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Community and Q&A

Unusual thermal envelope intrusion at roof-penthouse

keithhoffman22 | Posted in Energy Efficiency and Durability on

Hi,

I’ve discovered that the ~1987 penthouse addition to my house is partially outside the roof thermal envelope . Since it is attached (naturally) to the remainder of the house it compromises the performance of the whole house.

In a nutshell:
The penthouse was floor joisted directly on the old tar and pitch roof. Since it is a penthouse, a portion of the flat roof remains exterior space. No air or thermal barrier was installed at the perimeter of the penthouse in the roof trusses below or at the roof plane These trusses are only filled 1/2 way with insulation (9″, R-30, in 18-20″ trusses). Additionally, the uninsulated portion of the roof truss bays are allowed to communicate with air in the rest of the house due to a lack of air barrier.

Goals:
The penthouse is unusably hot and cold. Conditioning it would be a ferocious expense in its current state. And it compromises the performance of an otherwise reasonably insulated building which we do care about.

Solutions:
I’m struggling to figure out an idealized solution to address the thermal and/or air barrier issues.

One option would be to cut access holes in the old roof deck (or from conditioned space below) and do my best to fill the roof trusses with cellulose (on top of the existing 9″ fb batts). I’m not sure what WUFI would think of a pile of cellulose in an unvented flat roof assembly. Gaps allowing communication of super heated ‘attic’ air to the rest of the house could be addressed.

Another option would be to gain access to the penthouse perimeter in the roof trusses from below, installing some combination of roll air barrier and sheet foam (a good air barrier), thus hopefully bringing the floor system and old roof deck into the air barrier.

A third option would be to insulate the floor system (~R-23) and perform some air sealing to prevent air communication. This is by far the easiest and cheapest option but it seems questionable to allow a 13’x10′ slice of unvented superheated roof space to be sandwiched inside what should otherwise be space and structure inside the envelope.

Has anyone dealt with this ‘penthouse’ problem? I have to guess this enveloper boundary problem exists for any penthouse addition.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Keith,
    First of all, the existing low-slope roof sounds like it is poorly insulated. You are going to need to develop a long-term plan to properly insulate the main roof. Here is an article that lays out the alternatives: Insulating Low-Slope Residential Roofs.

    Second, it sounds like you understand the options for making sure that the air barrier follows the actual thermal boundary of your building. Sometimes there is no alternative to ripping out enough of the existing finishes to gain access to the area where the work needs to be done. Air sealing work often involves laborious insertion of rigid foam rectangles sealed at the perimeter with canned foam.

    But you can't really decide where to put your air barrier until you decide how you will fix the insulation deficiencies of your main roof.

  2. keithhoffman22 | | #2

    Martin,

    I've read that article before and re-read it now. The gold standard seems to be (for my zone 5b climate anyways):
    ~R-20 on top in the form of foam or mineral wool boards
    R-30 to R-50 in the roof trusses in the form of air permeable batt insulation

    However, in a retrofit situation, that's almost impossible. It involves the expense of demoing one (or two if it is an older building - at least some will have a tar roof under epdm) existing roofs off, laying a large quantity of foam, re-roofing, potentially demoing out the living space drywall (with the concomitant discovery of bad electrical, hvac, etc), batt insulating that space, and reinstalling a new ceiling. In a medium to large home, you have likely described $50k+ of work. If the roof doesn't require replacement, no amount of energy savings will ever recover that much money. That's a huge PV array, to use your favorite example. Additionally, on an older building, if the roof isn't rotten yet, you have to ask the question of the value of a preemptive strike to eliminate moisture based rot that hasn't caused any failures yet ...

    I think a better question in the world of retrofit (as opposed to passive/net zero new construction) is what can we get away with that won't cause the roof to fail and will save energy and money while making the house more comfortable.

    There is a huge market of people who can only get an ROI off DER-type insulation efforts if they do the work themselves piecemeal. The gold standard leaves that group out in the cold.

    Anyways, rant off. You can't help that most people can't afford a huge costly re-insulation effort on a flat roof that doesn't need replacing and probably need a 2nd mortgage to afford it while replacing the roof.

    But it does make me curious: in what conditions can you get away with it and in what conditions can you not get away with it?

    If you have a black epdm roof in a sunny winter climate, can you get away with it? But not with a white roof?

    What about in a climate with a low winter RH (Rockies) vs high winter RH (northeast)?

    How about if you have a leaky air barrier, like in many 1970s homes? Does that make moisture condensation better or worse than in a tight home?

    What about if you have polyE behind the ceiling, like most homes from the 60 or 70s? Or can you get away with it if you don't have it?

    If you are wondering what I mean by get away with, one question might be: are you making the assembly more or less vulnerable to condensation based rotting.

    I realize you deal mostly in best practices and code compliant practices. It's a good approach. But I'd bet 90% of the residential flat roof structures out there aren't code compliant. And yet there roofs are still there 35 years later ...

    I may try to WUFI or pay some to WUFI this question.

    As always, I appreciate the amazing service that this website is. It's just a challenge sometimes to strike a balance between best practice now in a retrofit and what you can manage/afford without scraping the structure.

  3. GBA Editor
    Martin Holladay | | #3

    Keith,
    In North America, tens of thousands of buildings are poorly built -- underinsulated and leaky. Moreover, many of these buildings have air leakage pathways that can cause condensation, mold, and rot.

    Many readers of GBA poke around their own home and realize that their home probably falls into the category I just described. That's a bummer. Unfortunately, there is no easy solution. Fixing all of these mistakes takes money and time.

    I wish that I could provide a magic formula for "doing it wrong and staying out of trouble," but I can't. If you have a building that lacks a clear air barrier, and that is insulated with the wrong type of insulation, it's very hard to predict whether your building will develop a problem or not -- especially without a site visit.

    The best way to fix a roof assembly that is defective is to do it right. In some cases, it may make sense to sell a building that is too expensive to fix, and to buy another building with fewer problems.

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