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ld_n_Tex
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Posts:2
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| 06/30/2009 1:41 AM |
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Greetings and thanks in advanced for well thought out advice based by sound principles to back it up.
I have recently dug into a Cooling issue with the second floor of my home here in the Austin TX area. It is about 4 years old and is suffering from an extreme heat gain during the summer months to the point that the second floor cooling unit is unable to cool past 81 or so degrees peak hours, this is with the first floor set to and maintaining 75. Because of ROI I am going to stick to a plan of things that are DIY to attack this, instead of the, “if I could start all over with someone else’s money approach”.
I have checked the splits on the a/c unit and it appears to be in good order, it was checked last year by a tech also (because of the same issue). I have inspected three walk-in attics that are off the second floor and the attic ladder access for several possible problems: leaky ducts (no), etc. What I have found: Knee walls with R15 faced batts and reflective half inch Tuff-R blue board. None of the seams are sealed at all, the board is pulled away from the studs a half inch or more at the top on one long wall due to not being fastened, cuts for framing joints are left open to the stuffed batt inside. In some thicker spaces there is a wide gap between the board and the framing, then a very deep air gap between the blue board and the batt in the cavity sometimes 3-5 inches. I was going to seal all of this blue board by: appropriately fastening it, applying reflective tape to the joints, and then filing the air gaps at the edges with foam to seal it and also seal as many perforations between the first and second floors while I was at it. There are also areas around the attic door frames that I can look past the frame from the exterior attic space and see the interior trim all the way around the door frame, seal them and place several layer of blue board on their pretty exteriors. The sloped ceiling issues: the batts are clogging the space between the rafters completely; there are 2x6 on one side and 2x8 on the other. On the sloped ceilings on top of the knee walls the batts fill the space to the roof deck, upon inspection I have found that most are not in contact with the many have an air gap of <2in. this is very visible from the upper attic area down. Crawling the length of one space, I also then found no blown insulation on top spaces in this area of the home and one closet that is adjacent to the walk-in attic with no insulation on top of it at all, of course the framing makes it impossible to reach. What I would like to do is prior to applying any more blown insulation is prep correctly: fix the batts to have an air gap between the decking, I think I may have a way to also get a radiant barrier above the batts, is this a good idea? Get the faced side of the batts in contact with the drywall. Then, since it is relatively cheap, add more blown insulation to properly cover the areas needing it. The second floor cools down immediately after sunset, so I think if I can get something to help it at peak (rb), seal up the things I can reach (which appear to be many), and fix the fairly poor installation practices I might have a shot at reasonable comfort? All the walk-in attics have soffit vent strips with at least one mushroom vent plus if the rafters are unclogged this will further vent them, so would this with radiant barrier in the rafters be a good fit, I have even considered the kind you staple between the rafters that has a layer of insulation sandwiched between two foil layers to try to get some additional performance, opinions are appreciated on these products. |
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Dana1
Registered Users
Advanced Member
Posts:702
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| 06/30/2009 5:33 PM |
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A bunch of issues (wish they were broken out by more white, space, not one run-on paragraph but I think I get it.)
1: Kneewalls have R15 batts + R3 foil-faced XPS board, but it's not all in good condition, lots of gaps, etc. The attic floors behind the kneewalls have an unspecified depth of insulation in some areas, but none in others. One closet next to a kneewall area has no insulation above it.
2: Cathedral ceilings have R19-ish batts in both 2x6 and 2x8 raftered sections, no reflective insulation.
Fixing the gaps is the right thing to do, and making as air tight as possible is right too. At your insulation levels it may pay to add radiant barrier to the rafters behind the kneewalls:
http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html#table4
(the number in the box is the max price you should pay for RB to be cost effective in your climate & insulation levels.)
But adding more fiber/other insulation to raise the R-value might be a better investment:
http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html#table6
As long as you provide space between the insulation and the roof deck to maintain air-flow from your soffits to the ridge for removing moisture and make the rafters air-tight to the inside, having the total R-value split between the rafters and kneewall/floor isn't a problem- it'll be semi-conditioned space, but the total R-value will be additive. (Don't bother with radiant barrier between the roof deck & fiber if the total R-value rafters + kneewall will add up to R30 or more.) You'll have to seal up your mushroom vent to max out the advantage here though. The vents don't really cool the roof deck/attic space, but they DO provide a path for moisture to escape. If you're feeling rich you can foam-insulate the entire roof deck from the inside, sealing all vents (and this works just fine, traditional methods notwithstanding.)
On the cathedral ceiling where the rafters are 2x8" with a 2" gap between insulation & roof deck, if you thing you can get radiant barrier in there easily it'll probably be worth it. On the side with the 2x6" rafters, forget about it- it is what it is. Radiant barrier there would itself get hot and conduct the heat through the insulation, for extremely minor benefit, if any. Disturbing the batting would probably create voids & gaps that worsen the problem. (Compressing the insulation there to created better venting flow reduces the R-value too. It'll still be about R3.5/inch when compressed, but it'll be fewer inches.) For radiant barrier to be effective at all you'd need at least a half-inch gap. Don't sweat the venting issue on those pitches either- air migrates all too well through fiberglass batting.
On the closet with no insulation above & inaccessible framing, driling 1-1/4" holes through the ceiling and blowing in cellulose or fiberglass is possible. If you're renting a blower for doing open-blow on the floors, see if they have a nozzle (or fabricate one) to neck down the 2.5-3" hose to 1".
On the open-blow attic floors, filling it at least 2-3" over the joist tops provides a good thermal-break. The framing joists are only ~R1/inch, whereas the blown insulation is R2.5-R4 (depending on what it is.) If you use blown fiberglass without also insulating the rafters with something (radiant barrier or "real" insulation), the blow-over needs to be a bit deeper, since it passes some radiant heat which gets absorbed inside the insulation itself. (Cellulose & rock wool have somewhat better thin-layer characteristics, and radiant-heats only at the surface, not from the inside out. In layers deeper than 8" the performance difference is somewhat academic though.)
For a graphic representation of the wierdness with 5.5" of fiberglass & radiated heat, see: http://www.fsec.ucf.edu/en/publications/pdf/FSEC-DN-6-86.pdf
Note figure 3: Without radiant barrier the temp of the insulation 1" below the top is higher than the attic air temp(!). With radiant barrier in place the temp 1" below the insulation is well under the attic ambient (which is why it's worth it on your cathedral-ceiling with the 2x8 framing.)
Basically it boils down to this: Add as much insulation as is reasonable where you have the space, only use RB where it's easy and there's not enough room to get at least another R6 easily. Then make it as air-tight as possible. (A blower-door test will surely find infiltration paths you'd never considered- always does. Some might be big enough to really count too.)
Doubling the average R value cuts the heat flux in half. The fact that you have some areas & big gaps that have NO insulation means that those are going to dominate the whole heat-gain equation until dealt with. (The roof decking + interior finish probably add up to a whopping R1.5-R2.) If you could get the whole place up to R36-40-ish or at least get the average to something over R30 you'll likely see a HUGE difference.
Of course this all assumes you don't have a huge south-facing window with no awnings, shutters, or shades acting as a big solar-collector for you. :-) It also assumes that your second floor walls (especially the S side) don't have big gaps in the insulation. You may be able to pinpoint insulation gaps with an infra-red thermometer, big gaps can be found just by feeling the wall with your hand. Gaps around window framing and any non-standard stud spacings are common, as well as around electrical wiring & outlet/switch boxes. What to do about those gaps depends...
Now supposing you had a friend with an infra-red camera...
(Let me know if that's the case, but I'm gonna go out on a limb and assume it's not. :-) )
If your duct joints aren't sealed with mastic (or FSK tape in good condition) they may be leakier than you think- even if you can't feel air moving. 15-25% leakage is typical for pretty-good but not goop-sealed ducts. $50 in mastic goes quite a way, and is as permanent as it gets in the duct-world. Where accessible it's worthwhile to hit it, but it's pretty clear your insulation issues are likely to be the largest factor.
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