What's the deal with attic fans and humidistats?

Hap_Hazzard

We got a new roof installed over the summer, and the roofing company recommended installing humidity controls on the attic fans. At the time I thought this was a good idea, because I've seen a lot of condensation going on up there during the winter months, but now that I've been living with them for half a winter, I don't think they're helping. The more I think about it, I don't see how they possibly can.

If the humidity gets high enough to trigger the fans (I have them set at about 80%.), they blow air out of the attic, which draws air out of the house, through lighting fixtures and other openings in the ceiling. When the warmer air from the house enters the attic, it gets colder, and the relative humidity increases. I don't see how this helps. I've started going around all the light fixtures and sealing them up with caulking and foam insulation, but it seems unlikely that I'll be able to find everything, and even if I do, that will just make the vent fans less effective.

I keep thinking something's missing, like soffet vents, but if they're necessary, the roofers didn't mention that. I just don't feel like I have a real grasp of how these things are supposed to work, and I don't think the roofers did either.

STEVEusaPA

If you have soffit vents, a ridge vent (or a gable vent, but not both), and a properly insulated attic floor, you don't need a fan.
It's doing exactly what it's supposed to do, make living quarters negative and pulling in fresh humid air.
It can also affect your draft in a bad way, on all natural draft appliances, possibly pulling products of combustion down the chimney and into the living spaces.

Hap_Hazzard

So what's the best way to fix this, given that I have two attic fans, no ridge vents or soffit vents, and an attic floor that has insulation but no vapor barrier?

Jamie Hall

You need soffit vents.

Hap_Hazzard

Jamie Hall said:

You need soffit vents.

First I'd need soffits to put 'em in. There's not much overhang. I think that's why the roofers didn't install vents there.

Hap_Hazzard

I do have an atmospheric boiler, but it's in the basement, and the house is pretty far from airtight, so I think the draft would be pretty loosely coupled to the attic fans. But I'm trying to minimize the amount of time they have to run. Before they installed the humidity controls they didn't run at all in the winter, but there was a humidity problem.

Maybe the best way forward is to take out the humidity controls and install a vapor barrier.

pecmsg

Gable Vents?

ChrisJ

You have two fans blowing out but no way for air to get in?

That seems kind of................you know......

KC_Jones

Just had a new roof put on last summer. Since I didn't have soffit vents they cut vents into the sheathing of the roof down low to function as the air intake, then added the ridge vent that I never had. The vents down low cut a slot in the sheathing, then put a vent similar to a ridge vent over the opening, then it was shingled over.

I'm surprised they didn't add anything or at least recommend, as far as I know all manufacturers warranties require proper venting. I had it all done and got 25 year warranty from the contractor and lifetime from the manufacturer.

For me the passive ventilation is the way to go, since there isn't much to go wrong. I can tell you for sure my attic was much cooler after they did this than it was with the old roof and no venting.

Hap_Hazzard

I got a lifetime guarantee on the roof too, and they did cut some slots in the sheathing, but they put a blue membrane over everything before they installed the metal roofing, so I don't know if any air is getting through that.

I'd go for passive ventilation too, if I had my druthers, but I already had a vent fan, and they insisted on installing a second one as a condition of the warranty.

Hap_Hazzard

So, anyway, suppose there was a way to get outside air to flow in when the fans run, I'm not clear on how that's going to bring the humidity down, especially when it's like 100% humidity and raining out.

STEVEusaPA

Got a picture of the outside of your house? Or should I drive over...lol
I don't think there's going to be any humidity reduction. It's more about letting the attic breathe.
Although technically if you're pulling lower humidity air out of your living space and dumping it into the attic, that's probably where you're seeing the moisture.
You have a lifetime warranty on the roof, but it's not going to cover rotted sheathing, mold and other issues not related to a roof leak.
Another, probably expensive, option is to spray foam the entire underside of the roof, now it's a conditioned space, ditch the fans.

Hap_Hazzard

I'm pretty sure insulating under the roof would void the warranty, because it would allow the sheathing to get really hot in the summer. The plywood would probably delaminate.

pecmsg

The RH really doesn’t matter as long as it doesn’t condense. That’s what I don’t understand.

mattmia2

Most of the function of attic ventilation is to keep the roof surface cool. In winter to prevent ice damming, in the summer to keep from cooking the shingles and sheathing.

There is a vent strip you can cut in at the edge of the roof that is a combination vent and drip edge. This might tend to pull snow in as well but there isn't a whole lot you can do without building a soffit.

The ridge vent near the bottom of the roof is likely to get covered and blocked with snow when you really need it to keep a heavy snow from becoming an ice dam (as do ridge vents). Ordinary vents that stick up from the roof tend to do better in snow. I have seen the low roof vents strategy tried, not sure how successful it was.

Steamhead

@Hap_Hazzard , does your house have a "close cornice"? Post a pic if you're not sure................

ChrisJ

> @Steamhead said:
> @Hap_Hazzard , does your house have a "close cornice"? Post a pic if you're not sure................

Closed cornice meaning the rafters aren't visible from the outside?

Steamhead

ChrisJ said:

> @Steamhead said:

> @Hap_Hazzard , does your house have a "close cornice"? Post a pic if you're not sure................



Closed cornice meaning the rafters aren't visible from the outside?

Illustration is here:

https://en.wikipedia.org/wiki/Cornice

ChrisJ

> @Steamhead said:
> (Quote)
> Illustration is here:
>
> https://en.wikipedia.org/wiki/Cornice

Hap_Hazzard

The roof is in two sections, because an addition was built about 20 years after the house was built. As near as I can tell, the original 1940 roof is a box cornice and the newer 1960 roof is a close cornice. In the picture, below, the newer roof is on the left.

The attic is also in two sections, with a hole big enough to crawl through in the partition. The most serious condensation issues, as it turns out, are in the newer part of the attic. So, I think it might be possible to put soffit vents in the older section, it probably wouldn't do much good.

Hap_Hazzard

Sorry the picture isn't very clear. It's dark out. I'll try to take a better one tomorrow.

Steamhead

OK, @Hap_Hazzard - it certainly looks like a close cornice. My parents' 1957-vintage house had this type of cornice on the rear shed dormer. When the summer sun came up and its rays hit the roof, the bedrooms and bathroom underneath flashed hot and all the window A/C units in the world couldn't keep the rooms bearable.

Eventually I figured out a way to get vents onto that cornice. Basically, we took a 1x6 plank and cut inverted U-shaped openings at the center of each rafter space. We mounted that plank to the fascia board (where the gutter attaches to the house) after painting it on both sides, and used a hole-saw to make holes in the original fascia to match the inverted U-shaped cutouts. We then mounted grilles in the holes- ISTR they were 2" diameter, and looked something like this:

https://www.homedepot.com/p/Master-Flow-2-in-Resin-Circular-Mini-Wall-Louver-Soffit-Vent-in-White-6-Pack-RLSC2/100090036?mtc=Shopping-B-F_D22-G-D22-22_10_ROOFING-Generic-NA-Feed-LIA-NA-NA-&cm_mmc=Shopping-B-F_D22-G-D22-22_10_ROOFING-Generic-NA-Feed-LIA-NA-NA--71700000044086327-58700004607994977-92700044043517442&gclid=CjwKCAiA98TxBRBtEiwAVRLqu1CTEgUIS_PN_tREaYEruzJRC8gErocptvRZ7oJnm__WIINc5JQjvxoC7ToQAvD_BwE&gclsrc=aw.ds

I've attached a back-of-the-envelope drawing of what this looked like- remember, I'm no artist.

Then we mounted the gutter to the board with the cutouts, and (somehow- I don't remember exactly) extended the roof to keep it draining into the gutter.

This setup allowed air to come up in back of the gutter and into the vents, where it cooled both the roof and the ceiling below, exiting through the gable vents. Problem solved- not just the heat retention, but also the chronically peeling paint which I'm sure was due to moisture buildup.

mattmia2

This is what i used on my mom's 1957 house with the same problem:
http://www.airvent.com/products/intake-vents/vented-drip-edge

With either solution there is some chance the air can blow over the snow in the gutter and bring it inside the attic.

EBEBRATT-Ed

@Hap_Hazzard as @mattmia2 said your trying to let the attic breathe. I deally you want the attic to be the same temp as outdoors. Ventilation or fan will keep the roofing cooler and avoid heat build up in the attic making th house cooler in the summer

In the winter you want the attic as cold as outside to prevent ice dams. Condensation isn't usually an issue if it can breathe

Hap_Hazzard

I should probably mention that the attic doesn't get excessively hot in the summer. If I set the temperature control to 100° the fans come on most days when the sun is out, but the temperature doesn't get much higher than 100°. Before the second fan was added it was a little unbearable, but it didn't make the room below noticeably warmer, probably because the attic floor is amply insulated.

The only real problem I have now is the condensation in the newer section of the attic, so I think I need to continue calking all the light fixtures and install a vapor barrier in the newer section, then see if I can find a way to get some fresh air in so the fans don't keep sucking air out of the house. Soffit vents might be doable in the old section, and a gable vent might work for the newer section.

Steamhead

@Hap_Hazzard , you need to add those vents. If you're exhausting air, then more air has to come in from somewhere. You don't want it coming from a conditioned space. Caulking will help but isn't the whole answer.

@mattmia2 , I wonder what would happen if the gutter below that drip edge got backed up and the wind blew hard enough? It would blow the water right into the attic.

Hap_Hazzard

Here's a better picture:


I might be able to fit the 2" soffit vents in the old section, but there's really not much soffit on the newer part. If I take off that vinyl trim I'll be able to get a better look at what I have to work with. Maybe I can open up the end of the cornice and then replace the vinyl trim to keep water from blowing in if there's an updraft.

Solid_Fuel_Man

Plenty of room for soffit vent. Just replace all that with perforated vinyl. You'll have to hole saw it slice any wood that the vinyl is covering, depending on the age of the house if it was originally vinyl or it's a cover up.

Hap_Hazzard

Oh, it's a cover-up. Hides a multitude of sins.

That's pretty much the approach I'm thinking of taking, only I'm not sure if I'll even need perforated vinyl. Plenty of air should be able to leak through between the vinyl soffit and the j-channels. If it makes the soffit flutter when the fan comes on I'll consider using the perforated stuff.

Zman

In some cases mechanically venting the attic is necessary. You do need to have adequate intake vents so the attic does not become negative to the house.

mattmia2

@Steamhead It seems like the approach of getting vents in the face of the soffit above the gutter would also allow water to blow in(Unless i'm misunderstanding where the vents were). Ideally if the drip edge vent is installed properly the slot would be in the sheathing and any water that gets sucked in would run out down the face of the fascia.

You could do what my uncle that was a glazier did and build out a soffit on the eaves...

JUGHNE

Often the tail end of the roof rafters that sits on the outside walls has a "birds mouth" cut on it to key it onto the top of the wall plate. That leaves little space between the roof sheathing and the top wall plate. Often that is packed full of insulation.
Removing that insulation to get air ventilation thru that point leaves the ceiling uncovered, even if the vent troughs are used, (the styrofoam ones that look like a reflector for a shop light), the insulation thickness is greatly diminished.
Here we use an energy truss that increases that height above the outside wall for more insulation depth.....but.

I have a rental with your roof/soffit construction, I put a power vent up high as possible and then the static turtle type vents down low about 2' above the gutter. It is a steep roof and have no problem with snow or ice dams.

This kept the insulation and cornice/soffit intact. Was the least amount of work.


On another note, a HO called to say his power vent would shut off during the day and start later that night. The vent fan was on one side of the attic and his only large soffit inlet on the other. The inlet was covered with window screen. Over the years the screen plugged with lint etc. The fan motor would over heat and go off on high limit and restart after cooling down.
Cleaned the lint screen off with a broom and he never called back.

The point being that he had no inlet air into the attic. House build in the early 70's.....so so tightness on ceiling leaks.

Missed a chance to change his attic vent fan and better yet missed getting called back as to why the new one would have done the same thing.

BTW, this was a no charge as I didn't do anything to justify getting paid for. Another small town thing one might do for old folk neighbor friend. So small that every one is a neighbor here.

Grateful to not go into the attic or on the roof.

STEVEusaPA

If you change the soffit to a vented vinyl soffit, you can problaby slip the baffles in from the outside.

JUGHNE

FWIW, some people who have done that here, with the baffle/trough every 4' or so will get frost spots on the ceiling at zero or below temps. With the thinner insulation and wind blowing into the vented soffit that produces those cold spots.

That was another reason for the lower static roof vents on my old rental roof.

mattmia2

BTW, the best way to seal up the light junction boxes is with plastic sheet from the attic (as long as they aren't can lights that need the airflow). Caulk/staple the sheet to the back of the ceiling and slit and tape or caulk it around the romex.

Steamhead

mattmia2 said:

@Steamhead It seems like the approach of getting vents in the face of the soffit above the gutter would also allow water to blow in(Unless i'm misunderstanding where the vents were). Ideally if the drip edge vent is installed properly the slot would be in the sheathing and any water that gets sucked in would run out down the face of the fascia.

You could do what my uncle that was a glazier did and build out a soffit on the eaves...

My vents were behind the gutter, not above it. The inverted U-shaped cutouts allowed air to reach the vents from underneath. This way, no water could get in them.

Solid_Fuel_Man

Can lights into an unconditional attic are about the most wasteful pieces of junk!