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I am living in a one-story house with flat roof. There is an 2-feet-high empty space above the ceiling and below the roof, and let me call it my attic space (although my house has a flat roof). The attic space seems not insulated (there is some pink fiberglass in the attic space, but only covers a small portion of it)

It is my understanding that the attic space should be sealed from the living space to prevent heat loss and moisture condensation. But in my house, we have a furnace located in a small equipment room, which directly connects the attic space and the living space. My attic space is also connected to the outside by several vent pipes and vent grills.

Equipment room with furnace If the image is not showing please see picture here

The furnace (See the picture attached) is standing on the floor of the equipment room. There is a white louvered shutter door (not air tight) separating the equipment room and the rest of the living space. The hot air duct (correct me if it's actually the cold air duct) directly goes into the attic space. As shown in the picture, there is no separation between the attic space and the equipment room.

I am concerned that the current configuration is causing significant heat loss during cold days, because warm air in our living space can escape by living space --> equipment roof --> attic space --> outside. Which of the following solutions would be better for my situation?

(1) Replace the louvered shutter door with an air-tight (and possibly insulated) door. If so, the furnace need to draw the air only from the attic space, whose volumn (around 2000 sq ft) might not be enough to provide combustion air for the furnace (92k btu/h). Should I worry about this?

(2) Install some panels between the attic space and the equipment room to separate them. It is relatively hard to do for me without removing the duct from the furnace.

(3) seal the attic space from outside of the house, and leave it connected to our living space. I am not sure if this will actually help to reduce heat loss.

Our local climate (North CA): very cool in the summer (never needed AC) and not too cold in the winter (almost never snows or freezes, but does need heating).

  • How is your roof constructed? It seems to me that it may be more practicable to insulate the roof here, especially considering it's a flat roof... – ThreePhaseEel Aug 8 '18 at 1:01
  • @ThreePhaseEel The top layer of my roof is modified bitumen. I'm not sure about the layers below, but I can see the rafters supporting the roof from a vent grill. When you say insulate the roof, do you mean to put insulation blankets between the rafters? Thanks. – user12075 Aug 8 '18 at 1:58
  • Can you see what the deck of the roof is made of? Modern compact flat roofs have insulation built in (they're a corrugated steel -- foam -- corrugated steel sandwich), while older ones may have just a wood or steel deck with the WRB and overcoat applied atop it (no insulation) – ThreePhaseEel Aug 8 '18 at 2:23
  • How does the furnace get its combustion air? Does it draw from a vent out the roof or does it draw through an opening on the side of the furnace? The former is AFIK always a concentric double vent where the exhaust goes out the center and the combustion air comes in through the annular space. Are there soffitt vents allowing air to get into the "attic" space? From what I can see I think you should not change anything until you fully understand the air flows for combustion air. Contact your local building inspection and ask about this. I suspect you should leave it as it is. – Jim Stewart Aug 8 '18 at 4:40
  • @ThreePhaseEel From what I saw, the roof decking is build with wood without insulation. The house itself is pretty old (1930s), so I guess it does not have the modern "insulation built in" roof that you mentioned. – user12075 Aug 8 '18 at 5:31

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