Attic Insulation, nearly flat roof

Discussion in 'Remodel Forum & Blog' started by jadnashua, May 19, 2013.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    I'm going to be doing some work at my mother's house and was thinking about adding some blown-in insulation in her attic. But, the back half of the roof only has a very shallow slope, so any depth would mean filling up the roof rafters on at least the lower say 1/3-rd. Will this be a problem? The house was built in the 50's, had probably R-9 fiberglass, and some additional was installed some time ago, but I do not know how much. I figure I'd add at least 6" of blown-in stuff, and maybe more depending on what's there. So, will filling things up to the rafters be an issue?
  2. DougB

    DougB Member

    In the northern climates, you need good air circulation in the attic to prevent ice dams. You want to make sure that you don't block the soffit vents. They make cardboard / thin rigid foam that you can use to prevent the insulation from blocking these vents. I'd leave at least a couple of inches from the rafters.
  3. Dana

    Dana In the trades

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    While venting flat roof (particularly in snowy areas) at the eaves is fraught with issues, filling the rafter bays with fiber insulation (even dense-packed) will lead to a punky roof deck in short years. If you do a full-gut of the ceiling as spray the bottom side of the roof deck with an inch of closed cell foam (maybe 2" if on the north side) you can then fill it up with cellulose or fiberglass without concern, as the foam would be a low-permeance non-wicking condensing surface. See RR-1001, and scroll down to Table 3. An inch of foam does world of good, even in colder climates.

    Flat roofs are best insulated from above, if you can. In southern New England R20 above the roof deck would be sufficient to allow you to fill the rafter bays with fiber per IRC prescriptive levels, but in northern New England (US climate zone 6) you'd need at least R25. If the rafters are 2x12s you'd have to go even higher for dew-point control. This will add 4-6" to the stackup, but you'd then be able to put down a peel'n'stick (Grace Ice & Water), or EPDM on the roof deck, and wouldn't necessarily have to rework the flashing where it meets any walls, etc , provide you use the proper slip-surface goods under the foam. Reclaimed roofing polyiso is pretty cheap- cheaper than virgin-stock batts at equivalent R-value per foot. There are multiple vendors in southern New England, search your local craigslist materials listings for "rigid insulation" (or check out insulationdepot.com.)
  4. historichouseguy

    historichouseguy In the Trades

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    Read up on "Flash and Batt". if you foam under roof and fill the balance with glas you need to have enough foam to eliminate a moisture issue.
  5. Dana

    Dana In the trades

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    The flash'n'fill approach to roofs works (which is what the RR-1001 link in my previous post was about), but it requires pulling the finish ceiling down to be able to apply the closed cell foam to the underside of the roof deck. Unlike the above-the-roof deck approach, you DON'T need the full R20+ in foam to get you there. Even though it would constitute a code violation to go with only and inch, in most of New England an inch is enough.

    But that won't fix ice dam issues. Preventing ice dams is all about having sufficient R, which in the maritime regions of New England tends to be R55 or so for sloped roofs, but may need to be higher for flat roofs where deeper (and more insulating) snowpacks can build up.

    Soffit venting on flat roofs is better than nothing, but there is no convection forces pulling air through, and it will NOT cool a flat roof sufficiently for ice dam prevention, and isn't always up to even it's primary task of purging moisture. To get adequate ventilation on a flat roof requires adding vent stacks to give it some stack effect, and the stacks need to be well above the snow-depth line.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    It's a raised Cape, the front roof is steep, and the back has a low pitch (don't know how much really until I get there). There are no soffit vents, two gable end vents and a full ridge vent. There isn't much roof overhang, and what's there is stuffed with a gutter. So, that's what I have to work with. They're old plastered ceilings, so they're not coming out! There's some insulation up there, but I don't know how much. I'm adding a bathroom vent fan, and hope there's enough to run the 6" vent duct plus insulation on top of the ceiling rafters! I considered a through-the-wall vent, but it would have had to go through tile in the shower, then, I'd have had to tear some tile and ceiling out to get the wire to it. The alternative was to go outside and come back in, but I'm sure my Mother wouldn't go for that, and I didn't really want to put more penetrations there. So, sounds like I must keep at least some air space above the existing insulation. I'll have to evaluate things once I get there to determine if there's enough room to make it useful to add any more. I don't think she'd want to pay to have foam sprayed in, and it would be a tough job. I'll let you know how things work out...thanks for your inputs and references. BTW, this is near Rochester, NY...far enough away from the lake to avoid the lake effect snows 99% of the time, but they still do get some big ones.
  7. Dana

    Dana In the trades

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    Rochester gets tagged BIG TIME from lake effect when the wind direction is optimally bad, are you kidding? (I've skied waist deep lake effect dumps in the back country as far away as the central Catskills in years past.) I suppose it's not likely to be as deep on average as it is on the lake fronts, but it's still serious-snowpack country.

    Rochester is in US zone 5, so if doing a foam-over on top of the roof deck you'd need a minimum of R20 to meet code. Given the miniscule amount of pre-existing insulation there, an R20 foam over on all pitches and sealing the attic would be huge improvement, even if you added nothing to the R10-ish batts on the attic floor. The heat leak at the soffits has to be pretty large and ice-dam prone on the low-slope end with almost no insulation.

    [​IMG]
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    The lake effect from Erie peters out about 20-miles away, and with the prevailing winds, about 5-miles north of the house (it's about 10-miles due south of the lake), doesn't get much from Ontario. Now, as I said, you don't have to go far to get plastered, but it's often a day/night demarcation line, and her house is mostly out of it (thank god). Adding insulation on top of the deck would have been a good option when they put the membrane on, but she's not going to be there all too many more years, then it'll be someone else's problem (she's 86). Those bedrooms are generally only used a couple of times a year as she does most of her living on the first floor. So, it would be more for my and my sister's occasional comfort (and maybe resale value) after probate.
  9. jefferson17

    jefferson17 New Member

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    Location:
    Bristol, PA
    I'm quite knowledgeable about insulation, among other things. Here's the thing - it's a balancing act of what you wish to invest, what a reasonable ROI might be for you, what is Practical in a situation, etc. So we can all offer advice but you need to weigh the costs and what is the best fit for you.

    First - as others have pointed out, you can't have spray-in cellulose or fiberglass touching the roof - air must be able to circulate around. Granted you may not have "proper soffits" but ... it's a good bet that there are some cracks around the edge and some air infiltration. So even if you DO go with blown-in - install some of those spacers that create gaps at the edges. A simple wind-powered attic fan can be installed for $50. They work remarkably well (I'm in PA and have a low pitch mansard roof - Victorian).

    I would really be hesitant to recommend closed cell insulation directly against the roof for this situation. THAT would make the entire attic part of the heated envelope. Were that attic a CATHEDRAL structure (and it's not - Cape, right?) then sure - that's a way to go. But with flat ceilings and an attic above - you do NOT want to spray foam on the inside of the roof.

    Here are some ideas for you to consider - and feel free to contact me with any questions, or to bounce ideas back and forth.

    IDEALLY ... you would vacuum out or remove what is currently in those bays up there. Then, SEAL (fire-rated foam only please) ALL penetrations (lights, holes, etc). Air infiltration is the #1 thing that costs the most. More than a lack of insulation. Air that goes up just carries the hot air with it in the winter - it's the "chimney effect". And be sure to seal whatever the attic entry is. Use foam tape, etc - whatever will do a really good job to ensure that air does NOT pass with the hatch closed. You may need to add weight if it's just a hatch panel - so that it pushes down on the foam, etc.

    After making the attic as air tight as you can from the house underneath ... THEN decide what the best insulation options are. Ideally, you would want say 3" of closed cell foam sprayed between those joists on the attic FLOOR - not the ceiling. But ... it sounds like this is only possible in PART of the attic and there isn't roof to do it in the other part? Closed cell isn't cheap but it is the best. You will expect to pay $1 per board foot per inch.

    Note: prices vary WIDELY for closed cell spray foam. You'll get guys that quote $1350-2400 for the SAME EXACT JOB. I would personally only hire an outfit that had a big spray rig (truck-based), and not some tanks that they carry around. The big truck guys invested a lot of money in equipment and training, and they will have the best prices. The tank guys ... who knows if they know what they are doing - it's hardly a regulated field!

    Technically, this solution this isn't compliant with R-38 code - but you don't HAVE the room for R-38 in places anyway. And insulation code hasn't caught up to the benefits of foam vs traditional insulation. After 3" of foam there is little additional benefit. BUT even a few inches of closed cell foam is VASTLY superior to filling up every inch you can w/ cellulose or fiberglass blown-in. Think of how a cooler is constructed ... pretty darn air-tight and a solid layer of foam ... like your fridge ... like your water heater! They don't need 12" of fiberglass ... why should your house?

    Another option is ... get a bunch of 2" thick 4x8 foam board panels from a big box store - foil side pointed toward the heated space (if only 1 side is foil). Carefully cut to fit slightly snugly between the joists and push it in - then put caulk or spray foam around the edges, so there is no air that can pass. Hopefully, there aren't too many wires in the way :) Think of it as poor-mans closed cell spray foam. It will be VERY effective - if you are willing to do the work or hire some guys to do it. Then you can (if you wish) put more insulation on top of that. Even 2" of foam, with edges sealed will be incredibly effective.

    Remember - first seal ALL penetrations. That is the most important thing. Then ... insulate the best you can, given the structure, your options, and your budget.

    Please contact me if you wish! Thanks! I hope this helps!
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