Insulating top of dormer

Discussion in 'Remodel Forum & Blog' started by jadnashua, Jan 29, 2011.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    I live in a townhouse condominium. I have a couple of dormers on each side of a Mansard roof. Long ago I added a lot of insulation to the accessable attic area and put a radiant barrier sheet under the roof joists. On the main part of the roof, I'll have snow for as much as a week after the other units have had all theirs melted off, so at least on that part, I'm fairly well situated.

    The problem is on the dormers...major icicles. I neglected to knock them down as early as I should have while they were smaller, and on big one fell and bent my satellite dish, so that's out of commission until I could find a good time to have them come out. I may have them move it slightly to provide more protection, but that doesn't address the issue with the dormers, and the obvious heat leaks up there.

    So, I think I'll tear the ceiling off the dormer bumpout in the bedrooms, but not sure what I'll find. I'm leaning towards putting foam sheets up there against the roof deck. There is a full ridge vent in the main attic, and the only inlet vents are under the dormers (maybe 20% of the width of the roofline). From what I remember, those did have foam channels, but no idea if they are intact (will see when I get the ceiling out).

    What might be the best way to insulate and plug any air leaks into this area while maintaining any ventillation and not introducing potential condensation and rot issues? When they reroofed a few years ago, they wrapped the entire dormer, top to bottom and up onto the main roof deck with ice and water shield, so I don't expect the ice dams to have generated water damage in there. This is in Nashua, NH.
  2. dlarrivee

    dlarrivee New Member

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    Spray foam or bust.

    You need to introduce more fresh air at the soffits and forget about venting the dormers.
  3. Dana

    Dana In the trades

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    dlarrivee- clear some of your stored-messages in order to receive more....

    Jim- the best approach is to get RID of the ventilation scheme and use closed cell spray foam at ~1-perm depth (will vary by vendor) against the roof deck, finishing out the R value with fiber, with NO interior vapor retarders. Higher ventilation will not achieve your goal here, but higher R and blocking thermal-bypass air flows can. In less snowy areas you can sometimes get away with the roof deck ventilators in lieu of bigger soffit vent area, but not in NH, IMHO. Even then, the ventilation does very little by way of cooling to even up the temp of the roof dect to reduce melting- it's primay function is to purge moist air to keep it from condensing. Insulating the roof deck with spray foam fixes the melting, and protects the roof deck from direct contact with any condensation events. With inadequate soffit cross section, the ridge vent can sometimes be counterproductive, actively drawing moist conditioned-space air in through any leakage in the attic floor boundary, without being able to dilute it with dry soffit-vent air. Sealing the ridge vent may help the overall picture if there wasn't a very serious attempt to air-seal the attic floor during construction.

    With generic 2lb SPF you're looking at ~2"/R12 in foam to get to ~1perm.. If you go with 2lb "mid density" Icynene(tm) you can put ~4"/R20 in foam to hit ~1 perm which may be preferable. (It's cheaper per unit R than 2lb SPF, but has a lower R/inch).

    For a sanity-check on this approach, see the simulations in this document.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Is this a DIY'er thing, or better contracted out? It's a small place, so setup per amount of material may make it less attractive to a pro, and therefore fairly costly. recommendations on anyone in SNH, or close, that would work there?
  5. Dana

    Dana In the trades

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    If it's less than 1000 board-feet of material but more than 500 board-feet, 2lb polyurethane will be pricier from a pro than with a DIY kit, but if you went with 2lb Icycnene at 4" it would be 2x the material. That would still be more expensive that 2" of SPF from a 700board-foot kit (seems to the upper bound of the TigerFoam/FomoFoam/etc kits, which all use the same chemical supplier, BTW.) While there are no rocket-scientists out there wasting their careers as foam installers, there's a bit of an art to it, and the temperature controlled spray equipment the pros use delivers a more consistent result than the DIY kits, in general. Wait for the right weather/air temps if you tackle it yourself- if you end up with shrinkage/bonding issue hacking it out for a re-do is a real PITA.

    Before either you or a contractor installs the insulation, take the time to pressurize/depressurize the house with a window fan when you have it all opened up and seal every micro-leak you find in there with caulk or 1-part foam. While 2lb foam is a great sealant, it's not guaranteed- one micro-crack from shrinkage at a rafter edge can become an air/moisture bypass, even if it's still working for you thermally. Nearly all moisture damage problems are from air-leaks, not vapor diffusion, despite the fuss made over gettting the vapor retardency dialed in.

    Contractor reference comin' atcha via PM.
  6. dlarrivee

    dlarrivee New Member

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    Great advice from Dana.

    My inbox is now empty, by the way.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Okay...I finally got around to blowing in insulation in the attic...while it will take awhile to get useful results from my utility bills, it does appear to be more comfortable upstairs now.

    Now, two more projects, the dormers and the rim joist. Figure that on the rim joist, I'll cut about a 1' strip of the ceiling drywall out so I'll have access, then seal the rim joist with spray foam. Now, the question is, do I insulate it as well with spray, or cut up some foam panels to fit, and seal them in with spray? I've only got about 40' of it to do - the front and back walls that face the exterior, the other two walls are internal and shared. If I go with bulk spray foam, it looks like I could get by with a small set - figure something in the order of 100board feet would be plenty. Thoughts on which might be better and product selection?
  8. dlarrivee

    dlarrivee New Member

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    For the record, I'm a qualified polyurethane spray-foam applicator, I've done 2lb and 3lb density as well as elastomeric coatings and various other spray applied coatings.

    I would urge you to use XPS and foam in a can (doesn't spray), instead of a small "spray-foam" kit (propane bottles, boxes etc)...

    I've used the Dow froth paks before in a repair scenario, and I can assure you that the results aren't great considering the cost involved.

    True spray foam (1000psi, plural component, rise before your eyes), is very finicky when it comes to liquid temperature in the drums, hose, hose whip... The stuff in small kits is the same, but you don't have $100,000 in equipment to apply it properly.

    The other warning I have for you is how relentless the stuff is when it comes to over-spray. Any polymer based items, plastics, painted surfaces, finished floors, etc. etc. will provide excellent adhesion for spray applied foam, and the only solvents strong enough to break the foam down will also likely wreck whatever the over-spray is on.

    Not worth the hassle IMO, cut yourself some XPS and use a professional style foam-in-a-can gun (replaceable foam cans), the one with a proper valving rod and tip that can be cleaned. They're easier to get into tight spots, and you wont have to deal with a clogged up plastic nozzle if you leave it for a few hours.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Thanks, sounds like a plan...I can rip a bunch of the xps panels to fit, then seal them in with the spray. I'd planned on that for the dormers, but all the small pieces for around the joists on the rim seemed like just spraying would be faster. Better safe than sorry, though.
  10. dlarrivee

    dlarrivee New Member

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    It all depends on the situation, I take it you want to do this yourself, or I would suggest pro-applied spray-foam.

    You would need to cut more ceiling for that too though.
  11. Dana

    Dana In the trades

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    It's pretty easy to do a sloppy cut'n'cobble on the odd bits and use a big-gap filler single expansion foam to make it truly air tight. Leaving a 1/4-1/2" clearance on the cut up pieces makes it go quicker, with no need for precision.
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Well, I started on the dormers. While they had shutes that should have maintained the air path from the soffit vents, the insulation jammed in there had collapsed it to nothing - they were cheap foam - the new ones have internal posts that keep it from collapsing.. I though it might have been filled with cellulose, but that doesn't appear to be the case. Since I don't expect to need a full can of foam per dormer, I'm going to wait to do that once I get at least another one ready. Went from a leaky R-19 batt to what should be two layers of xps foam in the rafter chamber with one on top for a total of R30 and no air leaks. Hopefully, this will at least lessen the ice that accumulates on the end of the dormer's roof.
  13. Dana

    Dana In the trades

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    That should pretty much do it. Most of the time adding R10 foam over the roof deck fixes ice damming issues in my neighborhood, and that's the same R10 thermal break over the rafters that you've added here.
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