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insulate finished attic while replacing the roof

Chicagobungalow12 | Posted in General Questions on

Hi all. We bought a Chicago bungalow about 2 years ago.  We are getting ready to replace the roof. We can’t figure out if it’s worth the hefty extra cost to insulate the roof while we’re at it. Currently there is little to no insulation as far as we can tell. Some very deteriorated rolled insulation is visible in some areas.

In order to add insulation, our contractor proposed to remove all of the original planks, placed rolled insulation, and install baffles and a ridge vent, and lay the new plywood over that. Then of course the shingles last. This does not include insulation for the knee walls. This is roof lines only. The attic is 2 small bedrooms and a small landing between. Fist question, does this plan sound right to everyone? Any alternatives we ought to consider?

The reason we may do this is that the space is already finished. It’s very very warm up there in the summer. There are 2 vents for the central air up there but no air return so it barely touches the heat and humidity of a Chicago summer. A window unit in the back bedroom helps a bit but not so much. We imagine it would be much more comfortable with proper insulation.

Does that seem realistic, for the insulation to make a major change to the climate upstairs? Or should we save the extra 5thousand  or more dollars and just re-shingle with roof on the existing planks? Either way, we will have them install the baffles and ridge vent as it does sound like that is the right way to ventilate a roof like this one. 

Finally, the heat in the winter is not a problem. It is usually a few degrees cooler upstairs than on the main level. But, it’s pretty comfortable for sleeping and doesn’t bother anyone. Is there any chance that adding the new insulation would actually change this for the worse? Could heat be too trapped in the attic in the winter? As a reminder, there is no air return up there. 

Thank you so much for you helpful thoughts on this. We are handy, but we’re out of our depth on this one. 

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  1. Expert Member
    DCcontrarian | | #1

    Insulating as you've described can be risky. If the ventilation is not done properly moisture in the air can condense on the sheathing and cause rot. The reason ventilation makes the roof last longer is that if the sheathing rots the nails pull out and the shingles fall off.

    Do you know how thick the rafters are? This will determine how much insulation you can put in there.

    Another approach -- although probably more expensive -- would be to put foam sheets over the sheathing when you replace the roof. This would allow you to put more insulation on, and wouldn't be so risky.

    1. Chicagobungalow12 | | #6

      Can you explain this "The reason ventilation makes the roof last longer is that if the sheathing rots the nails pull out and the shingles fall off." You lost me there. Thanks!

      1. Expert Member
        DCcontrarian | | #8

        In the wintertime, the indoor air has enough moisture that if it comes into contact with something at outdoor temperature condensation will form. You can see this if you set out a glass of ice water.

        The underside of the sheathing is going to be at approximately outdoor temperature. No matter how well sealed your ceiling is, air from inside the house will leak up and hit the sheathing. When that happens, moisture will condense on the underside of the sheathing. If the roof is ventilated, that moisture will dissipate. If the roof isn't ventilated the moisture will accumulate, and the prolonged wetting of the sheathing will cause it to rot. It also causes the roofing nails to rust. When the wood rots and the nails rust the nails lose their holding power and the first wind to come along blows the shingles off.

        The purpose of ventilation is to dissipate that moisture. I'm old enough that when I was young I was taught that roofs needed be ventilated because heat caused the wood in the sheathing to crumble and ventilation kept the sheathing cool. In the past few decades we've learned that's not true, the reason the wood crumbles is because it's getting wet and rotting.

  2. user-5946022 | | #2

    Strongly consider DC's suggestion of exterior foam. Your contractor installs rigid foam on top of the existing decking, and another piece of decking on top of the foam so the roof has something to nail to. There are also premade products out there with the foam already attached to the decking so it is one job. When you do this, the contractor must do a very good job of sealing up the soffit vents that vent the roof, and must also seal the foam, so no moist air gets to the top piece of decking under the shingles. The ridge does not get vented. I would be a bit concerned in your case about difficulty sealing off the existing rafter bays from air movement, because air is not just coming through the vents, but also from the knee walls you referenced. Others here will have better ideas.

    1. Chicagobungalow12 | | #5

      There is just no way to make sure everything gets perfectly sealed without gutting the finished living space and starting completely over. So this approach wont be an option for this particular case. Thanks for your thoughts.

  3. Expert Member
    BILL WICHERS | | #3

    The easiest/cheapest option is probably to do as your roofer suggests, but BE SURE that vent baffles are installed on top of the batts and immediately below the roof sheathing. You need a continuous pathway for airflow from the soffit vent to the ridge vent, and you need it in EVERY rafter bay. If you insulate with batts in these types of assemblies and don't provide ventilation, you'll probably end up with mold and rot problems at some point in the future. You're also limited in how much insulation you can fit, and my guess is that after you put in at least a code minimum 1" vent channel, you'll probably only be able to fit R19 or so worth of insulation, which is a lot less than code minimum R49 in most areas.

    If you can fit exterior rigid foam, that would solve a lot of problems. Go this route and you can put baffles in the rafter cavities, then polyiso OVER the roof sheathing, a second layer of sheathing, and then your finished roof. You typically want about 50% of your total R value in that exterior rigid foam in this case for a safe assembly, which would mean a MINIMUM of about 3" of polyiso (R19), and ideally more. The downside is that this can mess up trim details at the eaves, so keep that in mind if going this route.


    1. Chicagobungalow12 | | #4

      Hello Bill, Thank you for chiming in. What you first mention (the easiest/cheapest option) is exactly the plan. Yes, we will be certain that the airflow has a continuous pathway in order to prevent moisture and mold issues down the road.

      As you said, "You're also limited in how much insulation you can fit, and my guess is that after you put in at least a code minimum 1" vent channel, you'll probably only be able to fit R19 or so worth of insulation, which is a lot less than code minimum R49 in most areas." You're exactly right. With this in mind, do you think the limited amount on insulation we can add is really worth all the extra expense? The removal of the planks and the new plywood alone will be quite a hefty add on. It's just so unclear to us if it would really change the climate of the attic living space enough. At the same time, we hope to only do this roof once and not again for another 30 years, so we do want to do it right. You're thoughts here are much appreciated.

      1. Expert Member
        BILL WICHERS | | #7

        You have the longevity of the structure to think about here too, not just indoor comfort levels. If you've been having issues with rotting sheathing, you really have to deal with that problem -- it's not optional. Structural concerns are ALWAYS first priority, everything else comes after that.

        Without doing anything on the interior, your options are pretty limited. You could either use vent channels and batts, for lowest R value and cheapest assembly, use cut sheets of polyiso with vent channels for some more R value, but a whole lot more labor and cost, or you could use batts without vent channels and continuous rigid foam over the sheathing, which will get you the best performing assembly but will also be the most expensive option.

        If you can't do foam over the sheathing, I would do batts and vent channels and hope for the best in terms of comfort levels. You'd essentially be building to the insulating standard of 50+ years ago, but at least with the vent channels done correctly, you should be OK structurally and safe from issues with rot. I would try to find a way to get at least 3" of polyiso on that roof over the batts if at all possible (and remember, if you put exterior foam on, then you do NOT want to use vent channels), since that is a safe assembly that is also a whole lot better performing in terms of R value.

        I would recommend trying to find reclaimed or factory seconds for the polyiso to save some money. You can use the cheaper fiber faced "roofing" polyiso here too for additional savings. My sorta-local (~180 mile drive for me) insulating salvage place has 4x8 foot sheets of fiber faced polyiso for $28/sheet. That's new material, factory seconds, and is pretty good stuff. You would be better off doing two layers with staggered seams instead of putting all the R value in a single layer, and you'd also do better to go with more total insulation, but at least this gives you an idea of what the material cost might be.


  4. ekibum | | #9

    I'm renovating an old home in Chicago on my own. It was my mother's but sat vacant with a leaking roof for years. So far I have replaced the entire HVAC, re-ran the electrical (Updated Knob and Tube runs), put in new plumbing lines for everything (except underground plumbing), Partially replaced plumbing stacks, I added in batt insulation and put up new drywall removing all of the old plaster and lack of insulation that was there on the first floor.

    I have two issues. The first I will address in this post:

    It is a bungalow and the upstairs attic area is very weather dependent. When its cold its freezing up there, when its warm it is HOT. I have an exhaust fan that I wired in that does kick on and try to mitigate some of the hot air. The HVAC does not have A/C, i use a window unit on the first floor.

    The ceiling is exposed meaning no insulation is upstairs yet and its just some wood separating the roof from the inside of the home. The roof is brand new, a fantastic job done by Green Attic Roofing company from Chicago early last year. It’s basically just shingles, but they also added in ventilation that was not previously installed properly.

    Basically, how does one go about insulating their upstairs bungalow? The ceiling on the first floor (so attic's floor) has no insulation in it, I was told not to add insulation because the heat naturally wants to rise and this would prevent warmth from reaching the attic in the winter, but I have some floor boards that I need to replace so I can pull them all up and insulation if I need to. I do want it to be a usable/liveable space. Do I simply just add batting in the ceiling, cover with panelling or whatever and call it a day or is there more to this? I know moisture build up can be a problem sometimes, so just looking for ways to eliminate this and do it properly.

    I did all of the work on my house by reading articles here on Green Building Advisor. I am in no way a professional so really looking for solid advice here. I am on a budget with everything as well. Thanks

    1. Expert Member
      DCcontrarian | | #10

      You'll probably get better answers if you start a new thread.

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