Best practices – fiberglass insulation
We are having some rolled fiberglass insulation installed in old work (2×4 studs) in what was once an unfinished attic. I know a Mooney wall would be the best option, but that is not an option for us now.
My question is, is it worthwhile to put a small bead of ‘Great Stuff’ foam insulation along the edges of each bay (where the 2X4 meets the sheathing) before the fiberglass insulation goes in? On the other side of the sheathing is aluminum siding. From what I’ve heard air gaps are kryptonite to fiberglass. If you can seal the air gaps, it’s not a bad insulation. The trouble is in the installation.
Replies
Go to the DIY Spray Foam Thread. Look at my post #10. That would solve your problems and you can do it yourself. Not rocket science.
Install the fiberglass right ... or switch to some type of blown cellulose (wet or dry).
Fiberglass ... cut like drywall only opposite +1/4" minus nothing. Tuck into corners of stud bay and then fluff back out flush with the interior finish. Fill ALL cavities and take care during the installation to DO A GOOD JOB.
A guy showed me a demo of fiberglass vs. cellulose where they blew air through both to show that fiberglass didn't resist air flow. Personally, I say bunk to an extent. Most wall cavities are never subject to direct air flow dynamics like the demo I saw ... they are enclosed spaces. While they have some cracks and openings that certainly allow some degree of free air flow, they are far from the demo that forced air through the entire one end of a cavity and allowed the air to flow freely out the other end.
Real construction generally provides a good enclosure for fiberglass batts. As such, they should provide the tested R-value per the manufacturer and be comparable w/ e.g. cellulose. Yes, there is some degree of inefficiency associated with fiberglass allowing more free air flow than e.g. cellulose or foams. No argument there ... it is just that most normal applications provide a good enclosed cavity for fiberglass. Properly installed, it will perform well (IMO).
If you have particular problem areas (e.g. penetrations or other gaps that allow air leakage to occur ... you should treat those w/ some special attention ... but to arbitrarily declare every linear foot of stud/framing as a 'problem', I strongly disagree. Also ... foaming the edge of all framing may tend to prevent a good full/snug fit of fiberglass into corners/or otherwise fill the entire cavity.
Gaps in insulation in a framing cavity are generally the enemy of insulation. Tuck and fluff is the key w/ fiberglass ... cut it well and install it properly like the manufacturers intended it to be done ... and you will get reasonable performance.
>>If you have particular problem areas (e.g. penetrations or other gaps that allow air leakage to occur ... you should treat those w/ some special attention ... but to arbitrarily declare every linear foot of stud/framing as a 'problem', I strongly disagree. Also ... foaming the edge of all framing may tend to prevent a good full/snug fit of fiberglass into corners/or otherwise fill the entire cavity.
>>Gaps in insulation in a framing cavity are generally the enemy of insulation. Tuck and fluff is the key w/ fiberglass ... cut it well and install it properly like the manufacturers intended it to be done ... and you will get reasonable performance
I agree with you. F/G can be done correctly and will provide reasonable performance for the cost. I have used it for years on many commercial and residential projects. I do use spray foam for sealing large penetrations and openings, but when I am unsure of the exterior wall wrap, I usea butly or urethane caulk at gaps between studs, plates and sheathing.
Most f/g insulation contractors in this area do not give that quality of workmanship on thier installs, so the product performance suffers. With that in mind, I now use blown cellulose for ceilings and wet spray cellulose for walls when I need to contract the prep and istallation onf insulation.
In our attic, we have three different types of walls.
1) Old work walls at the gable ends.
2) Some new work walls along the side
3) Old work walls at the gable ends that are outside (towards the eaves) of the kneewalls
For #1 - the contractor put in fiberglass insulation with kraft paper facing.
For #2 - the contractor put in unfaced fiberglass insulation, and then covered with with a poly vapor barrier
For #3 - the contractor put in unfaced FG insulation, and then used no vapor barrier.
Anyone have a clue why he did different things in those three sections? What I find very curious is the distinction in the old work walls. The area behind our knee walls is conditioned space, so I don't know why there wouldn't be a vapor barrier there.
These are just swag's.
1) he didn't know what was on the other side of the sheathing board on the gable end walls so he opted for a minimally effect vapor retarder with the craft face insulation to avoid having a double vp situation arise.
2) he approached thes walls as new work and placed a highly effect vp inside and under the wall board to keep interior moisture from nigrating to the attic area.
3) he had a sudden brain fart and realized he didn't need a vp between two conditioned spaces:)
When you say the attic is a conditioned space do mean it is unvented also?
Maybe a crude sketch would help put things in perspective. Just draw up a plan view, without demension, label areas, and what is new and old, as well as what/where the different insulation techniques are used. Scan it and then post it as a jpg file.
Some one here will figure it all out.
Uh ... it was the 'best' way to do it (for him, not you). He had no clue (clueless, not clewless) what he was doing? I am not clear on all your situations you describe. Are you saying the contractor did all of these on the same job/at the same time (as opposed to occuring over many years)?
If he is a good contractor ... ask him ... he will tell you. If he is not, ask him anyway ... you may easily detect BS ... and if not sure of the answer ... come back to Breaktime and get some viewpoints.
I agree with Clewless. I would only add that high density batts (R-15 vs R-13, or R-21 vs R-19 for 2x6 walls) are far superior for getting the snug fit. Few lumberyards stock it (no big boxes do) but most should be able to order it.
JUST USE A LATEX CHAULK
the book from taunton in their for pro's by pro's series on insulating and weatherizing a house goes into great detail and pretty much step by step of exactly what you want to do.
you are correct i believe with sealing the air gaps, its been a while since i read the book, but i do remember thats just what the author does, including all wiring & plumbing holes/penetrations.
the book would be a great reference to make sure you got everything covered. sorry i don't remember authors name.