Help with my half-assed attic
sulltikk
| Posted in Energy Efficiency and Durability on
GBA enabled me to GC the energy aspects of the gut-renovation of my 1960s ranch (Maine coast, zone 6a) to a Pretty Good House level- completed in 2022. I thought I could live with the attic I knew I was half-assing- it was fatigue, budget, schedule, etc. But I have a bad summer attic overheating situation (a near-constant “heat blanket” that can reach 130 many afternoons) that I believe compromises indoor comfort for the 4-5 weeks of our peak summer. I am seeking suggestions for relief.
The attic ended up a mongrel, for sure, with very little ventilation and (only) decent floor insulation. I had a contractor spray foam the long overhanging soffits to eliminate thermal bridging to the living space- too bad, it’s done, can’t bear to think of excavating any for venting. Besides, I have inadequate ridge venting from the roof contractor’s metal-bending and installation. There’s a new carpenter-built vent in each gable, about 2 x 2ft sq, but free space is reduced by screening and slanted weather baffles.
The roof has 3/4 ply deck, full-cover stick-on membrane, medium gray metal roof on horizontal sleepers (very little air movement btwn them). The roof gets direct sun all day long. No insulation currently in the 2×6 rafter bays. The attic floor got R-60 blown-in cellulose (has settled a bit since install) on top of a well-detailed poly vapor barrier. The one blower door test we did, after completion, was 1.53 ACH50. Ducts for kitchen, baths, and HRV sit on attic joists and are covered by the cellulose. I haven’t measured attic humidity at all, but I expect the good vapor barrier keeps that to a minimum year-round. Assumption??!
On warm or hot days I manage the windows (but most are awnings, so limited air change), screen doors, and two floor fans to help reduce interior temperatures and cool off somewhat overnight. But then the cycle begins again.
I’ve read what I can find on both GBA and BSC, but because it’s a mongrel, I turn to the Forum for help. Access to the attic is limited to a 2×2 hatch and the gable vent spaces via ladders. Improvement options seem to include:
– Least expensive and least intrusive, simply add to the existing attic cellulose (can easily add 5” more) and continue experiments with windows and fans in the living space
– Spray foam the 2×6 rafter bays. If so, open, closed-cell, or a combo?
– Netted loose-fill fiberglass or cellulose in the rafter bays. If so, most suitable netting type?
– Installing one or two powered gable vent fans (the air-tightness at attic floor probably makes this ‘safe’?)
– Something else? Rigid foamboard installed across the rafter faces?
– Least expensive and least intrusive, simply add to the existing attic cellulose (can easily add 5” more) and continue experiments with windows and fans in the living space
– Spray foam the 2×6 rafter bays. If so, open, closed-cell, or a combo?
– Netted loose-fill fiberglass or cellulose in the rafter bays. If so, most suitable netting type?
– Installing one or two powered gable vent fans (the air-tightness at attic floor probably makes this ‘safe’?)
– Something else? Rigid foamboard installed across the rafter faces?
In all cases except powered gable fans, should I cover the gable vents and try to seal the ridge to make it unvented? Also, I really fear a contractor puncturing the vapor barrier and not repairing it, so I’ll have to manage that.
Thanks for knowledge and viewpoints!
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Replies
It sounds like you "had" a vented attic, but spray foamed the eaves and that blocked off the soffit vents. That's a problem, since it severely compromises that attic venting -- even if you still have a ridge vent. The attic can act to trap heat when it's not vented, but the primary purpose of attic ventilation is to remove excess moisture, not to keep the attic cool inside. In practice though, attic ventilation does help to keep the attic temperature a little lower, but it will still be hot.
R60 cellulose on the attic floor is pretty good. I doubt you'd notice much improvement going up from that, and at R60 you're already at, or even a bit over, the max allowable weight for the underlying drywall to support (I'm assuming you have a drywall ceiling under those attic floor joists that is supporting all that cellulose). I'd be reluctant to add significantly more cellulose over the entirety of the attic floor, but adding more selectively over the "above joist" ductwork may help. I'd be especially concerned with the HRV ducting since those units often have very long run cycles, or run continuously. You want all those ducts well air sealed too for both the HRV and the regular HVAC system.
If you already have a ridge vent, I'd consider adding some roof vents low down on the roof, out near the eaves but within the attic from the perspective of the spray foam you did out at the eaves. This would give the attic some intake area, and the ridge vent would then exhaust hot air. I see NO REASON to consider a power attic ventillator hear at all -- passive should be fine IF you have BOTH intake (usually soffit vents, but low roof vents in your case) AND exhaust (ridge vent) vents. It would be relatively easy to add a few roof vents down low, and I'd put them at the outer corners of the attic (if you look at the attic from above as a square, with the peak vertical in the middle of that square, I'd put roof vents out near each corner and the ridge vent would go right down the ridge in the middle of the square). That's a starting point to test if you see any improvement from increased ventilation, and it can be done entirely from the roof with no need to go into the attic.
Note that you can use the inexpensive Acurite outdoor temperature and humidity sensor to monitor your attic's temperature and humidity remotely. I'd put one of those in the attic BEFORE making changes so that you can see if things improve AFTER you make changes. Be sure to use lithium batteries in the attic sensor (I use the Energizer Ultimates for this sort of thing), because the extreme temperatures in the attic tend to make most other batteries die pretty quickly. You can get the sensor and remote display for about $20 on Amazon here: https://www.amazon.com/AcuRite-00424CA-Digital-Thermometer-Temperature/dp/B00WXIR94M/ref=sr_1_2_sspa?crid=UXVP4WBS0X29&dib=eyJ2IjoiMSJ9.FwlJayHxhgceG8x-3dVSl5v5GCiHEeCMrJY7mWA0Ft57FwtRpCs_Yr7ZNGxJw2ZtertJminCBAVsO_p183XCn_Bj-wq9LWh9pmK3bxHoJAnjeHswCDd3dlqttphh6xsRo7yzVZKheF1I2dcx-awMttn5cq1Cf31YnpCczvSOixOWtgWHoNH6Aj8I1U7Y-XvvPSzpGemOmbAQCP0OTvt2fOk_i-LTPBrslOPmjfdSmTf9RAlBCu9HlIv1tvYgU0Q2nz_KdUc_UJJVbu8kFyVS5ut1XEEtJEeVz9YAXa1Zntw.wQaad--cu1HsXc11ucjKVHFS7nKIczLuc29ijQYbkM8&dib_tag=se&keywords=acurite%2Boutdoor%2Bsensor&qid=1744393642&sprefix=acurite%2Boutdoor%2Caps%2C187&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1
All that said, all the insulation you have on the attic floor should shield you from most of that attic heat. I would check for air leaks and poor insulation in other areas of your home before putting a big investment into attic work, aside from adding the basic venting I mentioned above (which will help ensure you don't have moisture problems in the attic). Blower door tests with a smoke pen can help pinpoint other air leaks, for example. An IR camera, or even a simple IR thermometer, can help you to spot poorly insulated areas of things like walls.
Bill
All that fun stuff said,
Bill, thanks much. Can you describe or share a link for the type of roof vents you’re suggesting? The turbine style?? Also, I consider the metal roof a real challenge to penetrate well and neatly- any experience on that one? Appreciated!
There is no need for a power vent, or even a wind-driven vent like a turbine style. Also, those turbine vents pull air OUT, you want something down low on the roof that will let air IN. You want a basic passive vent like this: https://www.homedepot.com/p/Master-Flow-60-sq-in-NFA-Aluminum-Slant-Back-Roof-Louver-Static-Vent-in-Black-SSB960ABL/100041904
Up on the ridge, you should use a regular ridge vent. Never use the "shingle over foam" style, those don't work well, you want something rigid like this: https://www.homedepot.com/p/Air-Vent-ShingleVent-II-1-4-in-x-14-in-Static-Vent-in-Black-Sold-in-Carton-of-10-4-ft-Pieces-Only-SHFVBL/308306134
Bill
Got it, have seen many of these around. Now let’s see about installing them well on a metal roof. Your comment about the added weight of more cellulose was a good caution. Thx.
Metal roof installations are more complex for these. There are some other members on GBA with experience with metal roofs that can give you advice on installing vents on that material.
Bill
METAL ROOF WORK- gwhat do I need to know about selecting, locating, cutting for, and sealing retrofitted passive vents low in my standing-seam roof? What to look for in selecting a qualified contractor? Thanks.
Can you post a sketch of your attic cross section and mark out where spray foam, fluffy is.
Venting has very little effect on attic temperature. A hot 2nd floor is almost always from air leaks since stack pressure works backwards in the summer and air leaks enter through the top of the house. That air usually comes from the attic so it will be pretty hot.
Thanks for reply. I doubt, or want to believe, that I’m not getting much air leakage from the attic in summer given blower door result. An obvious (to me, ha!) problem in main living floor is the awning windows’ limited ability to vent much during summer weather. I never had them before, love them 7 mos a year, but not the best in fair summer weather. So I manage all windows and doors plus Vornado circulator fans to bring in some cool outdoor air.
What was your blower test number? From that you can calculate roughly how much leakage you are looking at.
The way I see it you like most people are thinking about your attic in the wrong way.
It seems every one gets upset when the attic gets hot in the summer. If it actually got to 130°, I am skeptical. In Maine you might get 130° for 3 hours on 10 or so days a year. Let’s say it is 80° indoors that give you a differential of 50° for 30 hours a year. In the winter you see a 50° differential when it is 70° inside and 20° outdoors that is 5 months of the year 24 -7. That’s 25,000 hours.
Venting an attic is not about equaling the summer temp differential it is about lowering the winter moisture levels in the attic. The amount of air we move is way to small to affect the summer temp much but is enough to make a huge change in the dewpoint of the attic air in the winter.
If you do have an R60 ceiling adding more will not make economic sense as very little heat will be flowing thru that much insulation. One would need to install another R60 for a total of R120 to make a measurable improvement by adding insulation.
My guess is in order to keep the roof withing 10° of the outdoor temps you would need to replace the air in the attic every minute or so. Moving air that fast without huge fans seems impossible and the fans would consume way more energy than you would save.
The current unvented unconditioned attic in your zone is a code violation and very risky. Mold and rot are a real possibility.
Walta
Walta! I have a cousin Roga! Thx for reply. My medium dark metal roof plus all-day direct sun plus poor ventilation heats up that attic. I put a remote Accurite up there last summah and saw those numbahs. (yeah, I’m OFD, St Peter’s parish.)
You raise the winter humidity question, which I haven’t thought about much. IF my attic floor poly air seal remains intact, what would be significant- I.e., dangerous- winter humidity sources? Thanks.
In my opinion what you have I call the free lunch plan some people do win when they play this game but predicting the winners in advance is impossible. I think the odds of winning go up if you live someplace that is hot and dry and down someplace cold and wet.
Yes a perfect air and moisture barriers would help but almost nothing is ever perfect and things that are perfect can only get worse over time.
Does your Accurite track the dew point?
Walta
You can get graphs over time including the dew point if you use the OUTDOOR temperature/humidity monitor, and you set it to talk to their web service (which requires an additional hardware device). The monitoring service is free, but the hardware device that sends the data to them used to be over $100 and doesn't appear to be available anymore (I was just looking for a current price and couldn't find any for sale).
Bill
Akos, per your Sat night question, the blower door test for the main living floor was 1.79 for the 9400 cf of space. (Full-house result was 1.53 for 15,600 cf, adds the lower level, 2/3s of which is below grade and has fewer windows and doors).
Your summer stack-effect note helps my available options. Only weak link I think of in main flr space could be the attic hatch (in the closet of a spare BR), it was carefully built and detailed, but I can do more.
My 10 lo-profile LED ceiling lights sit below the attic vapor barrier, and I sealed the 4 HRV duct heads and the K and bath vent fans to it carefully.
Should I turn off the HRV on hot days to prevent more hot exterior air coming in? Open windows and doors seem to make that OK.
I currently can’t or don’t exhaust enough interior air on the hotter, more-still days, a contributor to the discomfort; the mostly awning windows limit that. I also have to remind myself that when it gets to or above 80 (maybe 15 days per summer, but a few more every year) , it’s going to get warm inside w/o AC, no matter my door/window/shades routine.
Have considered adding a mini-split for late-day and sleeping comfort. Am now looking into passive vents low on roof as Bill W suggested, though installing and sealing those well through the metal roof and sheathing is worrisome.
Attic hatches are notorious leaky spots. The usual way to deal with those is to make the hatch cover out of something relatively heavy and flat (I used MDF in my own home), then glue some XPS panels to the back of the vent (polyiso and EPS work too, but EPS can be more crumbly, so less durable, if you use lower density variants). Use weather stripping around the perimeter of the hatch, and use draw latches for closure so that the hatch is drawn tightly against the weather stripping to ensure a good seal.
Any lights you have in the attic floor for the level below need to be well air sealed. Those are another classic spot for air leaks. Those lights might also have relatively thin layers of insulation above them in the attic, so they might become "hot spots" when the attic temperature gets high in the summer. The usual way to deal with those lights is to "box over" the light in the attic with rigid foam, then blow in your loose fill insulation over the box. The box is there to provide an air barrier, but the loose fill insulation over the top of the box provides most of the R value. Note that if you have a vapor barrier like poly in good shape, that can double as the air barrier here. The trick with poly is to make sure there are no holes in it if you want it to function as your air barrier in addition to being a vapor barrier.
Bill
Thanks Bill. I think I have both of those pretty well addressed. The hatch box was done well and has a v hvy insulated top. I have 2 sm improvement in mind for it, tho. The LEDs are the new, v lo-profile ones that install under the air barrier.
Perhaps the community can weigh in on whether to stop the HRV on hot days, and ideas about improving on my interior ventilation practices.
1.53 Ach for an old house is not half assed, that is as fully assed as it gets! Good job.
So put some numbers to this, assume an N factor of 25.
1.53ACH is about 400CFM at 50PA, so stack leaks is about 400/25=16CFM. All that air will enter near the top, so you are brining in 140F attic air into a 70F house so about 1000BTU. So that is not much.
I think the bigger issue is unlike a house with a flat ceiling, any attic room has all 3 sides exposed to the attic so you get a much larger effective ceiling area. Assuming the walls are 2x4 so around r10 with say 500sqft of knee walls and about the same in sloped ceiling/flat ceiling you gain about
(140-70)*500/R10=3500BTU through the walls. Probably about the same through the sloped/flat ceiling.
That does up to some real heat and I can't see you loosing it without an AC unit especially in a climate with a lot of humidity. Won't work with passive ventilation even with night time cooling since over the day the place will bake by the time the sun sets.
The good news is that it is still on the low side so a 3/4 ton minisplit (or one of the inverter window units) will easily do the job.
Getting the attic space cooler will help but you won't get it cool enough though regular venting. Power vents create other issues and best not used.
Hugely helpful, espec the physics, much appreciated. Hope you don’t mind a couple of further questions. You’re right, I forget to include the contribution of the other attic surfaces. Each gable wall- 2x4, new Zip sheathing and white cedar shingles, but uninsulated just like the rafter bays- get AM or PM sun, adding to heating. BTW, house dimensions are 44’ x 26’, roof pitch is moderate (dunno its ratio).
If it’s true the gable vents aren’t helping much w/o soffit venting, should I seal up the gable vents (2 @ 4sf ea) to limit some hot air intake?
And, an early suggestion to my post was passive vents installed low on the roof corners (prob also a few along edges?) to help the so-so ridge vent (not done well by roof contractor) contribute more. Besides finding an exc shop to do the retrofit install well, will that help attic venting much?
I’d consider repainting the exc condition med-gray roof panels w/a max reflective paint and color if that would help. Any thought there? Thanks again, and also for the AC comments.
Venting is there it move moisture out of the attic space. Your gable vents are needed for that purpose and you can't remove it. I've been up in properly vented attics in the summer time and it is still unbearably hot, so venting won't help reduce the attic temps.
Simple things you can do is install some batts or staple perforated radiant foil on the underside of the rafters.
Painting the roof will help in the summer but you'll end up loosing the free solar heat in the winter. In heating dominated climates this ends up being a net energy loss over the year.
With summer cooling season getting longer each year, I would bite the bullet and get the AC unit. If the unit is installed in a hallway upstairs, you can upsize it and it will cool the whole house including downstairs as long as the stairway is open to the space bellow.
Thank you again! AC investigation begins…..