30 over the top plate for a R-30 wall) and it must be airtight to the point where heated air can't get into the attic and spoil all your ventilation work. How airtight? Well, I'd be using air barrier tape to seal the ceiling drywall if I were building a vented attic.

However, with ducts in the attic, and a conditioned room in the attic as well that extends to the rafters at some points, the challenge of building a thermal and air barrier between the conditioned space and the unconditioned attic/roof deck just went from merely "hard" to "ARE YOU KIDDING ME?" Ducts leak, despite all the work you do to mastic every last joint (that only gets your leakage down to a few percent). Worse yet, the existing drywall isn't air barrier sealed I bet -- even with sealed and insulated ducts, a drywall joint that has not been taped specially to serve as an air barrier will still leak air. Never mind that door to the unconditioned attic (err...storage space)!

Second, the underside of the roof deck needs to be vented to the outside air. This is how the roof deck is kept at the outside temperature, no matter how much snow is piled atop it. In practice, this usually translates to venting the whole attic, although it is possible to use an a rigid board insulation (foamboard or rockwool) sandwiched between the rafters and a layer of drywall attached with long screws into the rafters and air barrier taped, effectively creating a vent space while conditioning the attic.

Furthermore, vented attics need copious quantities of attic ventilation as ventilation's a lousy way to dispose of heat -- if I were building a vented attic, I would strongly consider using deep/wide enough trusses for the roof that the requisite top plate insulation does not come up to the bottom of the rafters, along with the "insulation wind baffle" from the illustration below (taken from BSI-046):

BSI-046 Figure 2

Given that the eastern 3/4ths of NC doesn't get enough snow loading for the R-value of snow to be a factor in ice damming, the commercial-style compact roof makes more sense in this application as it moves the attic into the conditioned space instead of trying to build a conditioned, air-sealed room within a vented-to-outside-air attic.

How do I actually do this?

Implementing this isn't hard, provided the details are minded. I would strip the existing roof down to the deck (which can stay provided it is in good condition), then apply an air barrier layer i.e. a fully adhered product or a liquid applied product, as sheet products get rips and holes in them that make them poor air barriers. Atop that, I would put in at least two layers of rigid insulation board (rockwool or foam) with a total R-value of R-50 or better and the joints staggered both horizontally and vertically to control airflow (otherwise, weird, complicated airflow networks make life hard). Finally, I would attach an overdeck or nail-base to the rafters with long screws as the insulation will resist the bending moments of the screws, then put down whatever actual roofing system I like for the application as everything's taken care of below it. See the illustration below, taken from the first link above (i.e. BSI-046), for a graphical view.

BSI-046 Figure 5

And oh, last but not least, once I fixed the roof, I'd remove the insulation from the attic floor -- it'd be superfluous once I installed the compact roof, as the attic is then properly part of the conditioned space.

  • Do not confused the ground snow loads published in the IRC with the actual roof design snow load. The snow load that the roof is designed for may be more or less than the "ground" snow load depending on roof geometry and other factors. – Hank Aug 16 '16 at 21:06
  • @HenryJackson -- this may be true for the sake of roof structural design, but the decision threshold when determining whether damming is a factor is based on ground snow loads, as explained in BSI-046 – ThreePhaseEel Aug 16 '16 at 21:09
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Yes, you should insulate the sides and the ceiling of your new room. Insulating a roof is generally not recommended, as the roof itself should remain at the outdoor temperature (which is why you insulate attic floors).

One thing: make sure you install ventilation baffles, aka "proper vent," (brand names like ProVent and Propavent) between the sloped parts of your ceiling and the roof. This is to ensure a good airflow along the roof itself -- assuming you have, as you should, :-) , a ridge vent or several vent ports near the ridgeline.

  • Why would you advocate for a vented attic when he has ducts in it leaking conditioned air into the attic and basically undoing all the hard work he puts into insulation? – ThreePhaseEel Aug 15 '16 at 23:25
  • @ThreePhaseEel Because it's the correct thing to do. Yes, he should insulate around the ductwork (true whether it's providing heat or cooling to the room), and no, heat loss from the ductwork doesn't "justify" doing the ventilation wrong. – Carl Witthoft Aug 16 '16 at 11:19
  • Also, how airtight do you think his attic room enclosure is? Because that's the other key to making a vented attic work... – ThreePhaseEel Aug 16 '16 at 17:20
  • Let me put it this way -- he needs to seal and superinsulate all the ductwork in the uninhabited spaces and air barrier seal the drywall in the attic room as well as venting the roof in order to make a vented attic work. (And it's the outside surface of the roof that needs to be at outside temperature, BTW -- insulated compact roofs are a standard-issue commercial assembly and have no problem maintaining their outer surfaces at outside temperature provided snow loads aren't extreme. Start with BSI-083 for more info.) – ThreePhaseEel Aug 16 '16 at 17:40

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