Graph shows ping-pong moisture with open cell foam in attic
Not really a question, but here’s a nifty visual from my attic-mounted temp/humidity sensor. Location is New Orleans, conditioned attic with open-cell spray foam on the roof decking.
The graph shows attic humidity spiking in the afternoon as the roof surface heats up on sunny days (Thurs & Fri) with somewhat low outdoor humidity. But the attic humidity stayed relatively low on Saturday through Monday, when conditions outside were extremely humid and overcast as we were impacted by Tropical Storm Cristobal. You can clearly see that we had much higher attic humidity on sunny days with 50-60% exterior humidity than we did with rainy days and 90% humidity.
If you didn’t know about ping-pong moisture, you’d think the inside of the house would be more humid on humid overcast days when the a/c doesn’t run, and less humid on hot days when the a/c runs a lot. And that’s true for the living space, but not the attic.
FYI, the open-cell foam was sprayed in the attic last winter, and I installed the sensor to monitor. I have one smallish HVAC vent into the attic, and you can see its effect as the humidity rises until about 1 p.m. because the a/c doesn’t run much early, then humidity plummets once the a/c starts running more frequently around 2 p.m. Generally the humidity in the house and attic is around 60%, which is typical for this area. I may add another vent into the opposite end of the attic at some point.
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Since your complaint is moisture in the attic and not moisture in your house. We must conclude you only went halfway to building a conditioned attic. I suspect you isolated the attic from the outdoors but failed to connect the attic to the indoor.
It is a bitter pill but you must take it.
Walta
See original message. There is an a/c duct into the attic, but the graph shows me it's not quite enough, so I'm going to put in a second vent. It's not a complaint, just showing the graph documenting the issue clearly given outdoor conditions. These little devices that do the remote measuring are pretty cool:
https://www.amazon.com/gp/product/B01AEQ9X9I/ref=ox_sc_saved_title_2?smid=A3FMIYHRLE9HNB&psc=1
You point out the big flaw in "put a HVAC vent in the attic to condition it". When the HVAC doesn't run, the attic isn't conditioned. Same problem with crawlspaces.
Standalone dehumidifiers don't suffer from this problem.
Agreed. Lstiburek & company recommend either an HVAC vent that operates intermittently, or a 24/7 vent from the living space into the attic at a much lower air flow. I went with the HVAC because it was easier in my case, but the 24/7 vent is still an option. Right now the air vent in the attic is not far enough from the passive air return to the living area, about 8-10 feet, so I'm going to duct one or the other to the opposite end of the attic (30 feet). That way I'll get more conditioned air spread around the attic, because some of it is probably short-circuiting right now. If that doesn't work, I'll add a second a/c vent to increase the CFM into the attic. By spray-foaming the roof and thereby moving the ducts and air handler into enclosed space, I've made the a/c work much better, which means it's a little oversized, which means I can spare an extra duct in the attic.
Another issue with my setup is that the dehumidification is needed most in the spring, when the a/c doesn't run as much, so that would be a another indication that a 24/7 vent might be better ... but that would require cutting another hole that might not be necessary, so I'm doing a wait-and-see. I didn't get a standalone dehumidifier because of energy use, but it's an option if other solutions don't work.
Please post your data as you make changes. But will need some fairly long time periods to make accurate conclusions.
With open cell spf you'll see moisture pool at the ridges of the roof. It was called the ping pong effect. Moisture would be driven out of the foam via daytime heating and then crawl along the surface and pool at the ridges. This was a hypothesis regarding why roof failures were occurring at the ridge of non-vented attics. Building Science Corp has an article on it and it was why they recommended some form of ventilation in the attic with ocSPF.
Closed cell, IF YOU MUST USE SPF, Cellulose on the attic floor OR rigid foam on the exterior side of roof solve these issues 100 percent.
I would think, in NOLA, you would want all exterior RECLAIMED rigid foam on roof and walls from a cost perspective. and while I haven't run the numbers I dont imagine it would take near as much as us northern guys.
Rigid foam on the exterior is the long-term plan -- it only takes a tiny amount here, around R-5 I think -- but I won't need to re-roof for 10-15 years (knock on wood for hurricane season). Maybe by 2035 there will be at least one roofer in town who knows how to install exterior foam board.
And no one here sprays the eco-friendly closed-cell blowing agent, either. So I did open-cell and planned from the start to engineer around the moisture problem, as Lstiburek et al. recommend. It's just a matter of figuring out which method works in my setup.
An update to the original message and graph. The new graph shows what happened when I installed a stand-alone dehumidifier. As you'd expect, attic temp went up a bit and humidity went down a lot and stabilized. The moisture ping-ponging has been tamed.
I got the dehumidifier after listening to the BS & Beer podcast on dehumidification. The podcast guests said that a dehumidifier is necessary almost everywhere in the southeastern U.S. Probably more so in New Orleans, I figure. I had plans to slowly test other possibilities as detailed above, with a dehumidifier as a last resort, but if experts say I need a dehumidifier, might as well do it now.
I went with an LG 50-pint model, which was well-reviewed at Home Depot. I installed it in the attic and connected it to a vent stack via a garden hose and PVC adapters. In the first two weeks it used about $2 of electricity per week, which is manageable. I can't say I've felt a humidity difference in the living area, but the humidity readouts on my Nest thermostats seem to be down by about 5%, to the mid-to-low 50's. I'm testing to see if I can now raise the thermostat by a degree and maintain comfort. So far, so good!
$2/week in electricity is not bad for that kind of humidity control. Thanks for the update.
6-month update: here are some current graphs of the temperature and humidity in my attic. You can see on the humidity graphs exactly when I installed the dehumidifier, and how consistently it has worked to solve the moisture problem.
Executive summary: Open-cell foam in Zone 2A, leading to ping-pong humidity problems in attic in spring when I stopped using the heater. Humidity problem did not improve as much as expected once summer came and a/c was running regularly, with one a/c vent adding conditioned air to the attic. Moisture problem was solved by $300 LG dehumidifier that costs about $2 per week to operate.
Thanks for posting. Just to add a data point, I'm in zone 4a with open cell foam in an attic, and my experience mirrors yours almost exactly. Where I live, for whatever reason, spray foam companies hardly ever use closed cell. 2 of the 3 I talked to wouldn't even give me a quote on closed cell. The one that did give me a quote priced closed cell at literally at 3x higher than the open cell. Overall, I'm happy with the performance of the OC and can live with spending a few dollars per month on a dehumidifier.
Hi, thanks for posting.
Questions
1. I have a dried in house (passive solar) and I want to start monitoring temp and humidity
I don't have internet. Can the sensor push store data to be retrieved later? Like maybe once daily?
2. Which lg dehumidifier did you go with? Thanks
1) I believe my Sensor Push communicates directly with my iPhone via Bluetooth. I have the basic version that was about $50, and the coin-cell battery has lasted a year (so far). There was a wifi option with a base station, but I didn't get that one.
2) I bought the dehumidifier from Home Depot for $300. My e-mail receipt identifies it as the "LG Electronics PuriCare 50-Pint Clear Bucket with Handle // Store SKU # 1004649823 // Internet # 310828348."
Many thanks!
I ordered two bluetooth hygrometers and I'm super excited to start logging