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Attic moisture …

humm9er | Posted in Energy Efficiency and Durability on


I have read the articles on attic ventilation and moisture and believe I understand that at its core any attic moisture is typically the result of air leakage from the house and/or basement/crawlspace moisture.

Zone 5A.

I have discussed my house previously but in a nutshell, I have 2 attic spaces, combined by a 3×3′ opening where the old house meets the addition. Attic space #1 has ridge and soffit vents, is air sealed with spray foam around all plumbing stacks, top plates and electrical boxes. It then has faced fiberglass batts covered with blown cellulose — the cellulose completely covers the joists. That attic has pink rafter vents to ensure airflow.

Attic space #2 has is over an old portion of the home. Non-vented. No soffit or ridge vent. From top to bottom the layers there are: r-30 unfaced batts, T&G pine boards, 1″ white rigid foam insulation (polystyrene maybe not sure?) tight between old ceiling joists (hand hewn timbers), drywall. The drywall and foam board under the attic floor is tight to the old timbers but it’s not 100% caulked everywhere so I am sure some air sneaks by. Attic door and walls along attic steps fitted with 2″ foil poly-iso. Attic door itself is gasketed and seams are taped shut. I air-sealed where I could with spray foam without pulling the attic floor.

No other ceiling penetrations in Attic #2 save the chimney which I already sealed with aluminum and fire-rated caulk — previously there was a 6″ x 36″ opening to the attic there. Yowza.

Yesterday the weather dropped here about 25 degrees and it snowed for the 1st time. I ventured into attic space #2 and noticed some of the roof nails had small amounts of condensation on them. I air-sealed 1 light-fixture and 1 smoke alarm that I had missed.


I assume attic #2 has survived 300 years unvented so venting is not something I should consider?

Does this point to the need for more air-sealing?

I did not install rafter vents in attic #2 because there are no soffit or ridge vents. The batts go right over the top plate to the eve of the roof. Is this okay or do the rafter vents still serve a purpose?

There is a bath fan vented to the roof in attic #1. The metal fan box is caulked to the ceiling but not covered in a rigid foam box. Insulated flex duct runs from fan assembly to roof vent. Is a rigid foam box over the whole metal assembly required, or does the metal box itself with the damper keep air from entering the house? I believe the flex duct is zip-tied but not sure if it’s taped — moisture source possibly?

Thank you as always!!

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  1. humm9er | | #1

    I was going to write a separate post, but this issue may be connected.

    50% of my house is heated with baseboard, 50% forced air. My forced air fresh air / combustion duct pulling air from outside the house is tied into my return air plenum on my main duct trunk. I then have 2 return registers in my basement.

    My understanding is this is an antiquated design that pressurizes the house and exacerbates the stack effect. Correct?

    Is it correct that I should have the combustion air duct disconnected from the return air plenum and simply have the duct drop down my basement wall and terminate near the floor?

    Thank you!

  2. Dana1 | | #2

    Having make-up air ducted into the return of fossil burner hot-air is required by code in some areas. Yes, it usually pressurizes the house slightly, but you can test how much pressurization there is relative to each room by cracking window and feeling just how much air is escaping. If some rooms seem much more pressurized than others, sealing both the supply and return ducts seam, joints and register boots, and checking to ensure that there is a low impedance return for every supply register should fix the worst of it.

    The dew/frost on the nail points is an indication of moisture in that attic space, and yes, more air sealing would be in order.

  3. humm9er | | #3

    Thanks Dana. 2 zones. Each has 8 supply and 4 return registers, plus the added fresh air duct.

    All seams have been sealed with UL tape.

    1 return register for each zone is in the basement -- I have slightly elevated radon levels. Given the fresh air duct can I close/ is there any benefit to closing those basement returns, bringing it down to 3 return registers for each zone from the living space and 1 fresh air duct?

    Thanks and any insight on requirement for foam box around bathroom fan appreciated, as well as need for rafter vents / insulation baffles in unvented space.

    Thank you!

  4. GBA Editor
    Martin Holladay | | #4

    If a duct brings outdoor air to your return air plenum, that duct is providing ventilation air, not combustion air.

    If a duct is open to the air in your mechanical room (or basement, or wherever your furnace is located), then that's a combustion air duct. Such ducts often terminate near the floor.

    If you have a duct that brings ventilation air from the outdoors to the return plenum of your furnace, that system will waste a lot of energy unless the duct is equipped with a motorized damper controlled by a FanCycler control or an AirCycler control. For more information on this type of ventilation system (known as a "central-fan-intergrated supply ventilation system"), see Designing a Good Ventilation System.

  5. humm9er | | #5


    Thank you for your reply. Yes, you are correct it is a ventilation air duct. My System 2000 boiler is about 40 feet from my primary trunk / return plenum. The boiler has a 4" pvc pipe coming off the boiler that's open to the basement -- this is the combustion air inlet I believe. Correct me if that sounds wrong.

    I read your link about designing a good ventilation system, thank you. I am not sure what the costs are with equipping the ventilation duct with a motorized damper controlled by fancycler / aircycler control -- any ballpark there?

    I also happened upon this article which suggests just letting the ventilation duct terminate near the basement floor. Any merit in that approach?

    Thank you!

  6. GBA Editor
    Martin Holladay | | #6

    Reuben Saltzman, the author of the article you linked to, is confused. There is no justification for referring to the outdoor air intake duct of a central-fan-integrated ventilation system as a "combustion air duct." This is a ventilation duct, and Saltzman is wrong.

    He is correct, however, that a central-fan-integrated ventilation system without a motorized damper and an AirCycler control wastes energy.

  7. humm9er | | #7


    Thank you. I am going to have a HVAC contractor out to the house.

    However, I want to be armed with as much info as possible since in my experience many HVAC and building folks do not look at issues with an eye toward energy efficiency (to put it mildly).

    My System 2000 is drawing combustion air from the basement via a 3" PVC pipe. I have air-sealed the entire rim joist, bulkhead doors, windows and ducts themselves. I am sure air is still getting in however from somewhere...

    I read your article on central-fan-integrated ventilation the damper closes after allowing in a preset amount of outside air? And this saves energy by reducing the amount of outside air the system has to heat, say from heating outside air 100% of the time to heating it 50% of the time.

    Is that correct? Installation looks fairly simply -- simply adding it into the 8" ventilation duct in front of the air handler.

    Does the damper do anything to reduce the pressure caused by the ventilation duct being tied to the return plenum? Regardless of Reuben Saltzman's incorrect terminology, is there a benefit as an intermediate step in removing the ventilation duct from the return plenum and making it "passive" -- simply open to the basement itself for my combustion duct to pull from as needed?

    Also, assuming it can be done from an air balancing perspective, am I correct that it would be prudent to remove my basement return registers altogether? It seems to me I don't want to be using basement air to heat my house ideally, from a health / air quality / energy perspective.

    Also, in an ideal world, prudent to run my combustion inlet out the side of the house?

    Thank you so much, your input is really helpful.

  8. GBA Editor
    Martin Holladay | | #8

    You wrote, "My System 2000 is drawing combustion air from the basement via a 3-inch PVC pipe."

    I'm not sure what this means. I know that a System 2000 is a residential boiler. But explain what the two ends of this PVC pipe are connected to.

    One end of the pipe is apparently open to the basement air. And the other end of the pipe -- is it connected directly to the burner on your boiler?

    If you are concerned that your atmospherically vented boiler is starved for combustion air, you should have the HVAC expert evaluate the situation to make sure that the combustion air is adequate. If it isn't -- for example, if the boiler is located in a small mechanical room, or if your basement is unusually tight, or if a worst-case depressurization test shows that your home has powerful exhaust fans that interfere with your boiler's combustion -- then you may need to install a combustion air duct, or switch to a sealed-combustion appliance.

    If you want to install a central-fan-integrated supply ventilation system, you need to buy two pieces of equipment: a motorized damper and an AirCycler control. After these pieces of equipment are installed, you need to have the control wired to the motorized damper and your furnace blower. Finally, a technician has to commission the system by testing airflow through the ventilation duct and adjusting a manual damper or the AirCycler control to prevent overventilation or underventilation.

  9. humm9er | | #9

    Martin, yes the 3" PVC is atmospherically vented with the inlet rising about 4' off the boiler itself into the basement. No powerful exhaust fans in the house.

    Okay I will assess with the HVAC person about the need to have the combustion duct run outside the house. Even though I made the basement a lot tighter than it was, I am sure that there are still leaks...correct me if that assumption is incorrect.

    Any feedback on my sense that return registers in the basement are not ideal? Seems to me that until I insulate the walls pulling cold basement air into the return is not ideal. Nevermind the health / dust factors. Again I will have HVAC tech assess but I'd like opinions. I have 4 returns for each of my 2 zones -- I'd love to close each zone's single basement return.

    Useless to move to passive ventilation duct as intermediary step (remove the ventilation duct from return plenum leave open to basement)?

    I may have them add an AirCycler...the system itself doesn't look too costly. Any estimation of how ROI / long-term savings here? Will it generate significant energy savings each winter?


  10. GBA Editor
    Martin Holladay | | #10

    If you want your basement to be heated in winter, it makes sense to include one or more supply-air registers, as well as one or more return-air grilles, in your basement.

    If you don't want your basement to be heated in winter, you can seal the supply-air registers and return-air grilles in your basement.

    This advice assumes, of course, that your duct system has an adequate number of return grilles upstairs, and that your return-air duct system is correctly sized. You don't want to starve your furnace of return air, so if you are unsure on this issue, have your return air ducts evaluated before sealing any return-air grilles.

  11. Expert Member
    Dana Dorsett | | #11

    A 3" PVC proximity vent is not adequate make-up air for the BTU output of any System 2K, even though it would be adequate if it were a sealed-combustion unit.

  12. humm9er | | #12

    Martin, I don't want my basement heated. Will insulate all my boiler pipes too (supply and return or just supply?). I already sealed the 2 air supply vents in the basement. I just wasn't sure on the returns. I assume 3 returns in the house per zone is adequate, but I want an HVAC tech blessing before sealing them. I just wanted to be sure my logic in sealing those basement returns is sound.


    Dana, I may have misspoke -- it could be a 4" PVC pipe off the air intake. That is I believe the inlet size on the air intake and what's spec'd by the manufacturer?

    I'm just not sure if my 2 basement supply vents (50' from the boiler) were placed to provide combustion air. If so, it seems silly to me to pay to heat it first.

    Sounds like passive ventilation not preferable to it being piped into return plenum?


  13. GBA Editor
    Martin Holladay | | #13

    It sounds like the air intake for a sealed-combustion boiler that was never properly hooked up.

  14. GBA Editor
    Martin Holladay | | #14

    Q. "Sounds like passive ventilation not preferable to it being piped into return plenum?"

    A. Sometimes it's hard to understand your questions. If you really want a ventilation system for your house, I strongly suggest you read this article -- it should answer your questions about ventilation systems: Designing a Good Ventilation System.

  15. humm9er | | #15

    Martin & Dana,

    In reading the System 2000 manual, it discusses the ability to atmospherically vent the unit, but if doing so it specs a large vent hole in the basement -- hence my curiosity about disconnecting the fresh air vent from the return plenum and letting it open to basement.

    Alternatively I can vent the combustion pipe to the outside of the house if preferable. HVAC tech visiting next week to weigh in.

    Thanks as always for your input.

  16. humm9er | | #16


    Sorry, I try to keep my questions short, which may make them harder to understand.

    I read that article and it was helpful. I am weighing a damper and aircontrol unit long-term.

    My question is, in the short-term, do you see merit in disconnecting my current fresh air vent from the return plenum, letting it terminate near my basement floor? This would facilitate air for my boiler to draw from.

    Thank you!

  17. GBA Editor
    Martin Holladay | | #17

    My previous advice stands.

    Many of your questions show a confusion between combustion air ducts and ventilation ducts. These are separate systems.

    As I said, if you are worried about whether your boiler has adequate combustion air, I strongly urge you to have an HVAC professional evaluate the situation.

  18. humm9er | | #18


    HVAC professional visiting next week. Thanks for your I said my concern is that many HVAC professionals don't take a keen interest in energy savings-oriented installations / recommendations.

  19. humm9er | | #19

    Circling back to the comment on attic moisture on the nail tips, I am going to get a hygrometer, but just discovered what I recently began suspecting: that my master bathroom vent fan is a fake -- it vents directly to my wall cavity into fiberglass insulation.

    Big bummer. I will work to correct with a properly-sized (CFM-wise) vent fan properly vented out the roof. It's a 12x12x12 bathroom so will require a large fan.

    With this latest cold snap I have seen some condensate on the bottom of my windows in unused areas of the house I keep at 58. I assume since the master bath is the site of 2 daily showers this false fan was a major source of trapped house humidity?

    Now we will have to see what the mold damage in that wall cavity is.

  20. Brian Knight | | #20

    Yeah Justin, that certainly sounds like a contributing problem to shore up. Cooktop venting is another area to consider and I'm always impressed with how much moisture my houseplants introduce to my small home. I keep most of them outside with good weather but I can watch the humidity reading jump dramatically when bringing them all in. My indulgent hot showers push it up the most and I usually open windows to help clear the humid air quicker, despite having a functional bath fan, though I doubt it's venting much more than 25-30 CFM.

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