Here in the desert southwest, most of our ducts are in attic spaces. A large majority of those are rigid in dropped ceilings that are open to the attic. All leak air into the walls, and many built pre-70 (or so) have open wall cavities with no blocking. I fix a LOT of these. I was gung ho at first with covering with rigid and sealing the edges, but this approach got very tedious and not very cost effective for all but the easiest of framing complexity. Many just have to many jogs and rampant inconsistency in framing placement.
Now, I often seal the heck out of the blocking or open cavities in the wall junctures with the dropped ceiling. After that is done I fill the cavity with blown insulation. This also insulates the ducts at the same time. Now I know this is a step below capping the drop performance wise, but I have created a continuous air barrier and the small kneewalls created by the drop are covered by insulation that will always be in contact due to the cavity being filled and the full insulation amount added on top of that.
I just wanted to see how other people are handling these issues. Thanks,
Any other approaches? Tyvek? Drywall? silly putty?
We do the same thing in the mid-Atlantic region. Covering the top of the entire drop seals both the house and makes duct sealing less necessary, and it also reduces the square footage of the thermal envelope so, in theory, reduces the bill. Your method of capping the walls and filling the cavity will seal the house and add insulation, but does not seal the ducts and makes the envelope larger - more square feet. The 2012 code (for new houses - but a good guideline) now specifically says that you can seal ducts while insulating with 2-part foam, and this is one of the very few instances that we would endorse this material. This might be another quick (although expensive) option that would allow you easier access to the open areas at the low end of the roof slopes, which I suspect are your problem areas.
Thanks Ed, typically don't have a problem with low end of roof as the run is usually in the middle of home and has gable ends. Good point about increasing the surface area of the thermal envelope, I didn't think about it that way. We are always combining this with a complete duct seal, but I suppose if we were doing a full air seal (including all top plates) I would then consider ducts in conditioned space. Often times the owner does not opt for the full air seal, but only doing sealing of the dropped ceiling junctures(due to cost). I could perform the higher price air seal and eliminate the need for a duct seal. I would not feel comfortable not sealing the ducts if I was just capping the dropped ceiling and not the top plates. Too many pathways through other walls.
This actually made me think of something else - I wonder if I am pushing leaks around by only sealing the dropped ceiling junctures. I am getting decent CFM drops, but maybe much of what I am capturing is being lost through wall cavities and out the tops of interior walls? The reason the air seal gets much more expensive is we only do full air seals if the customer chooses to vacuum the old blown insulation (or remove batts). I have found that it is near impossible to seal all wall tops and other pen's with existing insulation in place.
How have you dealt with complicated framing surrounding the dropped area? I have just found it near impossible in many situations - or just plain old not cost effective.
Send me your private email and I will forward some pictures. The very first thing I would do is the attic work- mandatory. Top of the house is where the action is. If it meant not sealing the ducts in the soffit and covering over the top of the soffit, I would do that instead of sealing the ducts and leaving the soffit undone. Yes there are many air paths through the house and we will even open the siding on the outside of a townhouse to seal the party wall vertically, but the top is where the action is. Did I mention that the top is where the action is? We have air sealed perhaps 12-15,000 attics since 1981 and have NEVER taken insulation out of an attic voluntarily (maybe 3-5 times, but only if the owner insists and is willing to pay a LOT extra for it - usually involves mold or bat guano and we have a guy that we hire to do it). If it is blown, use a small leaf rake - 12" across - to sweep it aside and then back after air sealing. If it is batts, just lift the part over the problem and fold it under itself to expose, seal, then put the batt back. Use your leaf rake to push the batts down for good contact with the ceiling below, then blow a bit over the top to even out and fill the voids. A foam gun with a 20-30" barrel helps. Remember, no matter how bad the insulation is, it is doing more good in that attic than at the landfill.