Does interior ceiling strapping defeat the effectiveness of fiberglass insulation?
I'm a remodeling contractor and am about to do another basement remodel. I will be insulating the walls with 2" of XPS, framing stud walls in front of the rigid foam, and then attaching drywall to the studwall. The underside of the slab is insulated, so I won't be laying foam and plywood on the floor of this project.
But, in order to leave the fiberglass batts in the ceiling joists undisturbed, and to make life easier for pulling new wiring, I was planning to install 1x3 furring strips to the underside of the joists (perpendicular, of course). But knowing that air movement can defeat the effectiveness of fiberglass insulation, I'm curious if the 3/4" dead space left between the drywall and ceiling joists/fiberglass batts will create a problem with convective loops?
Are convective loops only a concern on vertical setups (walls), or do I need to consider them a risk in ceiling applications as well?
Asked by Anonymous
Posted Fri, 03/05/2010 - 10:31
Edited Tue, 03/09/2010 - 07:17
Posted Fri, 03/05/2010 - 10:31
Edited Tue, 03/09/2010 - 07:17
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I don't understand the connection between basement walls and insulated ceiling joists.
Posted Fri, 03/05/2010 - 19:03
You answered your own question.
Because it will be a dead air space, it will add insulative value (about R-1) rather than create a problem
Natural convection requires two things: a vertical channel and a delta-T. The air on the warm side rises and the air on the cool side falls, creating a convective loop.
Not only is there no vertical channel for natural convection, there should also be no delta-T, since both sides of the air space are conditioned.
However, if there is any potential for exterior air to communicate with that air gap, then there can forced convection from wind pressure. Or, if there are any forced-air ducts in the ceiling with poorly sealed joints, there can be mechanically -forced convection.
Posted Fri, 03/05/2010 - 19:21
Justin,
Your question is a good one, and one that has been debated for years. Many experts recommend that the interior air barrier (usually drywall) should be in direct contact with the insulation layer. Such a recommendation precludes strapped ceilings.
In your particular case, I'm confused about why you care about the performance of insulation installed between an insulated, conditioned basement and the conditioned space upstairs. If both sides are conditioned, then the insulation is irrelevant.
As Robert pointed out, there will be no problem with a strapped ceiling if the air space is truly dead. Problems arise, however, because such spaces are rarely dead. If you were strapping the top-floor ceiling under a cold attic, it wouldn't be unusual for the perimeter of the air space to suffer air leakage, undermining the thermal performance of the ceiling.
I don't like to see ceiling strapping under fiberglass batts. I have no problem with ceiling strapping under rigid foam, however — especially if the foam has taped seams. Rigid foam provides a great air barrier to seal the underside of the insulated joist bays.
Posted Sat, 03/06/2010 - 05:17
I should have been more clear in my question. I'm not actually concerned about the thermal properties of the FG batts in the ceiling in THIS case, but using the strapping got my mind spinning about the potential problems in OTHER situations (ie: a bedroom ceiling) because I know that many builders strap ceilings as a rule of thumb.
I appreciate the answers, and they confirmed what I had suspected: that convective loops aren't a worry in horizontal situations.
Posted Mon, 03/08/2010 - 09:30
Justin, an energy auditor we've worked with (http://www.wesriley.com/) has done something like 4,000 home energy audits. He says that he almost always sees major air leaks when even large amount of cellulose are dumped over batt insulation in attics.
He always recommends pulling out the existing batts and air sealing the ceiling with spray foam before installing new cellulose. He has certified over 900 Energy Star homes and said that he won't certify them if they have batts over strapping in the ceilings because they always present excessive air leakage.
Posted Mon, 03/08/2010 - 18:15