Humidity control: what to do first?

[Hohn]

So if you condition the interior space, I suggest you also apply a poly vapor barrier to retard the moist air passing through the walls.

See post above. Vapor barrier in the midwest is almost always a bad idea unless you have somehow installed it in a way that it can never have condensation at any time all year round.

That's next to impossible to do with our climate.

 

[Hohn]

Ventilation. Lot's of it.
Zim

Probably needs the opposite. Sending the air inside to the outside means more air (and humidity) pulled in from outside.

That's counterproductive.

The only real option here is seal things up as air tight as possible and run all the dehumidification you can to try lower the dew point inside as low as you practically can.

Even if you have just a sheetmetal box, you can prevent moisture as long as 1) the interior dew point is always less than all surface temps, and 2) the surfaces are always warmer than any dewpoint they contact.
The last part is tricky when you're cranking up air conditioning on the inside to try to be comfortable and you have ripping hot metal sheets on the outside.
If you have a sheetmetal box that's leaky and crank up air conditioning inside, you'll get condensation (and mold) on the cold interior surfaces closest to the outside where that high dew point air is touching them before its been dried.

 

[Hohn]

Dehumidification is a really important part of any summer comfort-- air conditioning alone is NOT sufficient. A/C does dry air out a good bit, but most of the time you'll find you are running the thermostat much cooler than normal because you want dryer air, not because you are hot.
A single basement dehumidifier makes such a difference in the entire house that we can run the thermostat at 77-78 in this summer heat and be comfortable when before we'd need to run 74 or so.

 

[racecougar]

Dehumidification is a really important part of any summer comfort-- air conditioning alone is NOT sufficient. A/C does dry air out a good bit, but most of the time you'll find you are running the thermostat much cooler than normal because you want dryer air, not because you are hot.
A single basement dehumidifier makes such a difference in the entire house that we can run the thermostat at 77-78 in this summer heat and be comfortable when before we'd need to run 74 or so.

That gets back to running a load calc to size the mini split appropriately for the OP's specific conditions (humidity included). A good inverter system will run low and slow, which is excellent for pulling humidity down.

 

[zimman]

Probably needs the opposite. Sending the air inside to the outside means more air (and humidity) pulled in from outside.

That's counterproductive.

The only real option here is seal things up as air tight as possible and run all the dehumidification you can to try lower the dew point inside as low as you practically can.

Even if you have just a sheetmetal box, you can prevent moisture as long as 1) the interior dew point is always less than all surface temps, and 2) the surfaces are always warmer than any dewpoint they contact.
The last part is tricky when you're cranking up air conditioning on the inside to try to be comfortable and you have ripping hot metal sheets on the outside.
If you have a sheetmetal box that's leaky and crank up air conditioning inside, you'll get condensation (and mold) on the cold interior surfaces closest to the outside where that high dew point air is touching them before its been dried.

I'm not a rocket scientist like some on here. This guy has what amounts to an abandoned house with no AC or heat.
If you introduce AC and or Heat that's blowing air of whatever temperature. Blowing air whether hot or cold is ventilation in my simple mind. It's moving air.
You call it whatever with all the fancy words or percentages or formulas or whatever.
I'm good with what I said.
Zim

 

[dcg9381]

In your opinion, what should I do first? Any contractors you would recommend in the NE IN/NW OH/S MI area?
Is double bubble useful? I have a source that will give quite a lot of it.

Maybe nothing. Depends.

I had one "uninsulated" pole barn with zero vents (other than the barn doors, which were very leaky). I'm in TX, so it's usually hot, but when we'd get a cold snap (very rare) following high humidity high temp, I'd get condensation on the steel. Because hot humid air inside, condensed on cool steel.

I think this is sorta what's happening to you. Ground temp is very cool, you're getting high temps / high humidity and water is condensing on your "cold" slab.

I wouldn't try to wrap it. That's not going to change condensation on the floor. If I was going to "wrap" it, I'd do it with closed cell foam, but again, won't help condensation on the slab. Probably cheaper than trying to wrap it ineffectively from the inside and less expensive than pulling the exterior and wrapping it.

What I'd try (if I wanted to fix this) is installing mini-splits set on "dehumidify". Bonus is they provide heat and cooling.

 

[mikedodge]

You could always buy a used known working dehumidifier and see what happens.

No matter what you do if the walls aren't sealed properly you're not going to get optimal performance out of whatever you're running.

 

[Rusty Pilot]

Thanks again gents. I was going to ask what VB everyone likes but sounds like that's a bad idea. I have a commerical dehumidifier coming this week. I will start to figure out where and how to air seal the barn. The garage doors are probably going to the biggest pain.

 

[Fav Onefour]

It's truly amazing how many different suggestions you get.

The dehumidifier will help.
Try a little experiment while you're waiting. Set a simple box fan on the floor and let it run overnight. The concrete in front of the fan will dry out before the rest because it's heating up to the air temp. It's a great little experiment to show how a cold floor will have condensation when it's below dew point. In the big picture, a colder floor is nice in summer except for the condensation. The dehumidifier will help change the dew point.

In our area slab condensation is generally a short lived scenario. It usually lasts a day or less each round and happens a few times each summer. We try to manage the condensation for safety on smooth floors. It's pretty amazing how well airflow across the slab works. Even the simple step of opening the big doors and letting air blow through changes the slab temp. The area around the inflow doors dries first as that slab portion heats up.

The other option is to never let the slab cool. Nobody wants to go that route.

 

[Hohn]

A lot of folks are suggesting more ventilation, but that may actually make the issue worse. A others have said, the root cause of the condensation is the temperature of the floor dropping below the dew point of the air. Just looking at the weather for Indianapolis, the dew points are often in the mid 70s (ugh). If your floor is cooler than this it will condense the moisture from the air. If you ventilate the garage and bring in more moist air, you are just bringing in more water to condense. Later in the season, when the floor warms up the problem will likely fix itself.

Sealing up the garage on its own may help a little bit, as the air the air that is in the garage will condense, but without new air coming in there won't be as much moisture coming in to condense, but probably not a huge amount on it's own. Warming up the slab by opening up the garage at the hottest part of the day may also help some, but makes the garage less pleasant to be in. Or you could actively heat the floor, but also not great for the working environment.

Your best bet is probably AC/minisplits AND air sealing. The minisplits will reduce the humidity in the garage and keep the dew point above the temp of the floor. Air sealing will stop more moist air from entering the garage and make the minisplits have to work less hard.

This.

There's two sides of this. One is slab temp being below dew point. Other is dewpoint being above slab temp.

So you could tackle the issue from other angle-- lower the dew point or raise slab temp.

Short of adding radiant floor heating to the surface (and heating a slab in summer time seems incredibly wasteful), options for raising slab temp are limited in effectiveness.

Which leaves lowering the dew point. The means conditioning the space with a/c or dehumidifiers. And of course, that means air sealing since you can't effectively condition your entire zip code.

The more enclosing you can do, the less conditioning you will need. YOu can brute force it with gobs of dehumidificiation and a/c to overcome a terribly leaky enclosure, but there's no better example of the virtue of "crying once" than effective air sealing so you can condition the space.

If your slab condensation is a transient issue (like most are) where it's just the early day period where dew point is high and ground temps are still at the overnight low, you might try a bandaid the reduce the slab heat loss overnight.


You could try something like 1" XPS on the slab with advantech or similar on top as a subfloor. The idea being that the insulation both keeps keep the slab warmer overnight (and above dew point in the morning) while at the same time the XPS is a good vapor retarder, making it hard for the humidity to reach a condensing surface on the slab. The low permeance of XPS can be your friend if used thoughtfully.

Worth trying for you?

 

[sizem]

I agree w/ Hohn plus a couple of insights on floor temp management. I completed my residential retirement home which was built on a concrete slab. I opted to isolate the concrete flooring from the living environment from the concrete pad. I used 2 inch thick high density expanded polystyrene board from Menards which had a compressive load capability of 25psi. I used a vapor barrier under the concrete. Over the foam board I used two layers of advantech w/ each layer had joints staggered from the lower layer. In essence, the advantech is a floating floor. This provides a basis for finished floors (not tile) or none. To determine if a particular machine would cause problems w/ compression (creep) I used a typical 45 degree angle (advantech + foam = 3.5inches. This gives a comfortable 64x increase in load (psi ) capacity. For any loads greater than 30x I build a wood pad to further distribute the load under the machine. To date (3yrs), no a single issue and humidity issues from flooring has been none existent.

For Rusty Pilot, I would recommend a two step approach while collecting data. Definitely monitor temp and humidity in each room and outside this will provide a HVAC firm good data if you pursue the air conditioning/building sealing/insulation/wall condensation management.
First, use a moisture barrier sealant on the flooring. Monitor impacts.
Second step, if needed, add foam/flooring as above.
Good luck!

 

[cpakalolo]

AC it is... anyone have any recommendations? I am not sure if a mini split would suffice. Just the quick google search suggests that a 5 ton unit is what I would need.

Favorite (or hated) brands?

I worked in HVAC for a couple decades. I couldn't even recommend a brand anymore since they have all changed so much. I don't like mini-splits because they leak, are hard to work on and they end up producing unpaid bills. If you really like diy, Mr. Cool makes an air handler system that you can install yourself. It takes a little ductwork if you want vents and it is never a good idea to let an air handler get to much air. The motor will overheat.

 

[cpakalolo]

I'd go further-- the vapor barrier would be a disaster.

Vapor barriers only ever make sense where you have a single seasonal dominant vapor drive. Think northern Canada where you'll have a massive vapor drive from warm inside humid air to frozen desiccated air outside.

Or if you are in Miami and trying to keep out warm humid air in an almost always colder/drier interior space.

Here in Indiana, with both seasons capable of extremes, a vapor barrier just means condensation and mold. Put it on the inside, you get mold in summer. Put it on the outside, it molds in winter.

Skip the vapor barrier entirely.

The order of operations is this:
- Seal out water FIRST.
- Then seal out air
- Then address vapor.


The order matters because there' no point is sealing out vapor if you have air bringing more of it in. And there's no point sealing out air if you have water ingress.

If you want to control the atmosphere inside a space, you must first enclose it.

The easiest option for most of us that makes sense and is fairly cost effective is a flash-and-batt. This is a skim coat of closed cell spray foam just enough to seal things and then stuff your wall cavities with kraft-faced insulation. The kraft facing is a vapor *retarder*, not a barrier. That means it lets things dry out, but slowly. It slows the vapor movement. And since the spray foam is sealing water and air, you end up with a surprisingly effective setup that doesn't cost a ton compared to other options.

I will tend advice on using Kraft facing on insulation. If you can avoid it, don't use it. It is not just flammable. It's like storing gasoline near a fire. I was soldering on a system at a gas station's walk-in coolers. One small spark, and the flames started. In about 3 seconds, the fire was six feet across the whole ceiling. If there wasn't a water hose within 5 feet of the fire, I would have burned down the place. I was a volunteer firefighter for a couple years and learned of how many times kraft facing made it impossible to stop fire spread in an unfinished basement. If you might leave it up uncovered, you are playing with danger. Kraft facing makes a lousy vapor barrier too.

 

[Hohn]

Kraft facing is not intended for open walls near hot work, you are wise to point out the caution required.

It's not a vapor barrier, it's a retarder. That's why it's better than any vapor here. I've already explained why a vapor barrier is a terrible choice.

If you aren't sure why vapor barriers are a bad idea, it's worth reading up on. Look for the writings of Joe Lstiburek, the leading formulator of modern Building Science.

You Don't Need a Vapor Barrier (Probably) - Energy Vanguard

If there’s one building science topic that confuses more people in construction than any other, I think I’d say it’s the purposes and properties of the various control layers. I’m talking about the materials that are meant to control the…

www.energyvanguard.com

 

[mikedodge]

It's truly amazing how many different suggestions you get.

The dehumidifier will help.
Try a little experiment while you're waiting. Set a simple box fan on the floor and let it run overnight. The concrete in front of the fan will dry out before the rest because it's heating up to the air temp. It's a great little experiment to show how a cold floor will have condensation when it's below dew point. In the big picture, a colder floor is nice in summer except for the condensation. The dehumidifier will help change the dew point.

In our area slab condensation is generally a short lived scenario. It usually lasts a day or less each round and happens a few times each summer. We try to manage the condensation for safety on smooth floors. It's pretty amazing how well airflow across the slab works. Even the simple step of opening the big doors and letting air blow through changes the slab temp. The area around the inflow doors dries first as that slab portion heats up.

The other option is to never let the slab cool. Nobody wants to go that route.

No kidding on the different suggestions, that comes down to the multiple different environments people live in and their scenerios.
I know for mine if I'm in there regularely and the doors are open humidity isn't a problem. When Im not there it is. I was looking at other ways of dealing with it originally but running a dehumidifier does the job and is a cheap solution.

 

[cpakalolo]

Do a real load calculation, and decide if you want the mini just for cooling/dehumidification or heating as well. If you want heat (which I am guessing you do) that will probably drive the sizing as I am sure your heat loads are much higher than your cooling loads.

Also, remember this is a garage, not your house, so you don't necessarily need to plan for the worst possible scenario. If it is -10F out, I am not working in garage so it is fine if the garage is only 50F. If you are above 3 ton, then two independent systems may be your best bet - it is more versatile and efficient than a multizone system, and not much more expensive.

Just so you know, you can do your own manual J calculation for how much a/c to install. If your building is finished, it can be calculated. It would be neat if you posted online where we could all help you together. It is somewhat quirky to pick the U values for the different materials. At work, I would have a PC with the latest software from Carrier or Trane even though they are similar. I'm retired. The thing I usually find on sheds is that when you are done and give a number they say: Hey it's a shed for cryin' out loud. Which means I can't afford that much and less will do. Usually in a shed, one heats and cools it to a lesser extent. I worked up a quote a couple of years ago for a man who had a lot of hardwoods. I figured up everything for him because he did me a favor on the price of some Brazilian rosewood. When I sent it to him he said: heck I'm just going to put an electric water heater for my radiant heating. Imagine building out a liquid heated floor just to do the heat of a space heater. I don't think it worked for him, but I tried to tell him what it would take. Sheds are big. They are cheaper to cool than heat usually. Unless you live south of the 40 degree latitude line, of course. In the last post here that I read he said that you don't calculate a shed for the highest load you will see. That is certainly true for heat and mostly true for cooling. It seems that when people have a family get together, it will always fall on a really hot and humid day. I have been putting A/C in garages for a long time. Quite often, we are putting the used one from the house in the garage. I do not recommend this. If you are doing A/C, why put that inefficient thing back in service? HVAC companies almost never install used units because they usually fail soon after and you have to see it again on a service call. The worst thing a furnace or a\c could do is to last 30 years. They go bad before they quit, slowly losing efficiency and capacity. So, sometimes you know what you need, but you install what the customer wants because he is paying.