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Community and Q&A

Real “minimum” insulation value for a Phase 1 retrofit

motoguy128 | Posted in Building Code Questions on

I have a 110 year old house with no attic insulation. It does however have plaster in lath, about 75% in good condition on the rafters, joists have foil faced fiberglass already, with 1″ deck boards over that. It have knee walls around 1/2 the permiter, a bonus room, semi finished over one area, and gables over 2 sections. knee walls are 4’x4′ triangular and go out to the copper lined box gutters (so you can see which seams leak) and soffit assembly. I can see light where many of the separate boards of the soffit, face board and gutter assembly meet. There’s a wall plate resting on top of the brick walls which I believe are 3 course, air gap behind outside course to the structural inner courses.

I want to eventually utilize the space, (11-1/2′ ceilings in the main open area with just 4 support columns below the hipped roof with widows walk) but for now I have a upstairs high velocity system, that long story short has some leaks and inadequate insulation, so it’s too small, and costing a lot of run. So I want to bring it into the conditioned space.

I have a steam humidifier that keeps up (3300sqft total, full stone basement) but it limted by single pane with poor quality storm windows. I have however seen frost under so exposed parts of the roof deck. Roof deck is original 3/4 or 1″ tongue and groove boards. Great stuff. Survived decades of water leaks on the original slate roof. All wood is full dimension lumber of course.

I need to 1) seal the attic and provide some insulation. 2) create vented space above rafters/plaster and lathe from all the hip and gabled roofs to the widows walk where i have a 800CFM exhaust fan. 3) install some soffit vents, but rely on existing leaks and covert one window to a gable vent. 4) insulate under all joists. Leave the plaster and lathe, it adds mass and structural strength . removing it is a lot of work, a big mess and I have to go back and tray and enforce the whole structure. Any radical structural change will probably cause walls to crack on the 2nd floor.

Here’s the problem. To meet code and keep the P&L, I’d need to get ridiculous and install 4″ XPS then 2″ polyiso, furring and then drywall. That would cost me around $9,000 in materials.

But for about $1000, I could install 1″ polyiso and furring strups (for strength) for now. (yes I know no thermal barrier…. but seriously, if fire reaches penetrated my plaster ceilings, the insulation and deck boards, I’m already done for. It would be temporary and I’d add more gypsum. Phase 2 is rockwool where I plan to have unfinished spaces, add 1.5″ XPS and a 2nd layer of furring then gypsum in finished spaces.

Here’s the thing, at R6 I’m already exceeding the rest of my structure which I estimates performs at R4ish, and my windows which limit humidity levels. In terms of ROI, above about R10 it’s going to fall like a rock. R38 seems pointless with a high mass home. ROI on $10k of foam for the attic would be around 20 years even without labor.

Any thoughts? What’s a minimum to prevent condensation on the foam with a vented cathedral roof assembly? I think R4-6 would be adequate.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Any thoughts?"

    A. Lots of thoughts -- and questions, too.

    First of all, a question: You wrote that there is "no attic insulation," but you also wrote that the "joists have foil faced fiberglass already."

    So which is it?

    Are you talking about the attic floor joists? If so, you have attic insulation. How thick are the batts?

    You wrote, "I have a steam humidifier." Humidifiers are dangerous, because they can contribute to wall sheathing rot and roof sheathing rot. Right now, disable the humidifier -- don't run it next winter. In the meantime, implement an aggressive air-sealing effort. If your home's indoor air is dry during the winter, that's a sign of an air leakage problem.

    You wrote, "For about $1,000, I could install 1-inch polyiso and furring strips (for strength) for now."

    Where would that be? Presumably, you want to install the polyiso on the underside of the roof rafters -- but you didn't tell us that.

    You wrote, "Yes I know, no thermal barrier."

    Here at GBA, we won't advise you to violate building codes, especially when it comes to fire safety. You'll need to cover any rigid foam that you intend to install with 1/2-inch drywall.

  2. GBA Editor
    Martin Holladay | | #2

    You wrote that in the widow's walk, you "have an 800-cfm exhaust fan."

    What's that for? Such a fan is an energy hog and potentially dangerous. Disable it or unplug it immediately.

    For more information, see "Fans in the Attic: Do They Help or Do They Hurt?"

  3. motoguy128 | | #3

    Sorry, that was "Widows Walk" it's a flat roof on the peak of the hip roof. I have a small acrylic boat hatch there for easy watertight access. I mounted a standard attic exhaust fan. I keep 2 of the gable end windows open for about 2 square feet of area. Plus all the leaks around the perimeter. In winter I seal up the fan and close the windows. I still get summer stack effect to the 1st floor and basement, so it's not sucking out conditioned air is any real quantity. There's no soffit vents currently. House by design was intended to have all windows open and you could just open the attic gable windows too. The slate roof probably reflected more heat than the cheap modern asphalt equivalent.

    it's seems to cool off faster with the fan running than just passive ventilation through it. I may try not suing it and see if it makes much difference. But it does consume probably 200W or so around 1/4 HP I'd guess.

  4. motoguy128 | | #4

    Let me clarify, yes the joists do have insulation, but it's maybe R12 at best and doesn;t resolve air leaks.
    The HVAC system is in the attic, plus I'd like to use the space some day, so would prefer to start down that path and make it semi-conditioned.

    I could use spray foam to seal the soffits, but then seal in leaks (better to let them breath.

    Humidifier only runs to a point where the single pane windows start condensing. Has outdoor temp reset. Usually 20-40% upstairs depending on outdoor temp. Downstairs is lower. I've done a lot of air sealing, but there are limitations in expense and reality. Its a 3200sqft 1905 brick victorian with stone foundation and 1 pipe steam system. If it was stick frame, it would be easier. Air leaks are from locations at the brick and around windows. Some leaks are part of the brick and window drainage plane. Seal it too much are you will rot it out. Its' actually not that bad. A lot of the leaks are windows (need new storm windows, but that's about a $8000 project since I have 42 windows not including the 3 attic windows. Some double hung windows are 4' x 7'!!!

    Yes, Polyiso under the rafters, over the plaster and lathe.

    No good way to retrofit the brick walls. Just have to focus on air leaks. I installed rope caulk on most the windows. We rarely open the windows. Most do open pretty well, nicely balanced on the pulleys and weights. Have the steam system tuned and balanced pretty well. Operates at <1oz of vapor, was designed as a vapor vacuum system.

  5. GBA Editor
    Martin Holladay | | #5

    You have a big old house with terrible energy details, so I imagine that you are spending an arm and a leg every winter to keep the place heated.

    Before we can give you advice, we need more information. Where are you located? We need to know your geographical location or Climate Zone. Since you are running a humidifier during the winter, I'm guessing that you live in a cold climate. But that's just a guess.

    With a house like you describe, air sealing measures and improvements in attic insulation are almost always cost-effective, especially if the homeowners are spending $1,000 or $2,000 a year for heating fuel. Only you can determine whether you have the funds to invest in energy improvements. If you can get a bank loan for energy retrofit work, you'll likely save enough money over the next 10 or 15 years to justify lots of improvements.

  6. motoguy128 | | #6

    Climate Zone 5. 6100HDD, -2F Design, 91F 77F WB summer. SO Zone 4 & 6 design temps, but Zone 5 HDD.

    I don't have exact numbers but boiler and heat pumps combined were around $2500 for heating last Fall/Winter/Spring. 1st year in the house. Heat loss I estimate around 90,000BTU/hr peak heating demand, 4 Tons total peak cooling demand. We keep it a comfortable 67-69 Winter, 73-74 Summer. Use very minimal setbacks, mainly for comfort. I use outdoor temp to stage the boiler. 720EDR (172k BTU) radiation on a oversized (sigh) 375k BTU input 80% steady state natural gas seam boiler. Boiler might outperform that number since I run it at 1oz or less, not the 2psi where its rated for commercial use. I also added a flue damper and it's has electronic intermittent ignition. Water heater is electric (I'll go to indirect when I replace the boiler in a couple years). I balanced the system (1 pipe parallel flow steam) and added 1 TRV so no zones are overheated or cold and heats up fairly evenly. When it's -10F and the wind and snow is blowing off the river, that nice even radiant steam heat is really something. Creates quite a strong convection current in the room.

    Air sealing 1) new Storm windows needed, 1 missing. 2) applied rope caulk and removable caulk to most all the windows and any air leaks around frames. Caulked at some of the baseboards. 3) Need to look at if I can spray foam the rim joist. Need to talk to a masonry expert. Not sure the wall detail where it interfaces the stone foundation. There's cut limestone on the exterior that goes right to the brick. But air leaks through weep holes into the wall cavity and then goes to the rim joist. Rim joist I assume is in front of the structural brick. 4) I've done extensive sealing around doors. There's 6 exterior doors, on upstairs to a back porch. 5) installed reflective insulation on wall behind each radiator on an exterior wall. 6) Insulated the 3" steam header in the basement.

    I think realistic is about a $1000/yr savings. So spending $15k over the next few years on insulation and storm windows would be worth it. Could replace the worst storm windows and those visible from street and just caulk the seams of the others you don't see. Maybe $4000 on storm windows, $5000 on insulation and finishing the attic (I think I have a way to do it in stages and still meet fire codes.) I don't need R40 until I condition the space since i have R15-20 or so already in the attic floor. More about sealing air leaks in the attic, then getting good controlled ventilation under the roof deck so it stays dry.

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