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Community and Q&A

Attic ventilation problem

brunojw | Posted in General Questions on

I am looking for some advice from someone who really understands attic ventilation. I’ve met with several roofing contractors and have received various suggestions on what to do. Having been through the discussions with the contractors I think I have most of the facts which I will provide below. Any help you can provide is appreciated.

House background and location.
1. House built in 1984 in the Philadelphia area
2. We are the original home owners
3. It is a 2 story colonial which faces south
4. The attic area is approximately 900 sq./Ft.
5. The original attic ventilation of the house consisted of (2) 18″x24″ gable vents on east and west sides of the house. The gable vents were midway between the roof peak and the floor of the attic. There were a few soffit intake vents (4″ x 16″) that were on the front and back of the house. Based on the net free air flow of the output gables (141 per gable = 2.0 NFA) and (12 soffit vents @ 26 per vent = 2.2 NFA) it looks like it was balanced and had sufficient total NFA.
6. The house sits on a hill and is open so there is normally a breeze on any given day.
7. There is one birch tree that shades the front of the house, but basically the house sits in the open on 1 acre of ground – not a crowded development scenario.

Change #1 to the original attic ventilation
1. 8 years ago we had new shingles installed. The old shingles were removed. The roofer said that in order to have the roof under warranty he needed to add a roof ridge vent. An opening was cut on the top of the roof to address this. There is a continuous opening 1/2″ on each side of the main roof rafter (total 1″ ridge opening)
2. There is a mesh rubber/plastic material to keep out dirt and bugs
3. At this point we did not detect any change to indoor temperatures in the house, particularly on the second floor. In the attic you could detect air flowing (not a formal test just an observation).
4. My estimate of this change in output NFA was that it added at least 2.5 NFA to the output. This would then calculate to a total of 4.5 NFA output and 2.2 NFA input. The total 6.7 NFA is well above the required venting for a 900 Sq./ft. area, but the balance changed from roughly 50/50 to 30/70. For 8 years we did not notice any change to the 2nd floor temperatures.

Change # 2 to the attic ventilation
1. 2 years ago we had the siding replaced on the house. The original siding was non insulated aluminum and the replacement siding was insulated vinyl.
2. The contractor recommended that we close off the gable vents since we had the roof vent and said he would add additional venting in the soffits to create more air intake (no statistical facts or calculations done that I know of to determine what the intake requirements needed to be)
3. The contractor drilled 2″ holes between each of the rafters and then installed a vinyl soffit cap across the soffit area that has little perforated holes in it. The work was completed in late spring. There were 57 holes drilled and when you calculate the total volume of openings created it was 1.24 NFA (1” x 1” x 3.14 x 57) – you half the size of the hole and multiply x pie.
4. However I found out recently that the net free area of the perforated siding was only 10 sq. inches per square ft. What this effectively did was block all the input ventilation. I believe I now have only .24 NFA in total. (original from vents was 2.2 NFA, added 57 holes = 1.24 for a total of 3.44). I reduced this by the NFA restriction from the perforated soffit siding (10 sq. inches NFA per sq. ft. = 10/144 = 7%) – So 3.44 x 7% = .24 NFA.

What happened next?
1. During the first summer we noticed that the upstairs of the house was significantly hotter in the summer than usual. I got suspicious that closing the gable vents cut off circulation to the attic causing the attic to be much hotter.
2. I installed a remote temperature and humidity sensor in the attic to begin to measure what was happening there. Here are some of my observations:
a) On a 90 degree day the attic temperature ranges from 140-150 degrees. The humidity levels are less than 20%
b) Once the sun sets the temperature in the attic reduces to almost the temperature of the outside, but this takes several hours after dark to occur
c) On a cloudy day the attic temperature is less than 20 degrees higher than the outside temperature
d) On a cold winter day (32 degrees or less outside temp) the attic temperature is very close to the outside temperature
e) The humidity levels in the attic when the outside temperatures are between 40 – 80 degrees range from 65 to 75%
f) If it rains for a few days the attic humidity levels are in the 80-90% range for days after the rain has left and the outside humidity levels have lowered to 60% or less
g) On one fall day where the winds were 20-30mph and the outdoor humidity levels was less than 30% the attic humidity was in the 60-65% range
h) The humidity level in our house on a winter day is in the low 30% range – this appears to be normal
I) The humidity in the basement on a winter day is in the high 30’s range – this appears to be normal
J) For example right now the outdoor temp/humidity level is 40 degrees/56% humidity. The basement temp/humidity is 60 degrees/38% humidity. The attic is 56 degrees/69% humidity. The house temp/humidity is 70 degrees/34% humidity.

Other points to note:
1. The plywood sheathing facing the south looks pretty new – some discoloration.
2. The plywood sheathing facing the north side of the house has some mold near the soffits mostly on the northeast side of the roof which gets the most shade. The nails are rusty on the north side.
3. Before I started to look into the air circulation options I read that you should try to solve any air leakage issues on the 2nd floor ceiling. So I spend a week applying spray foam to seal all electrical wire holes, areas around plumbing vent pipes, 2nd floor light fixtures, spaces where drywalls butt up against each other and I also enclosed (2) recessed lighting fixtures in insulation board and applied spray foam to seal it. We have a chimney, but I did not find any areas that may be compromised since the chimney is on the outside of the house. I also redid the entry hatch to the attic to insulate above the hatch and placed a gasket around the hatch to make a tight seal when closed. The results of this effort did not show a material change in the before and after humidity readings.
4. I made sure the insulation was pulled back and not blocking the soffit intake holes. I can see daylight where the holes exist.
5. I’ve received several proposals on what to do ranging from redoing the ridge vent to increase the cut from 1/2″ to 3/4 inch on both sides of the main roof rafter to cutting a 3/4″ strip across the bottom of the roof to install a Airvent Edge-Eave intake vent to ‘balance the ridge vent”. I have a proposal to cut a 2″ slot across the front and back soffits and reinstall the perforated vinyl siding (which to my calculations does essentially nothing to add intake to the soffits). And everything in between. These proposals range around $3,500.

1. Do the calculations I’ve provided make sense?
2. It seems to me that the perforated vinyl soffit covers that GAF and CertainTeed sell are worthless, other than cosmetic beauty, and are an attic ventilation problem waiting to happen since the NFA is unbelievably low.
3. Has anyone had any experience with the Air Vent Edge-Vent system? Is it a good solution?
4. I’ve come to the conclusion the entire problem is the intake? Would you agree?
5. If I install the Air Vent solution that would provide 4.5 NFA in both the intake and output for a total of 9.0 NFA. I seems balanced and would provide much more airflow that required (which I think is a good thing). Is this thinking valid?
6. If I remove the entire soffit and replace the vinyl soffit siding then I get 1.7 NFA in the soffit intake (36’ x 4” opening = 1728 sq. inches. Then add other side of house for a total of 3,456 sq. inches or 24 sq. ft. Multiple by 7% NFA of soffit cover = 1.7 NFA. This solution would have 1.7 NFA input and 2.5 NFA output which would not be balanced. If I block part of the ridge vent output to equal the input this would leave a total of 3.4 NFA. This would meet the 3.0 minimum NFA for a 900 sq. ft. attic. However, getting a contractors interest on this project given the amount of labor involved to remove the capping, siding, gutters, soffit and then reinstalling everything would be tough. This assumes there is enough space to attach the J channel for the siding.
7. I don’t think adding a power exhaust vent would help because there is not enough air input. Is this thinking correct?
8. I am not thinking about removing the perforated soffit since there are now (57) 2” holes there and adding a continuous soffit cover would not match the color which would have to be painted.
9. Does anyone have any other solutions that may work?

Any help would be greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Long post. Can you please tell me what your problem is? (You didn't explain your problem clearly.)

    I'm guessing that you have two problems:

    1. You top floor (under the attic) gets hot in the summer.

    2. There are a few signs of mold on the roof sheathing on the north side of your house.

    Is this correct?

    My advice is to stop obsessing about attic ventilation and vent calculations. To learn why I don't think that venting matters much, see this article: All About Attic Venting.

    Problem #1 is probably caused by an insufficient thickness of insulation on your attic floor. [Later edit: see Comment #6, in which I change my opinion based on the information that the attic floor is insulated to R-30.]

    Problem #2 is probably caused by air leaks at your ceiling plane (your attic floor).

  2. iLikeDirt | | #2

    But… but, the temperature of your attic is irrelevant if you have lots of insulation, right? Right!? ;-) And gosh, your attic is well air-sealed and everything. Next you'll tell us that it's well-insulated too and by golly that's gonna be a fun conversation to have with the people who believe that air sealing and insulation are the only hammers that exist in the toolbox of someone trying to fix attic problems. Just a warning, you're entering touchy territory. That a is a significant faction here that believes that attic venting is all but irrelevant. I hope these people take the opportunity to listen to you and open their minds to the possibility that they may be wrong instead of insulting you and trying to convince you that you're crazy.

  3. brunojw | | #3

    The problems/concerns in my mind are the humidity level in the attic in the non summer months (main concern) and the mold that I observed on the north side of the house. The second concern is the higher temperature on the 2nd floor after the second change (soffit perforated siding) to the ventilation design. The higher temperature on the 2nd floor was not an issue before the 2nd change. It heat upstairs was VERY noticeable immediately after the change.

    I read the article that Martin suggested (which I read before my post) and one thing I've found out talking to various contractors is that everyone has their own, and often different opinion, on what the problem is (if any).

    I actually spent time sealing the air leaks in the ceiling as I mentioned earlier after I read the article that Martin suggested. I am sure I did not do as good a job as a professional, but I think I did a pretty good job. I did not notice any improvement to the lower humidity levels in the attic. I also noted that the humidity in both the living area of the house and basement are in the 30% range while the attic area is in the high 60's and into the 80's sometimes. Even when the outside humidity levels are very low 30's as well. This seems to me to indicate a lack of the higher humidity air in the attic to escape??

    Why I was focusing on the venting is because if the outside and inside humidity levels are in the 30's why would the attic be in the 70's? If there was better input I would have thought the attic temperature would have lowered from the 70's range. Another note, when you are in the attic on a windy day (30 MPH) you cannot feel any air movement. If I would set off a smoke bomb it may take days to clear.

    I have R-30 insulation in the attic which I am told by the insulation contractors is fine for this area.

    I know this thread was going to create multiple opinions, but I am hoping I can get some consensus on the subject.


  4. iLikeDirt | | #4

    You're going to be told to add more insulation because R-30 isn't enough. I followed this advice a year and a half ago and brought my attic up to a bit more than R-50 after having it professionally air-sealed. I gained nothing--NOTHING--in terms of summer comfort or summer cooling bills. I strongly suspect that adding another R-20 on top will give you the same non-result. By all means do it and be another data point.

  5. brunojw | | #5

    One more thought on the subject. I cannot find any stats on what the actual air replacement is in the attic using the NFA rations. So for example, if there is X cubic feet of air in the attic and the air displacement of that air through the input/output areas takes 3 hours wouldn't it stand to reason that if you did have temperature/humidity issues in the attic that the air being replaced in the attic from the outside would resemble more of the temp/humidity of the outside air being drawn in thereby "flushing out" the high temp air or high humidity air?????

  6. GBA Editor
    Martin Holladay | | #6

    First of all, I want to make it clear that I don't know exactly what's going on in your attic. Diagnosing these problems properly requires a site visit (and probably use of diagnostic equipment). All we can do is provide advice based on the information you are sharing.

    You live near Philadelphia, and you have some mold on the roof sheathing on the north side of the house. The sheathing is clearly damp. What is the source of the moisture? In almost all cases like the one you describe, the source of the moisture is indoor air. There must be a pathway that is allowing warm, moist indoor air to contact your cold roof sheathing. (Occasionally, especially in the Pacific Northwest, attic ventilation can bring enough damp outdoor air into an attic to cause mold on roof sheathing. But this mechanism for mold growth would be very unusual in Pennsylvania. That said, nothing can be ruled out. If the source of the moisture is exterior air, that would be an argument for reducing, not increasing, the rate of attic ventilation.)

    So I still think that the most likely reason that you have mold on your roof sheathing is air leakage from the interior of your home into the attic.

    You've told us that "the upstairs of the house was significantly hotter in the summer than usual." If you have R-30 insulation on your attic floor, it's unlikely that the temperature of your attic has anything to do with this problem. (That said, your contractors were mistaken when they told you that "R-30 attic insulation is fine for this area." In your climate zone -- Zone 4A -- the 2012 IRC requires a minimum of R-49 attic insulation.)

    I don't really know why you describe your upstairs as hot. Was the weather different? Do you even have air conditioning? The phenomenon you describe could be caused by many different factors.

  7. charlie_sullivan | | #7

    You might find that it's easier to understand the way the moisture in your attic works if you look at it in terms of dew point rather than humidity. Suppose the attic is at 30 F and 70% humidity. That's a dew point of 21 F. Your inside air would likely have a dew-point of 45 F. When that goes into the attic that will increase the humidity up there, because it has higher absolute moisture content than the 21 F dew point air it is displacing.

  8. brunojw | | #8

    First of all thanks for the input ....................

    Response to Martin - Yes we have air conditioning. After the 2nd ventilation change it didn't matter if the outside temp was 85 or 95 degrees you could tell it was much hotter on the second floor. I agree that it is difficult to diagnose the situation completely, but I am looking for help on the next move. I don't want to do the try this, no that didn't work, now try this. I've seen those postings already on several blogs.

    Response to Charlie - I understand the dew point in fact I was keeping a log of the outside, inside house and attic dew points to calculate if there might be frost in the attic since getting into the attic is problematic due to the small opening in a closet. I am not debating the fact that some moisture is finding it's way into the attic. Are you saying that no ventilation improvement will help to lesson the humidity in the attic. I guess I am visualizing a bathroom with no ventilation and the humidity before a shower is 40% and after a shower is 90%. If I left the door closed I would think it will take quite a long time for the humidity to decrease. If I left a 4" crack in the upper window in the bathroom before, during and after the shower I would think the humidity would rise, maybe not to the 90% level, but would decrease more rapidly than leaving the door closed.

    Response to all - would you suggest that I have a HVAC person come to pressurize the house to see how much and where air might be going into the attic?

  9. GBA Editor
    Martin Holladay | | #9

    You wrote, "It didn't matter if the outside temp was 85 or 95 degrees you could tell it was much hotter on the second floor."

    The situation you describe is fairly typical for single-zone forced air systems. For more information on this problem, see Keeping Cool in a Two-Story House.

  10. iLikeDirt | | #10

    Martin, you're missing that this change in comfort happened after the work was done that reduced the ventilation. It's not apparently something that Jim has lived with since he's owned the house. I think we should be willing to consider that this perceived change is somehow related to the work performed and not an illusion or a random freak coincidence that happens to have perfect correlation with related work in a nearby area that caused that nearby area to be hotter (which would logically increase the delta-T across any insulation separating that area from living spaces…).

    FWIW, Jim, you can do this air sealing work yourself with a fog machine and a box fan. Seal up the house and blow air into the house with the box fan, then turn on the fog machine. It's even cooler with colored fog. Then you can seal the leaks in the attic floor in real-time as you see the smoke escaping. No more smoke = no more ceiling leaks. Once you have this done, you can confidently refute the argument and alleviate the worry that the moisture load is caused by interior humid air escaping into the attic. After than, if your attic is easily accessible, grab some cheap cellulose from a big box store and install it yourself to R-60 or more. This will at least help you in the winter. Then once the summer comfort problem stubbornly refuses to disappear, you can refute the argument that it's caused by a lack of attic insulation.

    After this is done, try enlarging the soffit vents; clearly they're too small. Remove those perforated covers and replace them with wire mesh grates or something. Don't sweat the precise net free air calculations too much. After that, if you're still not satisfied, you might consider a solar attic fan with a west or southwest facing solar panel. With your perfectly-sealed attic floor, you won't have to worry about backdrafting any gas appliances (if you have any) and since it won't use any electricity, nobody can argue that it will cost you more to run than it will save you in AC bills (comfort is never mentioned in the argument).

  11. GBA Editor
    Martin Holladay | | #11

    If we assume that Jim is accurately describing his attic floor -- one that is insulated to R-30 -- what is your explanation for why his second floor is hot? And how, exactly, do you think that increasing the rate of attic ventilation will help? Are you proposing that a change in the attic temperature of a few degrees (due to a reduced rate of ventilation) is enough to make his upstairs hot? While also contending that adding insulation from R-30 to R-60 will be useless to slow this heat flow? That is a contradictory position to take.

  12. iLikeDirt | | #12

    All right, here's my hypothesis for what's going on.

    Due to the diminished ventilation, the increased heat in the attic has resulted in a higher delta-T through Jim's attic floor insulation, which has increased the rate of heat transfer through it and into his second floor ceiling. This is exacerbated by his single-zone AC system that does not cool the upper floor more than the ground floor as it should, so the increase in heat on the second floor is more noticeable than it would be with a superior air distribution system.

    If you're with me so far, we've established that the increased attic temperatures have resulted in slightly more heat gain through the second floor ceiling. This is mathematically necessary given a higher delta-T through the insulation. If the attic is hotter, there's more heat transfer through the insulation. No way around it.

    Now let's look at the comfort effects, not just the numbers. The human head feels heat very strongly, just like how cold feet make you feel hold even if you're wearing a sweater. An increase in temperature of only a few degrees in the ceiling drywall due to the already established greater heat transfer results in a comfort penalty that is surprisingly large compared to what one might expect basic on the numbers alone.

    So here's another refinement: let's add time into the equation. When the attic was well-ventilated, not only was the temperature in the attic lower, but it oscillated more with diurnal cycles. During the day it got hot, but it cooled down at night as the hot attic air was replaced with cool night air. But now, with little ventilation, when the attic is hot, it stays hot for longer through the night. The temperature inside of it remains higher and its temperature oscillations are dampened--it doesn't cool down at night as much as it did before. As a result, the delta-T and the heat transfer through the insulation is now higher later in the day, continuing longer into the evening. Before, if the attic temperature was 100 degrees or less by 10 PM, there was a delta-T of maybe 22 degrees (with the thermostat set to 78), yielding a heat transfer of 0.73 BTUs per square foot. Now, if the attic temperature is still 120 degrees by 10 PM, the heat transfer is 1.4 BTUs per square foot. That's twice as much heat transfer. So just when you're trying to fall asleep, you're subject to more heat entering your bedroom than before--perhaps enough to be noticeable and make it harder to fall asleep.

    So the answer is to add more insulation, right? Should be a no-brainer. Go from R-30 to R-60 and you'll halve the heat transfer at all hours of the day. Sounds perfect, right?

    Here's why it might not be. Imagine that the outside temperature falls below 78 degrees at night. Now suddenly the flow of heat reverses direction: heat is being pulled out of your house. Your whole house is a heat sink to the exterior. This is even more pronounced on the roof due to the phenomenon of nighttime radiational cooling. If you have good attic ventilation, your attic can take advantage of this quickly. But what if you instead have poor ventilation and really thick insulation? Then the rate of heat transfer *out* of your house is likewise slowed. During the shoulder seasons when it cools down a lot at night, this can be especially noticeable, and where I live in New Mexico, it's a big factor during the summer, too. It may be 100 and sunny during the day and 68 at night. Before I added more insulation, my house cooled down a lot more at night than it does now.

    And if you're dumb enough to have blown fiberglass as the top layer of your attic insulation like I do, there's an extra penalty since fiberglass is not IR-opaque (thanks Dana), so it's not as effective at blocking radiant heat into the house during the day when it's hot, but very effective at blocking conductive heat transfer out of the house at night.

    This is to say nothing about the moisture accumulation in the attic.

  13. GBA Editor
    Martin Holladay | | #13

    The mechanisms you describe are unlikely.

    If Jim really does have R-30 insulation an his attic floor, the temperature of the attic isn't going to affect the comfort level on his upper floor -- especially since he notes that the problem occurs whether it is 85 degrees or 95 degrees outside. That indicates that this is not an example of a struggling air conditioner burdened by heat flow that exceeds what was expected by Manual J calculations. This is an example of an unbalanced air conditioning system -- one that makes the first floor comfortable while the second floor is still hot, regardless of outdoor (or attic) conditions.

  14. brunojw | | #14

    Nate, from your response to your post in # 3 above it sounds like adding more insulation is not the answer. It sounds like the way forward is to reexamine the sealing that I have already done on the attic floor (see the comments from my original post under points to note # 3). If I cannot find any additional places to seal then is sounds like I need to increase the air intake in the soffit area. Would you agree?? Martin would you agree??

    Nate I am not clear on the way to perform the fog test. Are you saying to close all outside doors and windows and then turn a fan on in the house? If so where should the fan be positioned? Where should the fog box be located as well? I know my wife will be concerned about the odor it may leave and also staining on the walls and home furnishings such as drapes, bedspreads, etc.

    If I were to hire someone to perform a test would they do the same thing or can they perform a pressure test to determine the extend of the air leaking?

  15. GBA Editor
    Martin Holladay | | #15

    I agree completely on the need to double-check your air sealing work. Here is a link to an article that may help you: Air Sealing an Attic.

    You wrote, "It sounds like adding more insulation is not the answer." While adding more insulation may not be "the answer" to your two problems (sheathing mold and a hot upstairs), I certainly recommend that you add more insulation after completing your air sealing work. Your existing R-30 insulation does not meet minimum code requirements; the code requires at least R-49 in your climate zone.

    Adding insulation is important -- not because it is "the answer" to your two problems, but because it will lower your energy bills.

    If you want to hire a home performance contractor, the contractor could either:

    1. Perform a blower door test to determine the extent to which your attic is within your thermal envelope. If the contractor compares the depressurization of the house as well as the depressurization of the attic with respect to the outdoors during a blower-door test, the level of air leakage through your ceiling can be determined.

    2. Perform blower-door-directed air sealing.

    Here is a link to an article with more information on the use of a theatrical fog machine: Pinpointing Leaks With a Fog Machine.

  16. iLikeDirt | | #16

    If the primary problem is an unbalanced air conditioning system, why did this problem suddenly appear after a contractor reduced the attic ventilation? Wouldn't it have been omnipresent? I grew up in a house with an unbalanced AC system that under-cooled the bedrooms on the second floor. The effect was always there. It didn't just show up one day. So why did this effect suddenly show up in Jim's house two years ago immediately after a contractor noticeably reduced the attic ventilation? Is that just a huge coincidence?

  17. brunojw | | #17

    Martin, I actually read this article before I sealed the attic. I'll reexamine everything again to make sure I didn't miss anything, but I went over the attic the first time with a fine tooth comb. I did find large gaps around 2 vent pipes that go thru the roof, and an area that had a 1'x1' opening where the drywall was missing between a 2nd floor wall. As noted I sealed all the electrical wiring holes, light fixtures, etc. So I'm pretty certain that I got all the large stuff. I did follow the drywall on the ceiling to seal where they joined. If there is an area that might be an issue it may be the outside wall where the drywall meets the exterior wall - it was hard to reach and maybe I missed a few areas, but I think overall I did a good job.

    Once I reexamine what I did and am satisfied. If I do get a contractor to perform a blower door test and the results indicate the attic is within acceptable levels do you agree that the next step would be more soffit intake ventilation? I am not seeing any other logic step after a blower door test.

  18. GBA Editor
    Martin Holladay | | #18

    Q. "Why did this problem suddenly appear after a contractor reduced the attic ventilation?"

    A. I don't know. A site visit might reveal something obvious that has been missed, creating an "Aha!" moment.

    Jim's report that his upstairs feels warmer than it used to may be a correct observation, or may simply be an impression unsupported by data. Or a coincidental occurrence -- for example, a duct connection that came apart -- might have led Jim to misunderstand the cause of a real (correctly observed) change in upstairs temperatures.

    One thing I'm confident of, though: if Jim has R-30 attic insulation, correctly installed, it's highly unlikely that his observation that "the upstairs is hot" was caused by a change in the attic ventilation rate.

  19. GBA Editor
    Martin Holladay | | #19

    Q. "If I do get a contractor to perform a blower door test and the results indicate the attic is within acceptable levels do you agree that the next step would be more soffit intake ventilation?"

    A. No. The next step would be to assess your HVAC system. I assume that you have a single-zone forced-air system connected to a split-system air conditioner -- but you haven't described your system yet.

  20. brunojw | | #20

    I do have R30 insulation. It is rolled fiberglass that sits a few inches above the floor joists. It had the backing on the floor side, but when I rolled it back to seal I know some if it ripped and probably compromised any vapor barrio, but that was after the issue already existed.

    The HVAC system is a heat pump with electrical heat backup. It is located in the basement. It gets checked 2x year for maintenance by the HVAC guys who installed it. That has not changed in the past 10 years.

  21. iLikeDirt | | #21

    Jim, where are you ducts feeding the registers for the rooms on the second floor? In the attic? Or entirely within conditioned space?

    Martin, what I am trying to express is that if the attic itself has a higher temperature in it than before (right Jim?), than it is mathematically, thermodynamically necessary that more heat is entering the second floor ceiling due to the greater temperature difference across the insulation than existed before. You can say that this increase should be negligible or so small as to be irrelevant given the insulation, but you can't say that it doesn't exist. Heat movement is driven by temperature differences. The greater the difference, the greater the movement.

  22. brunojw | | #22

    Nate, the registers are on the floor on the second floor. There is no ductwork in the attic.

    Yes the attic is hotter than before. The temp on a normal sunny summer day are in the 140-150 degree range.

    One comment. When I read articles that say your attic should not be hotter than 20 degrees more than the outside air temperature is that all B.S.? All those articles point to a balanced ventilation system.

  23. brunojw | | #23

    One more bit of info also. Only my wife and I live at home so we do not have a family of 5 taking showers, etc. We have a first floor laundry room where the dryer is vented to the outside. We have a cooktop with a hood vent vented to the outside. My wife and I crack the bathroom windows 3-4 inches at the top before and after our showers even when its 20 degrees outside. We try to be humidity friendly.....

  24. GBA Editor
    Martin Holladay | | #24

    You wrote, "I do have R-30 insulation. It is rolled fiberglass that sits a few inches above the floor joists."

    Please explain what that means. Are the batts above the top of the floor joists? Meaning that there is a 6-inch to 9-inch air gap between the ceiling and the bottom of the batts? If so, that's a big problem. Your attic insulation must be in direct contact with your ceiling to be effective.

    I assume that your heat pump has a forced-air distrubtion system (ducts). Is it a single-zone system? If so, it's not unusual for your second floor to be much hotter than your first floor during the summer. There are a variety of possible fixes. Many HVAC contractors insist on installing two totally separate cooling systems -- one serving the first floor, and another for the second floor. That's not cheap, however. It would probably be easier to just add a ductless minisplit to your upstairs hallway.

  25. iLikeDirt | | #25

    Okay Jim, so we know there was no change to your HVAC system that could have caused this, and we know that there is no ductwork located in the attic that's contributing to the problem. There is an additional data point we need: the number of cooling degree days for the summers when you have experienced this problem vs the prior summers. If the numbers are very similar, then that rules out the possibility that it's just hotter out and you somehow failed to notice this (???). Find the numbers here: degreedays[dot]net

  26. brunojw | | #26

    Nate, I am not sure what parameters to enter. What should I use as the baseline in degrees? Also, should I run the daily, weekly or monthly report?

  27. GBA Editor
    Martin Holladay | | #27

    So -- is your insulation in contact with the ceiling?

  28. iLikeDirt | | #28

    Choose whatever base temperature is the default and the monthly report. It doesn't matter much. What you're looking for is a huge increase in the number of cooling degree days between the last two summers/years and prior years. If one doesn't exist (or is even negative) then your discomfort is not caused by hotter temperatures.

  29. brunojw | | #29

    Martin, I thought I replied. It looks like it was not posted.....The insulation is in contact with the attic floor. I was just pointing out the insulation was not compacted.

  30. GBA Editor
    Martin Holladay | | #30

    Is the "attic floor" the same as the ceiling below?

    If you have a drywall or plaster ceiling, and then floor joists above that, and then an attic floor (perhaps made of boards or plywood -- and if the fiberglass batts are installed above the attic floor instead of in contact with the ceiling -- you have a problem.

  31. brunojw | | #31

    Ok I'll need help here. I ran a baseline temp of 75 degrees monthly. Here are the results:

    June - Sept 2013 = 824, 2014 = 721, 2015 = 607
    July-Aug 2013 = 258, 2014 = 286, 2015 = 218

  32. iLikeDirt | | #32

    There you go. So in 2014 and 2015, it was generally cooler, not warmer than 2013 when you didn't have this problem. So it's not a heat wave or other weather change that would explain why your second floor suddenly became uncomfortable during the summer of 2014.

    Let's see, what options are left besides the diminished attic ventilation… ;-)

  33. iLikeDirt | | #33

    Jim, can you confirm that the bottom layer of attic insulation is in direct contact with the drywall or plaster & lath that constitutes your second floor ceiling?

  34. brunojw | | #34

    Nate. Yes it absolutely does

  35. iLikeDirt | | #35

    Perfect, thanks Jim.

    In summary: we have a homeowner complaining about greater heat and discomfort in his second floor, right under the attic, after a contractor made a change that reduced the ventilation in his attic. Is that why it's feeling hotter? Let's break it down point by point here.

    We know that the weather wasn't hotter.

    And we know that the ductwork or other parts of the HVAC system weren't touched around that time--and that they are professionally serviced on a regular basis.

    And we know that the air sealing or insulation haven't worsened around that time--in fact, the air sealing has been improved by the homeowner himself.

    And we know that the one thing that was changed right before the problem manifested was that the attic's ventilation capacity was substantially reduced.

    And we know that after this work, it became hotter inside the attic (confirmed by the homeowner).

    And we know that the speed and amount of heat flow is driven by delta-T.

    And we know that a hotter attic results in a higher delta-T through the insulation and second floor ceiling.

    And we know that the attic now doesn't cool down at night as much as it used to, so that delta-T stays higher than it used to for longer periods of time than it used to.

    And based on all this, we know that there logically, mathematically, thermodynamically must be greater heat flow down through the ceiling during the day, and for longer into the night than there used to be.

    Based on this, I feel confident saying that the diminished ventilation is the culprit of the increased heat load, regardless of what corrective action should be taken to remedy it (e.g. more attic insulation).

    And if I am correct, then we have just discovered a case where the temperature of the attic is not completely irrelevant and attic ventilation can noticeably affect interior thermal performance.

    For any who disagree, I look forward to hearing just which of the above sentences that support this conclusion is incorrect.

  36. GBA Editor
    Martin Holladay | | #36

    We're going around in circles here. Nate and I disagree. I have presented my reasoning, and I stand by my reasoning.

    Nate's theories differ from mine. You can follow my advice or Nate's advice; the choice is yours. Good luck.

  37. iLikeDirt | | #37

    Martin, I respect you greatly. Your contributions on this and other sites have taught me an enormous amount. My wife is tired of hearing your name around the house. But nobody knows everything and I believe that you as well as the expert consensus may be wrong here. And I haven't seen you actually refute anything I've said yet. The closest you came was to speculate that that Jim's likely unbalanced AC system was to blame even though we established that nothing about his system changed before the problem occurred, nor was there a heat wave or a mechanical problem that would be overwhelming its former ability to satisfactorily cool the second floor. I have explained my argument in simple, logical, step-by-step terms. I am more than willing to be proven wrong, especially by someone as learned as you are, but if you cannot do it, then I will stand by my reasoning, because it makes the most sense to me given the dimensions of the problem that is being described.

    Jim, regardless of whether you take my advice, I advise you to first take Martin's advice and improve the insulation of your attic to R-60 or more. Preferably, don't use fiberglass for the top layer, which is slightly transparent to infrared radiation--the kind you're trying to block. Make sure the top layer is mineral wool batts or cellulose. If your attic is very accessible, this should be cheap and easy to do yourself, and it will pay dividends in the winter, guaranteed.

    See what happens the summer after you do this work. Does it help much? Does it help at all? If it does help, does it help by an amount you consider satisfactory? And if not, then I recommend restoring the attic venting like I'm proposing, and then see which change had the bigger effect. Oh, and please report back! If I'm wrong, then at least it will be an honor to be schooled by the master.

  38. AlanB4 | | #38

    "My wife is tired of hearing your name around the house"
    Should we ask why? :P

  39. iLikeDirt | | #39

    LOL! Just boring her to death with green building enthusiasm.

  40. brunojw | | #40

    Thanks Martin and Nate for your input. I am going to first reexamine the sealing that I did and probably get confirmation with a blower door test. I'll see what this results in. I'll look at adding the insulation and see what the results of that is. I'll report back sometime in the summer to let you know what results I see.

  41. charlie_sullivan | | #41

    I didn't read all the way through the long initial question until now. After getting all the way through that, I think there is evidence that
    1) There is still some air leakage into the attic, which contributes to raising winter attic humidity.
    2) The soffit ventilation has been impeded by the vinyl pieces that were recently installed. This contributes to high summer attic temperatures and slows drying of the moisture introduced in the winter by #1.

    The discussion has focused on air leakage into the attic and on the insulation and how that can help reduced the effect of attic temperature. While I agree that those are the right priorities, I think it is also worth considering the ventilation. Generally, the attic temperature shouldn't matter too much if there is good insulation, but 140 to 150 attic temperatures are quite high. The heat flow through R30 insulation with 140 on one side and 80 F on the other side is the same as if you had 100 F in the attic and 80 F in the room, with R10 insulation, for example. Increasing the insulation is a good thing, because it will help your energy costs all year, but considering the ventilation is also worthwhile, given how high the attic temperature is in the summer, and given that there are also moisture issues.

    The NFA numbers can be helpful to get an idea of what is going on there, but they don't really provide a solid way to know for sure what's going on. It sounds like the configuration around the top of your siding and your soffit is kind of complicated. I'm not very familiar with the different special pieces provided by vinyl siding manufacturers. So it's hard to know how your actual configuration affects the ventilation. Some pictures might help, but I'm not sure we'd be able to see enough of what matters. A sketch, or perhaps a link to a manufacturer's sketch, might be better.

    Without knowing exactly how it's configured, I can make up stories about what might be going on, which might or might not be related to reality. One such story would be that the air from behind the vinyl siding has an easier path to flow up through the soffit into the attic than outside air does. In the summer, the siding is heating by the sun, and so that air coming up behind the siding and into the attic is hot. In the winter, that air might include moisture that came from the inside of the house through leaks in the walls, and then came up into the attic. But that might be impossible, based on your actual configuration. I don't know.

  42. Expert Member
    Dana Dorsett | | #42

    Charlie has it right- there has to be air leakage either at the ceiling plane, or at the top plate of the stud walls. If the mold on the roof deck is showing up at the SOFFIT end it means that humid wintertime air is escaping there.

    I suspect that this only happens when the air handler is running, which is somehow pressurizing the upper floor air relative to the outdoors driving conditioned space air into the attic. When the venting was wide open with the gable vents that air flow was higher than it is now, and the air conditioning was cooling the attic by displacing it with cooler conditioned space air. Now that the venting is restricted and the ceiling is a bit tighter there's less conditioned air flowing into the attic, and it's running hotter. During the heating season that same air-handler driven pressure differential drive humid air into the attic giving it a moisture burden than it wouldn't otherwise have (as well as blowing heat out the venting.)

    With air leaks in the ceiling plane all wood in the attic takes on a lot of moisture, and it can take months for it to dry out even in decently vented attic configuration. When the roof deck and attic temps rise, it drives the moisture out, but when the roof deck cools overnight to the outdoor dew point it pulls a lot of that moisture back into the wood. As long as no new moisture is being brought in, it should dry out eventually though.

    Blower door testing & rectification of air leakage at the ceiling plane is clearly called for. You can seal 90% of the small to medium sized holes and still get tripped up by one hard to see big 'un.

    The higher solar heating of the roof deck year round drives moisture off the sunnier sides much more rapidly than on the north or shaded sides, and keeps the wood drier. Almost all roof deck condensation/adsorption problems in the field are on north facing or heavily shaded pitches. It's no surprise to find mold on a shaded northeast pitch even when the rest of the roof deck looks clean. It typically shows up on a sunny south facing pitches only when there's a significant air leak from the ceiling or wall's top plate proximate to the moldy patch.

    Having gable vents in combination with soffit to ridge venting short-circuits the air flow, and ends up with more air flow at some soffits than others. From a moisture management point of view it's better to have ~50% more soffit vent area than ridge vent area, since that reduces the stack effect drive pulling air through leaks at the ceiling plane. If the soffit vent area has now been blocked to where the ridge vent area is bigger than the soffit vent area, it increases the stack-effect draw through the ceiling plane. With bigger soffit vents than ridge vents, the stack effect is only as tall as the elevation between the lowest air leaks in the house and the soffits, whereas if the ridge vent area is bigger it's that much taller, and the attic is more depressurized relative to the upper floor. (But while the air handler is running, an unbalanced or leaky duct system can trump all stack effect bets until the blower stops.)

    R30 fiberglass batts are less effective at rejecting summertime heat than R30 cellulose. Fiberglass is somewhat translucent to infra-red radiating in from the hot roof deck, and the temperature in the batt an inch or two below the top surface peaks at temperatures well above the AIR temp in the attic, since there is R4-R6 of insulation between where that radiated heat got absorbed and the attic air. That means you're insulated against a higher temp with less insulation. Cellulose is opaque to IR, and the temperature profile through the cellulose is fairly uniform. After chasing down and truly fixing all of the air leaks at the ceiling plane, overtopping the fiberglass with 3" of cellulose would fix that problem, and restore full performance of the fiberglass to it's labeled R-value. The total R-value would still be below current code min, but adding 3" of cellulose would at least bring it up to IRC 2009 code min levels for US climate zone 4A, Philadelphia. It's a pretty easy DIY, well worth doing, but in locations where it's subsidized it can be cheaper to let the pros handle it.

  43. GBA Editor
    Martin Holladay | | #43

    Thanks, Charlie and Dana. I think a consensus is emerging: the most important measures are sealing air leaks at the ceiling, and adding some cellulose insulation to the attic floor.

    I understand that there may be a small benefit to increasing air flow at the soffits. But until the air leakage through the ceiling plane is addressed (and, ideally, the insulation levels are improved), focusing on soffit ventilation is likely to yield disappointment.

  44. Expert Member
    Dana Dorsett | | #44

    Consider running the following quick experiment:

    With the air handler off, use a silk thread or spider web as the visual flow meter, and crack the hatch to the attic to where you can just detect the upward air flow. Then have somebody turn on the air handler, and see if and how much the flow changes (or if it even changes direction!) That might give some idea of how much of the attic moisture problem is air-handler driven vs. stack effect driven.

    Of course, one bathroom fan illegally (under current code) vented into the attic could be 99% of the moisture problem. Even if not actively running it's a humidity conduit- they all leak.

  45. iLikeDirt | | #45

    Dana, would something like construction paper or another flat and opaque material laid over fiberglass be as effective? I have this same problem in my own attic with about 10" of blown fiberglass over cellulose and mineral wool, but no room to add more insulation in much of the attic due to a shallow pitched roof. What about a multi-layered radiant barrier like this?

  46. brunojw | | #46

    Dana and Charlie, Thanks for the additional perspective. As a homeowner trying to fix this is very frustrating. I want to spend "good money" to fix the problem and not guess at a solution (if possible).

    Here is a link to the perforated vinyl siding soffit material that is over the soffit area. It is the triple soffit 3 1/3.............

    Regarding the bathroom exhaust fan... I actually built a box around the unit with insulated board after looking on the Internet about how to seal it. It is sealing around it with expanding sealing foam.

    I'm going to reexamine the sealing I've already done to see if I missed anything. Then I'll have a blower door test run to validate if/where any leaks exist and then go from there. Have them sealed and then have some additional insulation blown in. If the entry into the attic was easier I'd do it myself but it's so tiny I can barely fit in and I'm 5'7" and 150 lbs.

    If at the end of the day if there is no improvement then I'll address the soffit area for more ventilation.

  47. brunojw | | #47

    Dana, I will perform the attic door test as you suggest to see what results I get. I'll post when completed.

  48. brunojw | | #48

    Ok> Here are the results when of the test that Dana suggested I do with the attic door. With the HVAC system off I lit a candle cracked the attic door and blew out the candle. The smoke was immediately draw into the attic at a quick pace. With the HVAC system on I did the same thing. My observation was that the smoke was pulled into the attic maybe 10-15% faster than with the HVAC system off.

    So I guess this would suggest most of the airflow is stack driven. I'm not sure how to interpret the result. Maybe Dana you can elaborate.

  49. brunojw | | #49

    Sorry meant to say 10-15% faster with the HVAC system ON.

  50. Expert Member
    Dana Dorsett | | #50

    If the air is moving faster with the HVAC running, then the HVAC is actively pressurizing the upper floor relative to the outdoors when running, and forcing conditioned air into the attic. (The vented attic is at roughly the outdoor pressure.)

    It may be pressurizing only some rooms and depressurizing some others, or it may be pressurizing the whole house, but it's part of the attic moisture problem. If this can be reduced by better balancing or sealing the duct system it's worthwhile, but it underscores just how critical making the ceiling plane truly air tight is no matter where you end up with the attic venting. The colder it is outside, the more the air handler runs, and the more conditioned space air gets pushed into the attic, taking it's moisture load with it.

  51. brunojw | | #51

    Dana, can you clarify what you mean by rebalancing the duct system. Are you saying that the upstairs may have more internal airflow from the HVAC system compared to the downstairs? And when you say some rooms might be pressurized and some not, does this mean that there is more internal airflow from the HVAC system in one room vs. another. Sorry to be thick...............

  52. GBA Editor
    Martin Holladay | | #52

    A forced air system is unbalanced if operation of the system causes the house (or a room) to be pressurized or depressurized with respect to the outdoors.

    In your case, your forced air system is delivering supply air to your second floor at a greater rate than is being returned to your furnace (or air handler) through the return-air duct. This might happen if the return ducts are undersized, or if the return ducts are leaky and are located in areas that are either outside of your home's thermal envelope or in framing bays that communicate with the outdoors.

  53. Dana1 | | #53

    It's pretty common to find duct systems without a dedicated return for every supply duct. If the supply duct is in a room doored off from an appropriately sized common return path the room becomes pressurized relative to the other rooms, and pressurized relative to the outdoors whenever that door is closed. If the duct design had undersized return ducts relative to the supply, that too can cause rooms to become pressurized.

    Return paths in doored-off rooms can often be improved by using a partition wall as a jump-duct, with a grill at the bottom of a stud bay on one side of the wall, and at the top on the other (for better privacy.)

    Leaky supply or return ducts can also upset the pressure balance.

    Adjustable balancing vanes in supply ducts can reduce the air flow into the pressurized rooms, but may leave the room with inadequate heat/coolth if you take it too far. Ducts used for both heating and cooling rarely work perfectly in both modes, and if it's dialed-in for heat, it may not be so perfect for cooling & conversely. It's usually a compromise.

    Where the ducts are accessible it's worth taking the time to seal every seam & joint of hard-piped duct work with duct mastic. Air handler seams should also be sealed, but where service access is needed it's better to use a temperature rated foil tape, not mastic. Caulking the register boots to the subfloor can also be important- pressurizing a subfloor cavity can sometimes pressurize an exterior wall's stud bay creating potential moisture issues in the wall.

  54. brunojw | | #54

    Boy and the plot thickens...........There is one bedroom on the second floor that we keep closed since it does not get much use. The heating register on the floor in that room is shut off. Is this an issue?

    Also two bedrooms have dressers in front of the return duct, but there is a 2" gap between the return register and the dresser. Is this an issue?

    The size of the return ducts in each second floor room are larger than the combined heating ducts in each bedroom. There is no heating or return ducts in the second floor hallway, but the hallway is not that large. The master bedroom return duct for some reason is much larger than any other room. It's double the size of any other 2nd floor room. It is positioned right next to the bedroom door. The door leads to the hallway and the master bathroom entrance is nearby as well. If I follow that large return duct to the 1st floor there is a same size return duct in the family room. The family room is the largest room in the house and has a fireplace in it. All the return air ducts are in interior walls.


  55. GBA Editor
    Martin Holladay | | #55

    Here's a link to an article on bedrooms without return ducts: Return-Air Problems.

    You can't determine the air flow rate through your return duct system, or the tightness of the duct seams, by looking at the grille size. It is easy to imagine that your return ducts might be pulling outdoor air into leaky duct seams, especially if the return ducts are located in framing bays or chases that are adjacent to a (leaky) exterior wall. The only way to determine air flow through your ducts is to test the duct systems for leakage and flow rate. (You need a Duct Blaster and a technician who is familiar with duct diagnostics.)

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