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Community and Q&A

Insulation in 1926 house

McKCei | Posted in Energy Efficiency and Durability on

I am flipping an adorable 1926 Cottage. There was no AC system so I installed one. I had to get creative as to installing the ducts. But it’s still HOT upstairs. I live in Central Texas. There is no attic space. The upstairs looks like an attic conversion, but it was built this way. The walls are stucco or plaster. It’s like cement on metal mesh. Then there’s about 8″ of airspace and then wood with the shingles on top. I did put two AC vents up there, but it’s still hot. Can I put spray foam insulation in the space between the roof and ceiling. I would have to drill holes in the stucco. I’m reading that’s it’s not a good idea, but all of those people are in Canada where it’s cold. I’m in Texas where it’s HOT!! Thanks so much for your help.

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  1. Matt F | | #1

    Did you do a load calc and manual D design for the creative ducts? Did you verify the correct CFM are being delivered to the space? Where are the supply ducts located? Inside or outside the conditioned space?

    That is going to be a tough place to insulate without extensive work. There are injectable foams, but they are open cell as far as I know, so you need a very good vapor barrier plan, which may be satisfied with some work to the existing interior.

  2. Expert Member
    Zephyr7 | | #2

    Sounds like you have real wet plaster on metal lath. that holds up pretty well, but be careful drilling holes -- the metal lath (usually like mini size expanded metal sheet) can make for exciting collateral damage sometimes.

    Do you have any existing insulation in that roof assembly? Do you know you have sufficient airflow into the space from your new ducts? Do you have excessive stratification (cold stays low with the heat higher in the space)? Is this a vented roof assembly (it doesn't really look like it, but it's hard to tell from those pics)?


  3. Expert Member
    Dana Dorsett | | #3

    As long as the roof deck isn't skip-sheathing, it's reasonably safe to install cellulose in there, which is far easier to remove than foam without gutting the place if it ever becomes an issue.

    What type of shingle?

    Got a ZIP code?

    For more on the comparative safety of cellulose in your area see Table 3 in this document:

    In Miami , Houston, and Phoenix (the first three rows in the table), a full fill of cellulose (second column) showed the roof deck stays below 16% moisture content year round, even in the higher indoor humidity case, whereas if spray fiberglass were used (first column) there is some risk in the humid gulf coast states. This is largely due to the substantial moisture buffering capacity of cellulose safely harboring seasonal moisture burdens without loss of function.

    At 8" cellulose would be good for about R30 (which may be current state code in central TX, though shy of the IRC 2018, which calls out R38). If dense packed to 3lbs per cubic foot it would also have a measurable thermal mass effect to enhance performance and would never settle. Even ~ 1.8-2.2 lbs lower density cellulose would have some mass effect, but might need topping off at the peak after decade or two.

    If those are recessed lights under the ridges it's worth yarding them out and replacing them with surface mount LED fixtures prior to insulating, especially if they are not rated for insulated contact.

    1. McKCei | | #9

      Thank you Dana. This is very helpful. I replied to the main comment, so I'm not sure it will notify you that I replied. Would you take a look at my response if you haven't already? Thank you!

  4. McKCei | | #4

    Ok, let me back it up just a bit. I do have a tiny attic space which is where I have the furnace. In that tiny space (which did pass inspection but probably just as it is such a small space), I did put some spray foam insulation in that part and going up the wall just up from that tiny attic space as it was accessible from the tiny attic space.
    I did have a professional HVAC co install the system so I would imagine they did the testing. For the ducting, in the first floor, we had to add some furr down in the bedrooms, but were able to add ducting in a closet upstairs to go directly through a wall for the vent in the living and down for the dining so it looks nice. Upstairs, for the one room, we went directly out from the tiny attic space with one vent. For the other room, we came out from that tiny attic space, but had to go through a built in drawer - out the front of it, so it is stuck in place now. It's kind of cute. I hope that makes sense.
    I guess the problem is twofold: one, the lack of insulation upstairs and two, the thermostat is downstairs.

    1. Expert Member
      Zephyr7 | | #6

      Having the thermostat downstairs will likely result in the upstairs running a bit hotter than the setpoint during cooling season. There are thermostats with remote sensors (like the Ecobee) that can help with this somewhat (they'll maintain the average temperature between sensors at the setpoint, which splits the difference between upstairs and downstairs in your case). You might also try setting your thermostat to run the fan more often to circulate the air, which may also help to balance out the temperatures somewhat. You'll lose a bit in terms of energy efficiency if running the blower more often though.


      1. McKCei | | #8

        Thanks Bill. That is very helpful.

  5. McKCei | | #5

    You guys are so helpful. thank you. The zip code is 78648. In fact, this is the listing:,-97.387963,29.476366,-97.882348_rect/10_zm/2_p/?view=public
    I have composition shingles. I did not replace the roof. I had a roofer take a look at it and he said I have about 7-10 years left on it. At one point, I spoke to another roofer who said it was possible to get extra thick shingles made to help with insulation or heat issues.
    I'm going to look for a picture of what I've got behind the wall. Thanks!

  6. McKCei | | #7

    More pictures:
    1. The first pic is of the attic space where the unit went before the spray foam insulation was put.
    2. The second pic is of a portion of wall that I was removing that shows the type of metal mesh with the stucco from the inside of the wall.
    3. We built a furr down over this duct downstairs.
    4. This is of the duct that came out of the built in drawer. Now it has the drawer front on it with the vent in the middle.
    5. This is the pic of the other bedroom where we put a vent coming right off of the small attic space where the furnace is.
    So if you look at picture 1, you see the wood on the top of the attic space. I believe this is what is just under the roof. 8" or so below that wood is the mesh with stucco. I want to fill that space with spray foam insulation. Will that work? Or will it deteriorate the stucco. Will it dry well? Thanks so much for your help.

    1. Matt F | | #11

      It is important to make sure your are getting correct CFM to the upstairs registers. I would not assume that they actually verified this. Did they check airflow with a flow hood? Did they do a manual J S and D?

      At this point I would just try to maximize flow to those registers. It looks like the upstairs may have been done with flex duct, but I can't see too well. I would make sure there are no bends in the flex. If there are turns done with flex, replace with elbows. Use metal duct for straight sections or pull the flex tight. You may need to upsize the ductwork.

      I would also check the system static pressure and see if it is reasonable to close the dampers to the downstairs a bit. Closing these if the system is already running a high static pressure won't help a lot.

      Do Dana's cellulose plan.

  7. GBA Editor
    Brian Pontolilo | | #10


    Rooms underneath the roofline like your are notorious for being uncomfortable and inefficient. You may be able to play with your mechanicals to make the room more comfortable, but you likely also have some air sealing and insulating work to do. There's a ton of content on GBA about how to deal with this situation. For starters, try these articles:

  8. Expert Member
    Dana Dorsett | | #12

    In the section with the stucco ceiling a spray foam solution would require a slow-rise pour which is usually quite a bit more expensive than open-sprayed, and comes with at least some risk of cracks in the finish (though that risk is low with metal lath & stucco or hard plaster.) If there are any blockages that create gaps or voids, you're stuck with them forever. The foam won't deteriorate the stucco, but any touch-ups or removal become difficult. If the mixture is off when they install it there will be outgassing, and no way to remove it to start over. Some foam installers are pretty good at aiming the gun up a wall or rafter cavity with open cell foam. But with sprayed open cell foam there is both a fire risk during curing and quality issues if doing it 8" thick at a pass. (That may true for half-pound pours too- I'd have to look it up.)

    These are all reasons why cellulose is a safer option.

    At 2lbs density or higher cellulose has the same R/inch of a half-pound polyurethane pour, which is pretty good. I can't imagine that a 2lb (closed cell) pour bringing it up to ~R49 makes any sense at all. It' won't improve creature-comfort performance over an R30 attic ceiling, and would be difficult to detect in the utility bills. With an 8" cavity to work with a quasi-dense-packed 2.5lbs+ could be achieved as a DIY working from the mini-attic spaces for dirt-cheap. If there's ever an major issue with the insulation (such as a roof leak saturating a large section) it can be vacuumed out with an industrial vacuum and replaced. When it's time to re-roof, 1" of foil faced polyiso above the the roof deck would bring it up to IRC code minmum performance on a U-factor basis (due to the R6 thermal break over the rafters).

    Even though there isn't much window area on the attic space, from a utility bill point of view spending the difference of a foam vs. cellulose insulation on low-E storm windows over the nice antique single panes (for most or all of the house) would make a far greater difference than going from R30 to R50 in the sloped attic ceilings, and is probably worth doing in any event.

    Storm windows on a charming 1920s antique don't have to look like bird crap on a Cadillac- even Larson's Premium Series (can be ordered through the Apple Ace Lumber in Luling TX, and most box stores) has a color option that isn't too far from the painted trim color in the Zillow pics.

    An indium tin oxide hard coat low-E coating on the storms will cut the solar gain by about 1/4, and since it's on a second pane it cuts the total heat loss out the window by about 2/3. So during the winter the windows will become a net heat gainer, and during the summer it's still a lower gain. This comes close to the performance of an IRC 2018 code min new window or replacement window at a fraction of the cost, and you retain the charm of the antiques.

    I'm not sure if Larson has a solution for your arched windows, but even if those have to wait, there is decent "payback" on putting low-E storms elsewhere, in both comfort & dollars (winter & summer.)

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