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  1. #1
    Join Date
    Feb 2013
    Location
    Boston area
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    99

    Default Old house, no insulation, mansard roof, confused by recommendations.

    I have an 1865 2nd empire outside of Boston (zone 4) with no insulation, single glaze windows, 2 flat-ish roofs. I've been sealing all the gaps I can find, and now that the roofs are being replaced I can think about insulating.

    But, I'm quite confused. I had an energy audit and the company really pushed having insulation blown into my walls for about $6000, and that was about all they said on the subject of weatherizing. Then I read that walls make little difference at all, and the gov't energystar seems to recommend R5 as just an afterthought for walls.

    Meanwhile that website says R38-R60 for ceilings. I gutted one part and I think I have 6" rafters, no attic just a very small pocket under very shallow roofs. I think closed cell spray foam is my only option, but even there I have more questions... Do I need to pull down the other half of the ceiling to get the roof insulated? And what about the sides of the mansard roof, do they need to be spray foamed, and thus I need to rip out the top half of the walls? Or is blown cellulose okay there since it's sorta a wall and sorta a roof?

    Then that gov't website recommended R25-30 for a floor...?!? Do they mean a ground floor, or a basement floor? I never even considered insulating the floor, should I? That would be easy, I think I have 12" joists. However, I have steam heat and the heater is in the basement with hot pipes everywhere, should I consider that a conditioned space or insulate all the pipes, too?

    Any help would be greatly appreciated!

  2. #2
    Join Date
    Jan 2008
    Posts
    2,203

    Default Re: Old house, no insulation, mansard roof, confused by recommendations.

    Please note that general website advice (even from the Government) may not apply to you directly. Your on-site energy audit would likely be the best advice to follow.

    As a rule, attic insulation does the most good and you can't have too much up there- especially in non-mild climates. Wall and window insulation is usually the next most important followed by the bottom floor (which is your basement if you have one and it is not being heated or cooled). Sealing openings is also important.

    Perhaps whoever did your audit can drop back by and explain things a little better for you, or if someone else offers an audit for free you can call them in- do get at least 3 estimates before having any work done, and have them all explain everything to your satisfaction before sending them away.

    Phil

  3. #3
    Join Date
    Dec 2007
    Location
    Tennessee
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    1,791

    Default Re: Old house, no insulation, mansard roof, confused by recommendations.

    It almost sounds to me like the energy auditors didn't know what they were doing or you have completely misunderstood them. There are a lot of factors that need to go into any decisions you make. You want to get the most bang for the buck as sealing up the leaks is a very good place to start.

    Let me explain the walls first. An uninsulated wall cavity has about an R-5 value. This is because it is a narrow void with walls on each side. Convection currents do form inside the wall cavity, but they are limited because of the narrow spacing. An uninsulated ceiling is only an R-1 because any heat that escapes is immediately lost through the vents in the attic space.

    Your Mansard roof sides are walls for all practical purposes. You should have some small soffits that are vented. You cannot blow insulation into the Mansard sides if there are in fact vents in the soffits. You will have to remove the interior sheathing and put in batt insulation and then recover your walls with new sheetrock.

    Considering the age of your house, I think you should really consider tearing out all the interior wall sheathing so that you can replace all old wiring and pipes. Pumping loose insulation into an unknown void that is that old could be dangerous and cause a fire. But as far as bang for the buck, that is the cheapest way to insulate those walls. I would leave the walls alone until I could do it right.

    On a two story house, the walls may account for up to 25% of the total heat loss for the house. Insulating the walls to R-11 or R-13 will save about half of that 25%. If you have to tear out the interior sheathing as I recommend, that will be the most expensive insulation as well.

    Your attic will account for about 35% of your total heat loss. Normally there is a quick payback for adding insulation to an uninsulated attic because you are going from an R-1 to an R-19 or higher. If you just go to an R-19, then you are saving about 95% of that 35%, going to an R-38 only gives you about 2.5% more. Thats called diminishing returns.

    In your case, you have to take down and replace the ceiling so the cost to insulate your attic is going to be considerably more than a conventional roof. In a conventional roof, there is enough ventilation that you don't need a vapor barrier, but a Mansard roof has less ventilation so I would recommend that you do use a vapor barrier. You do need to leave some room above the insulation so you are limited on how much you can install. Be sure to check all your wet wall openings to the attic or anyplace that vent pipes or any other pipe or wiring goes into the attic that it is well sealed. If not, cold air will come down those interior walls and they will loose heat just as fast as the exterior uninsulated walls do.

    As for the floors, I would not insulate them at all. Instead look at the space around the edge of your floor joists where they sit on the foundation walls. This is where most of the heat from your floors and basement is lost. This is the rim joist, fill those cavities with insulation, then the basement will be warmer and you won't need to insulate the floors. If you get really ambitious, use foam boards like the insofast (insofast.com) to insulate the basement walls. You won't need a lot for that, 2" should be plenty. Don't worry about the basement floor unless you want to live down there.

    BTW, those single pane glass windows loose almost as much heat as the walls do. Storm windows or replacement high efficiency windows can cut that portion in half or a little more.

  4. #4
    Join Date
    Dec 2013
    Location
    New Hampshire
    Posts
    27

    Default Re: Old house, no insulation, mansard roof, confused by recommendations.

    Boston, MA is zone 5 not Zone 4. Insulation recommendations are based on a "typical" house. They are based more on the "typical" cost of insulating than on the relative heat loss through that part of the structure. The loss of heat through a square foot of ceiling isn't much more than a square foot of wall insulated to the same R-value. Putting a foot or more of cellulose or fiberglass in an accessible attic is just less expensive than framing out walls a foot thick or using expensive insulation with high R-value per inch. New construction minimums for Zone 5 are: Ceiling R38 ( R-30 in limited areas), Wall R-20 or 13 +5, Basement wall R-10. For retrofit situations don't get hung up trying to meet recommended levels if areas of the house are atypical bringing insulation up to recommended levels is often not cost effective.

    if the walls have no insulation blown in cellulose is usually cost effective and strait forward. If your house shows signs that it might have knob a tube wiring the contractor should refuse to insulate it. Other inadequate wiring issues should also be addressed before insulating. It's easier to fish wires through empty walls. $6000 seems high for blow in cellulose most of the work is for drilling and patching holes which can vary with siding type or if its installing from the inside through holes in the plaster. Adding an addition R-5 rigid insulation to the wall is only recommend if the exterior is being resided or the interior gutted anyway.

    Mansard roofs are a bit different. Blindly blowing in cellulose could easy cause condensation problems and leaks from ice dams. I wouldn't blame the auditor for trying to avoid it. Their profit margin for blow in wall insulation is good and predictable by comparison.

    4 ways to insulate attic:
    1. If there is vent space connecting upper and lower rafter bays, soffit vents and roof vents. Remove plaster on attic walls and ceiling, install baffles in rafter bays to maintain vent space, install insulation in ceiling and wall joists.
    2. When re-roofing blow in cellulose though roof sheeting, cover with house wrap, nail 2 by 2s over old sheeting to create an airspace, nail on new roof sheeting, with a vent strip under the drip edge and vents at the ridge.
    3. When reproofing install ~ 4 inches of rigid foam on top of the sheathing. Gut the attic and install another 6 to 8 inches of un-faced fiberglass in the rafter bays.
    4. Gut the attic as needed for access and spray the underside of the sheeting with closed cell foam.

    Vented approaches have lower risk of leaks due to ice dams and are generally preferable where possible in snowy regions.



    Floor recommendation of R-30 is for over a space that is open or vented to outside. Insulating the ceiling of a basement with a boiler is counterproductive. instead insulate the wall or just the upper portion of the wall if you have no intention of finishing the basement.

  5. #5
    Join Date
    Feb 2013
    Location
    Boston area
    Posts
    99

    Default Re: Old house, no insulation, mansard roof, confused by recommendations.

    Thank you so much for your help! Some details I left out:

    I am in the process of ordering some Harvey storm windows, so that part will be taken care of soon.

    As for old wiring and plumbing... well there is pretty much none. No plumbing in the walls, no electricity at all on the third floor and hardly any in the rest of the house (3 outlets per floor, some lights). What is there is a mix of BX and romex. So, I guess I am lucky...?

    We are living in the house and trying to avoid tearing out the plaster and lathe. Most of it is pristine.

    The Mansard roofs are not vented (minus the animal holes being fixed today). The lower roof (that I am going to insulate first) is rubber, so I don't think I need to worry about ice dams. The upper roof is shingled has a shallow pitch, so I guess I better insulate that really well. Luckily (?) I live in the 21st windiest city in America on top of a hill, so even in a blizzard not much snow accumulates. Even part of my lawn is often blown clean.

    There are no ceiling joists in the lower roof. There are 6" rafters and strapping attached to the beams.

    We were considering making room a cathedral ceiling, but have changed our mind so we should have a lot more room for insulation. I am worried about the cost of closed cell foam but will get estimates. Since we are really zone 5, the recommended R49-R60 would take 12-16 inches of fiberglass. There is only 9 inches of space at the sides of the lower roof so that might be under insulated, or I could lower the bathroom ceiling and hide the plumbing soffit as well. The bedroom ceiling next to it, one of the few ceilings not cracking and peeling... I can't really see how I will insulate without tearing it down.

    Based on your advice, I think I will have cellulose blown in the walls, hopefully I won't be able to hear everyone on the street afterwards. I won't bother with the floor. I eventually will do something with the basement walls but first I need to do a lot of patching and pointing. I don't have rim joists I don't think, just a big beam that may be notches for joists, but I have been finding small gaps and spraying foam into them. I also should cover the basement windows with plastic. One wall is above ground, I'll blow insulation into that, too.

  6. #6
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,791

    Default Re: Old house, no insulation, mansard roof, confused by recommendations.

    This will be more expensive than cellulose but may be a lot better and that would be a slow expanding foam. You get better insulation and it takes care of the vapor barrier. If you do use foam, it can be almost impossible to remove if you change your mind later and want to do something else.

    If you go with a spray cellulose, then you will need some type of vapor barrier applied to the plaster. There are vapor retardant primers that you can paint on or you can use a vinyl wall paper. You could use either the slow expanding foam or cellulose on the lower section of the Mansard roof. It is essentially a slightly slanted wall that looks like a roof. It should be treated as a wall.

    For the upper section, I think you just have to remove the ceiling plaster and install fiberglass with a vapor barrier and just R-19. Again, look for any penetrations to the roof cavity by any pipes coming up the walls or open topped (wet) walls. These need to be sealed. If at all possible, I think you should consider some type of vent for the upper roof cavity. Your roofer should have some options.

    I'm guessing that the estimate for the cellulose is so high because this is a pretty big house.

  7. #7
    Join Date
    Dec 2013
    Location
    New Hampshire
    Posts
    27

    Default Re: Old house, no insulation, mansard roof, confused by recommendations.

    There are a few reasons that a roof should not be insulated as if it were a wall.

    1. A well ventilated airspace is very useful in preventing ice dams.
    2. Even if that roof slope is too steep too steep gather ice and snow it may provide a useful air space to ventilate other portions of the roof.
    3. Unlike siding; the roofing materials used at eaves, on low slope roofs, and not uncommonly on steeper pitches are vapor barriers. Placing a vapor barrier to the exterior of a vapor permeable insulation will cause condensation problems in winter.
    4. Adding a vapor barrier on the interior side may stop the condensation problem, but with a vapor barrier on both sides of the roof assembly, if what would otherwise be a minor leak should occur, rot will set in before it has a chance to dry.

    That said: blowing cellulose or spraying foam into unvented rafter bays with a greater than 45 degree slope that are covered with a vapor semi-permeable roofing isn't terrible.

    I don't, however care much for the practice of spray foaming the underside of the roof sheathing, especially on low slope roofs.

    For low slope roofs your better providing at least a little ventilation or using rigid foam on top of the sheathing so the the roof structure can dry to the inside.

  8. #8
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,791

    Default Re: Old house, no insulation, mansard roof, confused by recommendations.

    The lower section of the Mansard roof is almost vertical, usually has no connection to the space under the upper section because it has a header and usually have wood shingles or shakes which are not a vapor barrier. They can and should be treated like a wall.

  9. #9
    Join Date
    Jan 2014
    Location
    NH
    Posts
    136

    Default Re: Old house, no insulation, mansard roof, confused by recommendations.

    Quote Originally Posted by skintigh View Post
    I have an 1865 2nd empire outside of Boston (zone 4) with no insulation, single glaze windows, 2 flat-ish roofs. I've been sealing all the gaps I can find, and now that the roofs are being replaced I can think about insulating.

    But, I'm quite confused. I had an energy audit and the company really pushed having insulation blown into my walls for about $6000, and that was about all they said on the subject of weatherizing. Then I read that walls make little difference at all, and the gov't energystar seems to recommend R5 as just an afterthought for walls.

    Meanwhile that website says R38-R60 for ceilings. I gutted one part and I think I have 6" rafters, no attic just a very small pocket under very shallow roofs. I think closed cell spray foam is my only option, but even there I have more questions... Do I need to pull down the other half of the ceiling to get the roof insulated? And what about the sides of the mansard roof, do they need to be spray foamed, and thus I need to rip out the top half of the walls? Or is blown cellulose okay there since it's sorta a wall and sorta a roof?

    Then that gov't website recommended R25-30 for a floor...?!? Do they mean a ground floor, or a basement floor? I never even considered insulating the floor, should I? That would be easy, I think I have 12" joists. However, I have steam heat and the heater is in the basement with hot pipes everywhere, should I consider that a conditioned space or insulate all the pipes, too?

    Any help would be greatly appreciated!
    You are ahead of the game because you don't have to rip out all of the mistakes from outhers trying to fix the problems. Most people feel the cold comming in through the walls and think that is the problem but it is only a symptom of the problem. The problem is the heat escaping through the roof and if you fix that first, you are headed in the right direction. If you want to know how I would fix it just ask.

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