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Community and Q&A

Insulating flat roof on 100-year-old brick row house

LMML | Posted in GBA Pro Help on

My recently purchased 100 yr old row house (in NYC – total 1200 sq ft),  sandwiched between 2 party walls, recently had its roof redone –  prior to purchase.  One of the next door neighbors confirmed the roof installation was a “warm roof” (foam panel install over the ply substrate w/ weatherproof barrier).    There are a couple of old vents/holes thru the tops of the  brick walls (with screen) at the top of the front & back of the house (presumably to vent what would have been a “cold roof” construction back in the day). 

The question is, now that its a “warm roof” should we still install insulation between the ceiling joists of the second floor (the ceilings below the warm roof)?
There would still be an air gap from below the roof substrate to the top of the ceiling joists. The wood roof structure was built with a slope providing a large gap above the ceiling joists at the front of the house and a lesser gap at the back of the house (although still more than a few inches above the joist tops). Again, presumably because construction was for a “cold roof”.

Is there any advice against installing Mineral Wool (rockwool) insulation between the ceiling joists there to help keep cooling and heating from escaping through sheetrock out through the vent holes and out the brick walls?

Thank you,

LM

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"The question is, now that its a “warm roof” should we still install insulation between the ceiling joists of the second floor (the ceilings below the warm roof)?"

    The amount of insulation that can be safely installed under the roof deck depends on the R-value of the foam up top. It's also limited by the depth of the rafters (not "ceiling joists"). NYC is in zone 4A. In that climate zone as long as the R-value of the above-deck insulation is more than 30% of the total insulation R, even with only a Class III vapor retarder on the interior (standard latex ceiling paint) the roof deck stays warm enough in winter to not accumulate too much moisture.

    Installing too much insulation on the interior makes the roof deck can make the average winter temp of the roof deck too cool, with potential for mold/rot if going way too far.

    If installing insulation under the roof deck, it needs to be snugged up to the roof deck, not at the ceiling level, with a vented air space between the insulation and roof deck. For insulation above or below the roof deck to be fully effective the space between the insulated roof deck and interior ceiling should NOT be vented to the exterior. It's fine to have an air space between the ceiling gypsum and insulation (or bare roof deck), but venting to the exterior creates a "thermal bypass", undercutting the performance of the insulation.

    1. LMML | | #3

      Thank you for your reply. The R value of the foam is unknown in that we have no paperwork from the roofing vendor that did the install for the previous deceased owners. Its only a presumption that the roofing company used an appropriate R-value for our zone.

      At this rate, it sounds like insulation above the sheetrock, between ceiling timbers on the second floor is not ideal?

      The center-line section drawing attached crudely illustrates the current roof/ceiling timber situation (not showing rooms/load bearing walls - the 7.5"H timbers don't span the whole length of the house of course). The roof originally being built with a slope and ceiling timbers parallel to the floor. At the front of the house is a large space (likely more than 2'-0" above the top of the ceiling timbers) and at the back of the house its much less, but space still exists...whatever the math would be for the required slope of the roof.

      Is it then correct to assume, with the "warm roof", that we would be better off insulating the brick walls above the ceiling timber line and sealing up the "vent holes" in the brick, permanently?

      Many thanks for the helpful info thus far.
      The details and nuances of a 100 year old house have been interesting (and sometimes fun to sort out), but going from old building methods to modern day energy efficiencies has presented some tricky hurdles.

  2. Deleted | | #2

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  3. Expert Member
    Dana Dorsett | | #4

    >"The R value of the foam is unknown in that we have no paperwork from the roofing vendor that did the install for the previous deceased owners. Its only a presumption that the roofing company used an appropriate R-value for our zone."

    That is not a good presumption. Sub-code stuff gets installed all the time in retrofit applications.

    To bring it fully up to current IRC and NY code with only the foam above the roof deck would take 6-7" of polyiso foam board, which can be a pretty expensive cost-adder (~$5 per square foot in my area, maybe more in your ZIP code) to a roofing job, even on fairly simple roof lines like a low-pitched/flat roof with few penetrating plumbing & flue stacks, etc. Odds are you only have about half that 3-3.5" .

    If they pulled a permit for the job there is probably a description on file with the city of what was installed. If you can get the name of the roofing contractor the company probably has a record of what was installed. Short of drilling from above or below to find out the foam thickness and type, see if there are any removable boots around the underside of a plumbing stacks where you might get a peek at or measure it.

    >"At this rate, it sounds like insulation above the sheetrock, between ceiling timbers on the second floor is not ideal?"

    Not ideal, but it still does something. If there's an an air gap vented to the outdoors above the ceiling level insulation the R-value ratio is less relevant. But venting on flat roofs doesn't function very well even in the best of cases, which is why it's safer to build it as an unvented assembly with the correct ratio of R-values. The fact that the venting hasn't been blocked off means that the above-deck insulation's performance is being undercut.

    >"Is it then correct to assume, with the "warm roof", that we would be better off insulating the brick walls above the ceiling timber line and sealing up the "vent holes" in the brick, permanently?"

    Yes.

    The drawing shows 7.25" (nominal 2x8 milled lumber) ceiling joists (that are probably not structural) well below the slightly sloped but undimensioned rafters. How deep are the rafters, and at what spacing?

    With a combined foam/fiber unvented roof insulation, if you have 3.5" of roofing polyiso above (R20) you could put as much as R39 of fiber insulation on the underside of the roof deck without much risk, and it would all meet code. With milled 2x12 rafters (11.25" nominal depth) filling it with rock wool would be too much, but with 3lbs density cellulose it would be about R41-R41, and even though it exceeds the R38 max by a bit, the hygric buffering capacity of the cellulose (much higher than rock wool) would protect the roof deck by wicking away and redistributing any seasonal moisture. Alternatively, R38 fiberglass batts compressed into a 2x12 bay would deliver R37 performance which would be pretty safe if it's held up on the interior side by a stout "smart" vapor retarder such as Intello Plus, or even PERFORATED aluminized fabric type radiant barrier. (Unperforated RB would create a moisture trap, and should not be considered.)

    With shallower rafters there are other options that still get you there with R20 above the roof deck, such as R30 rock wool, which is designed to fit into 2x8 framing.

    When we know the thickness and type of above-deck insulation it would be possible to advise more precisely.

    1. LMML | | #5

      Thank you again Dana. Your notes make sense so far, and are much appreciated.
      Still must measure the roof rafters to confirm their depth. However, it looks like you are correct about the noted polyiso at the top roof surface, the mini-split vendor installing the units cut through the roof for the PVC feeder tube, the remains were as you noted.

      At this time can my take away be: 1) if rafters are 2" x 12" (nominal sizes noted), they should get R38 fiberglass batts w/ Intello Plus (or variations you noted). 2) if rafters are the 2x8's, they can get R30 Rockwool...however, would the Intello Plus smart vapor retarder application apply as well? It seems it would, no?

      BTW, Intello Plus looks like a really cool product. Thank you for that info.

      Thank you again, in advance.

  4. Expert Member
    AKOS TOTH | | #6

    You have to have realistic expectations from the energy improvments of adding more insulation to your roof.

    Assuming your roof already has around R20 of polyiso above.

    With a standard torchdown above it, the roof gets up to around 150F in the summer. With a 1000sqft roof and 75F indoors, you gain ~4000 BTU through the roof.

    In the winter time with 15F design temperature, the 1000sqft of R20 roof looses ~3000BTU.

    Both of these are relatively small amounts, adding insulation can half that, but you are just reducing a small number to even smaller. It won't budge your building's energy use that much.

    The important part in your case is to seal up the existing vents and insulate the perimeter around the roof. This will definitely make a big difference.

    So if you can insulate for low cost, than by all means, go for it. If it costs a lot of money, there are probably better places in your building envelope to spend that money.

    1. LMML | | #7

      Many thanks Akos!

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