insulating top plate

Discussion in 'HVAC Heating & Cooling' started by ctreefer, Sep 27, 2012.

  1. ctreefer

    ctreefer New Member

    Messages:
    30
    Location:
    CT
    Well, while I'm thinking of insulating the basement I still need to add another layer to the attic (right now we're at one thickness of Fiberglass in the rafters about R19). I bought rolls and rolls of R30 unfaced over a year ago before the energy credits ended but unfortunately due to a back injury they remain up in my attic still in rolls stacked high. Well, the back is better (mostly) and I want to get moving on getting this laid down over the old stuff. Lots of questions and I know there are lots of differing answers to this so here goes....

    1. current stuff they have in the rafters is faced facing the outside. This is actually placed over the original batting which is some kind of rock wool in felt paper packs. Should I remove the paper facing of the stuff that is currently in the bays?

    2. house has no soffits (no overhang and no ridge vent therefore.) When I had the house inspected before we bought it the inspector commented on the fact that the insulation went all the way to the soffits and I should pull it back and put baffles in. At the time I didn't realize I had no soffits so I didn't argue the point. Now I'm not sure what to do. If I insulate all the way to the end of the roofline I worry someday an inspector well tell the people we're selling the house to that the insulation is wrong. I'd love to extend the roofline and add soffits (did this to our old house on one side) but fear I'll never do it to this house. We did have lots of ice damming 2 years ago. I know I also need to seal the top plate but man that sounds like a PIA. pitch of roof is low and back won't like it much. I'm thinking about just insulating all the way into the edge and adding baffles just so I avoid this conversation down the road. What would you do?

    Hmmm. I thought I had more questions but I guess for now that's it.

    I plan on trying to find all wire holes and sealing around them as well as the ceiling fixture outlets to prevent air transfer from the main living area as well.

    When I do the basement I'll be insulating the bottom plate as well with rigid foam and great stuff.

    Just as a note our walls are not insulated. Brick house with plaster walls. I don't plan on ever doing anything there.


    thanks
  2. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    Pulling a few pieces back and carefully inspect the roof deck for signs of moisture discoloration/ damage where it had been covered. If there's none or it's minor you've kinda dodged a bullet.

    Putting in a chute will prevent damage to the roof deck, but it would create a heat leak there, aggravating ice damming issues (which are also a PITA).

    Ripping down a bunch of reclaimed roofing iso at roughly the right widths & lenghts to stack up on top of the studwall plate with a loose fit, loos enough to sealing in place with a FrothPak or TigerFoam kit would raise the R-value at this critical point cutting the ice-dam potential. Leave at a 1/2"-1" breather space between the roof deck an foam to above the fiber layer wouldn't be quite code (IIRC code requires 1.5") If the brick veneer is vented to the attic with a ~1" gap, preserve that gap width at the roof deck with 1" foam board, and cut'n'cobble between that and the top plate.

    If you're putting unfaced R30s over the ~R19ish batts there's no reason to remove the facers. They may in fact have mold growing on the fiber-side of the facer that's best left in place, but once you have R30 above them they will stay warmer, above the dew point of the conditioned space air, and would no longer collect seasonal moisture to support further mold growth.

    To get the performance out of the R-30s they have to be very snugged up to one another with no perceptible gaps or compressions, and a layer of vapor-permeable housewrap over the top as an air-barrier also improves performance (unless they're the high-density "cathedral ceiling" type.) Even without the air barrier you'll be cutting the average heat loss out the attic in half, but at the temperature extremes the R30s will be underperforming their ratings by quite a bit. (But even if they're only getting ~R25ish performance in a 10F attic that's still pretty good compared to what you have!)

    Just for yuks, what's the wall stackup? If its brick/1" cavity/15# felt/ply or plank sheathing/2x4 stud-bay/lath/plaster it's possible to safely insulate the studbays, and that would cut fuel use by a significant double-digit fraction. Brick veneer over an uninsulated studwall adds up to less than R3, but with a cellulose or blown-fiberglass fill you'd be at about R10 with thermal bridging factored in, cutting the conducted wall losses by 2/3. It may be possible to insulate most of it from the attic blowing into 1" holes drilled in the top-plates, which would at least stop leaks from from the conditioned space into the walls from convecting/infiltrating to the top to deposit moisture and add to ice damming aggravation. You want to preserve the air gap between sheathing & brick, but fill the stud bays with fiber at a sufficient density to substantially block infiltration. Even low density cellulose would do that, with new-school fiberglass it takes ~1.8lbs density, which may be hard to do if installing that way, but even if it's only that density in the top 2 feet of the cavity that's HUGE from a convection loss and moisture transfer point of view.
  3. Windows on Washington

    Windows on Washington New Member

    Messages:
    33
    Location:
    Washington, DC
    What is the venting? Gables?

    +1 on getting the outside wall plate sealed. Just stuffing the chute will only lessen the insulation value at that critical locations and exacerbate the ice damming.

    Dana's idea of ripped down ISO is a good use as the R-Value per inch is pretty much the max you can get out there.

    If the roof deck is tight and low, odds are you are going to hate trying to pack that outside wall although it would make a huge difference.

    Never sure why folks like the roll out so much when loose fill cellulose is so each for attic applications. If you get the froth pack for the outside walls, do as much of the interior top plates as you can too.
  4. ctreefer

    ctreefer New Member

    Messages:
    30
    Location:
    CT
    Dana,
    To be honest I'm not sure of the wall stack up. I'm guessing there is a gap and there is sheating with felt but I've never had to open up a wall to investigate. I know looking from the inside the sheathing is tongue and groove pine.

    I didn't realize how a brick house is built and as a result I was having difficulty understanding what you were asking. I ended up drawing what I think is going on in there so if you could comment on the drawing and tell me where to add more ISO etc that would help. If the wall is made the way I drew it I'm guessing there is some form of venting going on between the facia and the brick as I know that isn't sealed. Would I be able to count on that as air flow and use baffles and cut a ridge vent into the roof?

    Also, not sure about the Froth pack or Tiger foam. Is the intention to seal around the ISO? Could I bring the ISO up to the baffle and just spray the surface there?

    Windows, yes, gable vents only. Yeah, I don't look forward to climbing in that space. I'm guessing if I throw a 4x8 sheet of plywood up there and a creeper it might make things a little more friendly working in that tight space. Waiting for it to get a bit colder so I can wrap myself up and not sweat to death.

    I chose the fiberglass mainly because I have other projects down the road in the attic (rewiring/bathroom venting and have AC installed) and I think its easier to move rolled material and work than to try and move the cellulose.

    Attached Files:

  5. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    There is a gap between the brick and the t & g pine plank sheathing, and probably 1-2 layers or #15 felt or similar. Ideally the top of the gap is vented to the outdoor air under the eaves (as in your drawing), but often it's vented into the attic. If it's vented into the attic it's important not to block it off, since it needs to be able to purge moisture or your t & g sheathing is at risk (even with the tar paper). Insulating the studwall portion of the wall rarely causes problems, and often solves some.

    There should also be weep holes in the mortar every few bricks along the bottom course of the brick too, to relieve any bulk-water and provide a vent entry point for convection through the cavity. In this climate it would be a BAD idea to inject foam into that cavity between the brick and wood sheathing.

    It's hard to say without closer inspection whether you have soffit venting behind the facia, or whether that vents the brick cavity to the exterior (or both/neither.) If you put up a ridge vent, you MUST have soffit ventilation, and preferably ~50% MORE cross sectional area to the soffit venting than to the ridge. Keeping the soffit vent area bigger than the ridge vent area keeps the ridge vent from depressurizing the attic relative to the conditioned space below, which lowers the amount of warm humid air getting pulled into the attic in winter. If you can't quite figure it out or it would take too much disassembly to figure out whether it has soffit venting, it's possible to ventilate through the roof deck there in at least a couple of ways.

    Stacking the polyiso where you have it drawn in pink, and sealing the edges to the joists/plates with 1-part foam or FrothPak, etc. is the right place for it. If you put an air-permeable insulation like fiberglass there you'll still have high convection potential, but with foam the air & moisture from the interior can't move up into the attic to condense (at least not quickly enough to matter.) Stacking it all the way up to where it meets the baffle gives you a higher-R than you'd get by overlaying fiberglass, in what would otherwise be a thinner low-R heat leak area. The higher the R-value there, the lower the ice-damming potential. (Insulating the walls with a high-density fiber or a foam insulation also lowers the heat loss and infiltration up from the wall cavity at that area.

    In-re cellulose vs. fiberglass batts on the attic floor: Cellulose has a much stabler R over temperature and fills-in better, with no gaps or compressions, which all add up to make it a better performer all around. If you need to work at the ceiling in some area it can alwasy be vacuumed up with a shop-vac in that area and bagged, to be re-installed with bucket & hand rake when you're done. The fire-retardents make it dustier than fiberglass when handling it, but unlike fiberglass the dust is non-toxic, and won't end up lingering in the air to make life miserable for asthmatics and others. (Fiberglass micro-dust getting sucked through unsealed recessed light fixtures when the central air conditioning or hot air heating is running is a pretty common source of indoor air quality problems- all the more reason to make the ceiling air-tight.) When you're done with all of your projects (or when you decide to insulate the walls), a 3"+ overtopping of the batts with cellulose is air retardent enough to restore the performance of the batts to near their rated-R (with the overall performance improvement far more than the mere R-value of the cellulose itself.)
  6. ctreefer

    ctreefer New Member

    Messages:
    30
    Location:
    CT
    thanks, more to think about. I'll try and get up in the attic over the next couple of days and look into what's going on by the top plate. My recollection of the only time looking in that area was I could see outside daylight at the edge of the brick where the trim met it (this was at one of the gable ends which is clapboard) so this wasn't a soffit area.

    One more question about something that has always confused me: Why is there air leakage occuring at the top plate? Is it coming from the basement? The interior of the house? If the basement is the issue would just sealing the space below the wall cavity be enough? (so much easier..) I've sealed all my outlets and will be sealing all my holes where wires etc go between floors. Windows are newer replacement units. Trim is painted all around so nothing to caulk. Replaced can lights with ones intended for insulation contact and sealed around all other light fixtures in the house. Not sure where else the warm air would get into the space unless its just getting warmth through the wall from the interior. At what point do I become concerned that my house is too sealed? ( I do have a wood burning stove insert I installed a couple years ago which I'm sure leaks some air around the fiber gaskets they provide for installation.)
  7. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    In your case, air can get into the studwall cavities from a number of places, including all electrical & plumbing penetrations of the wall, under the bottom stud plate, between the seams of the t & g sheathing etc. Only post 1980 was much effort given to making air tight walls, and then only amongst a microcosm of builders highly interested in energy efficient buildings. (T & g sheathing & no cavity insulation is not the hallmark of a high-efficiency builder interested in air-tightness.)

    But even air-tight empty studwall cavities lose/gain quite a bit of heat via unimpeded convection currents within the stud bay driven by warm air rising on the cold surface, falling on the cool surface, and via direct ultraviolet radiation from the warm side to the cold side. Stuffing it full of high-density fiber or any-density foam stops convection losses and most of the radiation losses (almost all of the radiation transfer is stopped, if cellulose, less so for fiberglass or foam, though all can perform well).

    The band joist and foundation sill is usually the largest untreated air leak, and is often larger than all window & door leakage combined. Caulking/foaming the air-tight recessed cans to the ceiling gypsum was good, as well as all the electrical/plumbing/vent/flue penetrations of the ceiling. If you have ducted AC or heat, caulking duct boots to the gypsum/framing behind the register faces keeps leakage from going up stud bays (even partition walls) as does caulking the electrical boxes to the gypsum behind the outlet plates, etc. (even on partition walls.) When you think you've got all the big (and small) holed nailed, a blower door test will find most of the extra 500 square inches of leakage area you never dreamed existed- you'll find air moving through cabinets (particularly under-sink cabinets), and under wall kick-boards, all sorts of places. But the key is to start with all of the big leaks first, or it wont' be tight enough to find all of the odds & ends stuff.

    It's almost impossible to retrofit a house to where it's "too sealed" and require active ventilation (and if you did, that would GOOD thing) without ripping the house apart and getting really SUPER serious about air sealing. Over the past year or so I've been advising on a friend's full-gut rehab of a century+ old 3 family in Worcester MA with some subsidy available for a "deep energy retrofit". This house had all the siding and interior plaster stripped, the stud bays were sealed at the wide-plank sheathing with closed cell foam, there exterior got a full taped-seam Tyvek treatment + 2 layers of rigid foam (seams taped, edges foam-sealed), all new triple-pane and very tight windows, caulked & sealed in place etc etc and it STILL leaked 464 cubic feet per minute under 50 pascals pressure in a calibrated blower door test last week. (About the equivalent of about 9 square inches of leakage cross section- a 3.4" diameter hole.) That's pretty damned tight- tight enough to qualify for a higher subsidy in his case, and tight enough to need active ventilation. Energy recovery ventilation (ERV) systems were designed in and installed from the get-go, since this was a design goal. If you made the place tight enough that you had less than 1.5 full air exchanges per hour (ACH) at 50 pascals in a blower door test you'd be a candidate for active ventilation. But it usually takes an effort to get it even down to 3 ACH/50 in a retrofit, especially for a plank-sheathed house. (My friend's project came in somewhere between 0.5-1 ACH/50.)

    One way to tell when you're getting there is by monitoring the indoor humidity in winter. If you normally run your bathroom fans to dehumidify the bathroom during/after showers, and run the range vent when cooking on the stove top, and your indoor humidity in January/February averages over 40% RH @ 70F you'll have copious window condensation when it's cold out even on low-E double-panes, and you need to start thinking more seriously about active ventilation. If you're in the low to mid 30%s you still have some tightening left to do. If you're (like most homes in southern NE) dropping below 30% RH whenever it's in the teens or low 20s out you have a LOT of room for tightening up the place.

    Dense fiber in the stud bays will impede any leakage paths through the walls, and it shows up in blower door testing. Dense-packing cellulose over pre-existing low density batts is one of many standard methods in the bag of tricks used by weatherization companies to improve ACH/50 numbers. Empty stud cavities are pretty low-hanging fruit on the building efficiency tree, and most houses have quite a bit more wall area than attic area, with correspondingly higher heat loss out walls.
  8. ctreefer

    ctreefer New Member

    Messages:
    30
    Location:
    CT
    isoimage.jpg thanks again for the info. I go to bed thinking about how to best address these issues. As it turns out I had to go in the attic last night due to a leak in the roof.....:(. Have to fix that when the weather clears. I'm thinking it may be easy enough to prefab a bunch of ISO for all the top plates as you mention. If I stack 2 on top of each other(3M spray mount together) and provide 1/2" clearance around the blocks and top baffle I can fill around the edges with spray foam. Please see image.

    do I need to seal all of A through F including B which is on the back side? After this can I run the fiberglass batts up to this and over on top as long as it doesn't get in front of the top edge of the baffle?

    It's hard for me to assess humidity in my house. I'm somewhat of a salt water fish/coral tank nut so I've got several tanks in the house that during the winter easily evaporate 1-1.5 gallons per day into the living space. The few original single pane windows are soaked all winter unless I use the shrink film over them. Most other double pane windows just have the usual condensation near the bottoms of the panes.
  9. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    The drawing looks right- go for it! I usually cut them to fit loose enough to fit the tip of a foam gun/can fully between the rigid foam & framing, and let the can-foam fill & seal rather than just edge-seal.

    You may find this discussion regarding insulation & air barrier & wall leakage etc relevant.

    Buy a few $10 AccuRite humidity monitors (online web-stores or at the evil Wal-Mart), and you'll be able to track the humidity in the tank room and elsewhere in the house. The li'l battery operated units look like this:

    [​IMG]http://ecx.images-amazon.com/images/I/51enRFUnxXL._SY355_.jpg

    A single AA runs them for about a year, maybe a bit more. They only display relative humidity, not dew point (absolute humidity), and a low room temp will deliver a higher number with the same air volume, but you can look up the dew point/100%RH temp on a psychrometric chart (or online psychrometric calculator) using the displayed RH & temperature. These are not precision scientific instruments, but they're pretty accurate when running at 60-85F room temps with the RH between 15% & 85%.
Similar Threads: insulating plate
Forum Title Date
HVAC Heating & Cooling Insulating pole barn Jan 4, 2012
HVAC Heating & Cooling insulating material info - please... Oct 6, 2011
HVAC Heating & Cooling Insulating square duct inside soffit Oct 28, 2010
HVAC Heating & Cooling What are the major benefits of insulating the refridgerant pipes Jul 27, 2010
HVAC Heating & Cooling Insulating radiant floor installation with exposed beams Dec 8, 2009

Share This Page