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I own a house built in the 1930s. I live in San Francisco in the Bayview district where it’s fairly sunny similarly to the Mission District. This house is all lath and plaster and because of which I notice they didn't use or have insulation at the time. I guess the lath and plaster thick walls and a tiny crawl space above the ceiling is sufficient to keep the house insulated. Now I want to knock down the ceiling to expose the nice beams, but that is taking that ceiling layer off which possibly will make my house colder in the winter and hotter in the summer. As someone noted, this is a “hot roof” which I’m not understanding what that means or the implications of which. Now, I plan on getting insulation sheets installed on the roof (NOTE: I'm getting my roof redone so I'll just have the roofer install the insulation sheets). Is this sufficient insulation to what I have currently?

  • If the lathe and plaster is in good shape you should consider keeping it. Atleast do not be hasty about embarking on such a modification, but consider it carefully. It sounds like the ceiling is attached directly to the roof rafters and not to ceiling joists. Exposing the roof rafters may not be a good idea because they are probably rough dimensional lumber and so would not be very attractive. – Jim Stewart Jan 15 '18 at 12:29
  • Lath. A lathe is a large floor mounted machine tool for making balusters. – Harper Jan 15 '18 at 15:40
  • The answer depends greatly on your local climate. Please update your post with such information. – isherwood Jan 15 '18 at 17:00
  • You're in Climate Zone 3, which stipulates an R-value of at least 30 for attics. You're building a "hot roof" by not having a ventilated attic above the insulation. That said, "any advice" questions are typically too broad for our Q&A format. Please edit to ask something more specific, or delete. – isherwood Jan 15 '18 at 20:37
  • Thank you @isherwood. I revised my question and asked specifically if tearing down my ceiling and installing insulation on the roof enough insulation. – josephnvu Jan 16 '18 at 1:31
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Your climate is similar to mine...never too cold and never too hot. That’s why not having insulation in your attic to date, isn’t critical. However, any insulation is a plus, but you’ll have several issues to overcome: 1) fastening vapor barrier to roof, 2) fastening of “roofing” on insulation board, 3) added structure height at eave and rake, 4) finishing of interior beams and ceiling material, 5) added weight to roof structure, 6) cost, and 7) Environmental issues.

1) A “hot roof” could mean a built-up roofing system. This means you have a low-pitch roof and you don’t have roof shingles. If so, tearing off the old roofing membrane and installing a new membrane on rigid insulation is easy and the vapor barrier is part of the system, including how the vapor barrier is secured to the roof. If not, it becomes a bit more complicated because you’ll need to add the proper vapor barrier on the old deck and secure it to the roof, then add rigid insulation, then add plywood on the insulation to hold the shingles, without any fasteners extending through the ceiling material.

2) The easiest method to fasten shingles to roof is with plywood. However, you may need wood stripping if you live in a “high wind climate.”

3) If you raise the height of the roof by a few inches, you’ll need new edge metal. Also, if it’s a historic district, like the mission district, you’ll need Planning Department approval for increased height.

4) After the ceiling is removed you’ll need to finish the beams and ceiling. All those nails / holes / etc. will need to be addressed.

5) You’ll need to calculate the weight of what is being removed and subtract the weight of what is being removed. Then, verify the structure is adequate.

6) $$$$$$$$ vrs. Potential savings from heating and cooling. Check with you local power company to see if they have rebates for adding insulation. They have people who can calculate the savings for 2” thick or 4” thick insulation and various kinds of rigid board.

7) That 1930’s plaster ceiling could have asbestos or those 900 coats of paint could have lead. You’ll need to have it tested.

Sounds simple...oh, and you are getting a Building Permit, right?

  • Well to comment on asbestos. To be clear, I actually would doubt there was asbestos since the house was built in the 30s. The use of that was more like the 50s. Lead, a very good possibility there, but we’re planning on redoing all the drywall anyway. – josephnvu Jan 27 '18 at 20:53
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I would say no this could not be done in a San Fran home dated in the 30' S (trying to save a few bucks may cost 10' S of thousands) for several reasons. First the electrical and duct work in the ceiling probably has been upgraded at least once and probably 2 times with lots of junction boxes being up there especially if the Original K & T wiring was left in the walls this was common in the 70' S , Depending on the type of heating (both water & enviromental air space heat) there may be asbestos covering vents and ducts. And last it would look like crap unless the roof structure was redesigned to allow for open beam design. Restaurants can do this because of the design of there roof structure , homes with open beam ceilings are designed and set up that way , taking down the ceiling will open a bigger can of worms than list of things I can remember. If you just won the lottery it can be done but the cost will be Big.

  • I do understand that it was about insulation and a way to save $. That's why I explained I don't think it could be done in a savings or cheap method. I started out in the north bay costs and regulations there are some of the highest / strictest in all the states I have worked. – Ed Beal Jan 16 '18 at 15:57

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