Humidity in Insulated Attic
I had spray foam insulation applied between rafters in my attic several years ago. My HVAC technician nephew living in Houston recently mentioned that insulated attics like mine are problematic in our hot climate because humidity can’t escape. I recently installed a hygrometer in my attic and have validated his statement. On sunny days, the attic temperature exceeds that of the house by maybe 5 degrees, but the humidity may be 10 to 15% higher. I’m expecting a much higher differential during hot, humid weather. Should I do something about this? I’ve considered conditioning the attic by adding a duct and return (this would be some work because of fresh air and venting requirement of a heater and water heater plus it would take energy to heat and cool) or putting a humidifier in the attic. I would love some feedback from experts. Thanks! Dorothy
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Hi Dorothy
I am not an expert...however, I have read about a few "foam between rafters" houses with large humidity swings....
What were the temperature and humidity readings?
Your comment about "venting" for a heater and water heater are troubling.
I hope you do not currently have an atmospherically vented heater and water heater inside the house or attic????
The temperature/humidity can start out similar in both attic and house at around 60 degrees and 60% humidity. As the day warms up the house may approach 70 degrees and 65% humidity whereas the attic may be 75 degrees and 82%. Both my heater and water heater are vented to the outside but I don't believe either have an dedicated in-flow line to combustion site. My house is almost 100 years old and foam installers said I didn't need in-flow because of air leaks inherent to an old house. Hope I answered your questions. I live in Baton Rouge, LA.
In hot humid climates a vented attic adds more humidity to the house in summer than it removes in winter. Air sealing the attic allows the absolute humidity (dew point, not relative humidity) to track that of the conditioned space, and the air conditioning can extract humidity from the attic even through 10" of open cell foam (but not closed cell foam).
The only hazard period for this approach is in winter, when the attic temperature may fall below the dew point of the interior air. Wood that is below that dew point temp will adsorb moisture- if that goes on uninterrupted for weeks or months it reach mold-potential. But in Houston the risk is pretty low, as long as the condition space has reasonable ventilation rates to the exterior. According to weatherspark datasets, the mean wintertime outdoor dew points are ~44F, whereas the mean outdoor temperature is 55F. (see: http://weatherspark.com/#!dashboard;q=houston%20tx%20usa open up both temperature and dew-point graphs, and zoom out to get a full year's worth.)
It should be pretty easy to ventilate sufficiently to keep the interior air's dew point below the 55F mean mid-winter temp, since that's ~58%RH @ 70F, a humidity that would be approaching mold-conditions at 70F. If you keep it under 50%RH indoors (recommended by health professionals) the dew point of the conditioned space is 51F, which is below the 55F mean outdoor temp. If you shoot for 45% wintertime max RH indoors, that's a 48F dew point, which is even more margin.
Pumping heated humid air via a heat duct into a ventilated attic would be moving in the WRONG direction, since it maximizes the humidity in the attic, which would have to be removed by the passive outdoor venting.
Dorothy (in Louisiana),
Do you know if they "sealed" your soffit vents?
I assumed that the spray foam was applied to the underside of the roof deck and that the attic is no longer "ventilated"....??
and what you now have is a "conditioned attic"
If so...here is a blog that is on topic.
https://www.greenbuildingadvisor.com/blogs/dept/musings/creating-conditioned-attic
check out question 4 and question 5
quoting the blog:
"If you hope to convert an existing unconditioned attic to a conditioned attic, the presence of any atmospherically vented appliances (for example, a gas water heater or a gas furnace) complicates the retrofit work. If you can’t afford to buy new sealed-combustion appliances, you’ll probably be better off leaving your attic unconditioned."
If...IF your attic is "sealed"...then
I think it was irresponsible for a spray foam company to simply assume that because your house is old that it is SAFE to leave non-sealed combustion equipment in place.
In response to John’s comments about appliances in attic: Soffit vents and gables are foamed so I have a “conditioned attic” (there is no ventilation to outside air). I had already read the article in the link you sent, “creating a conditioned attic”. I think the concern with respect to combustion air to appliances is carbon monoxide levels increased by back-drafting. I can fix this by installing a duct to fresh outside air to both appliances if I have a problem. Can’t I check for a problem with CO monitors near appliances and in the main house? I already have one in the house and it has never suggested high CO. Thanks for reminding me of this risk.
In response the Dana’s comments about humidity: I understand the first part of your response but I start to get confused on the sentence referencing to Houston. What do you mean by “reasonable ventilation rates to the exterior”? My attic and house are sealed from the outside. Are you suggesting adding a vent? Your suggestions of ideal indoor humidities in the range of 45% - 50% make sense except I am not sure how I would accomplish that in my climate. Would you mind clarifying please? Thanks again for helping.
Dorothy,
John Brooks is correct. You cannot have an atmospherically vented water heater or furnace in a sealed, conditioned attic. If you have that type of appliance, they must be removed and replaced with sealed-combustion units.
Adding an outdoor air duct to introduce air near the appliances is no solution. For one thing, that means that you are introducing hot, humid air into your sealed space -- exactly what you are trying to avoid.
Dorothy, some comments.
CO is a risk anytime a combustion appliance is indoors, and while the hardware store CO monitors are better than nothing, they only go off if there's a serious problem. You might consider investing in a low-level CO alarm, which costs more and goes off at lower levels.
Your heater (furnace?) and water heater are in the attic, and are vented to the outside. I assume you are saying that they are gas appliances and are what we call "natural draft" appliances, meaning they use indoor air for combustion and have a draft hood where the flue pipe starts. It would help if you could take a picture of both and post them, but for now I am going to assume that's what you have.
A possible issue with those appliances that needs to be ruled out is "spillage", meaning some flue gas doesn't make it up and out the flue, especially when the unit starts from cold. If that happens, the unit is putting some flue gas (and humidity) into the attic. This can certainly be a CO risk although not always, because the unit may be burning clean and generating very little CO. Anyway, a check of those appliances is warranted. Let's see photos so we can comment intelligently.
You mention supplying outdoor air to the appliances via a duct. If you install a duct to supply air to natural draft appliances, all you are doing is punching a hole in your insulation and letting outdoor air in. Much better would be to replace the appliances with sealed-combustion appliances that have a factory air connection as well as a flue connection. That way you don't need to let a bunch of outdoor air in, and your appliances are much less likely to interact with your indoor (breathing) air. You would probably get more efficient appliances as a bonus.
As far as natural humidity levels go, in the south (where I am not and have no experience) the air tends to be humid, especially in summer. You cannot remove humidity from the house by ventilating it, you have to remove it via A/C. You can only remove humidity from the house by ventilating IF the outdoor air is somewhat drier than the indoor air. From what I understand, in the south, you're going to be relying on A/C to dehumidify. Up here in the north, we use dry outdoor air to remove humidity (and we pay the cost of heating that cold air once it comes inside... that's the price of dry indoor air).
Anyway... long post... but your humidity puzzle has several possible pieces to it, and it can take some effort and experience to sort them out. You have to figure out if there is a source of moisture like a wet basement, a collection of fish tanks, a lot of wet laundry on indoor dry lines, someone cooking without the range hood, someone showering without the bath fan, and etc. You have to determine if the HVAC system is operating in a way that will remove humidity (A/C needs to run relatively long periods to do this). Is the HVAC serving the attic or just the house (if air isn't being circulated from the attic, humidity may accumulate). You would benefit from having a skilled energy auditor/home performance contractor evaluate the whole thing. Finding a good one can be challenging, but they are out there.
I can't remember if you mentioned the type of foam. What brand and type is it, and how thick is it? Is your roof asphalt shingles, wood, metal, or..?
Oh, and it looks like Martin was typing while I was.
Yes, my appliances are natural gas. See attached photos. I will look into sealed-combustion units and in the mean time will get a low-level CO alarm.
My AC does a good job of keeping humidity below 60% in warm months. You make a good point about sources of moisture. I am diligent about minimizing sources inside the house but my house has a dirt crawlspace that is surely a moisture source. I solved a drainage issue in the fall, and am working on cleaning the area out so I can apply a membrane over the dirt. I have read countless articles about crawlspaces and making them “conditioned” spaces but that is a huge challenge again due to high rainfall and humidity here. The space is so limited that I can barely crawl around in it. If I can get the moisture level down, I will close the vents. That should help with the moisture through floor issue with my home. But, lowering moisture levels significantly will be work and I’m pretty sure I don’t want to run an HVAC duct and return to the crawlspace.
In answer to your last questions, my HVAC is servicing the house not the attic. I’m not sure I want to add a duct and return to attic because it is probably not as air-tight as the house. But, I can without too much trouble but appliances would have to be changed out first. My roof is asphalt shingles. Foam is 4+ inches (rafters are 2”x6”) and I don’t know type. I will look into Energy Auditor but it’s kind of like the 3rd world down here.
Thanks for the pics. I'm sure you will get useful comments based on them. I gotta run but will take a closer look later. I would *definitely* go after the crawl space vapor barrier--very important--and I don't know if I'd seal the vents.