Does Heating Air reduce moisture content? (For HVAC Folks...and other smart people..

[Philster]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Part of the confusion comes from the fact that in winter, the air all around the area of a cold weather town is cold/dry. It's the nature of the beast.

So, if you heat cold/dry air and don't add moisture to it -- moisture that hotter air can handle -- then it's hot/dry air. I mean, it's winter, so the air you are starting out with is dry. Sure, warmer air holds more moisture, but where's it coming from unless you humidify it?

In summer, we are starting out surrounded by hot/warm air, which starts out with more moisture, 'cause hey, it's the nature of the beast. The good thing is that just cooling it gets it to drop its moisture... so cooling also dries the air, provided the cooling system runs long enough and isn't oversized.

 

[bigdee]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Part of the confusion comes from the fact that in winter, the air all around the area of a cold weather town is cold/dry. It's the nature of the beast.

So, if you heat cold/dry air and don't add moisture to it -- moisture that hotter air can handle -- then it's hot/dry air. I mean, it's winter, so the air you are starting out with is dry. Sure, warmer air holds more moisture, but where's it coming from unless you humidify it?

In summer, we are starting out surrounded by hot/warm air, which starts out with more moisture, 'cause hey, it's the nature of the beast. The good thing is that just cooling it gets it to drop its moisture... so cooling also dries the air, provided the cooling system runs long enough and isn't oversized.

Good answer.....heat does not remove moistue. The heated air in your home is simply attracted by the natural law of physics to flow to a low pressure area (colder air) and in the process carries the moisture with it. Think of the cold air outside your house as a giant dehumidifier!

 

[Tim Frank]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Also, attic insulation should not have the vapor barrier against the living space. People will use paper-backed batt insulation in this application, but that doesn't make it right. Insulation against the living space (attic structure, not exterior walls), should always be loose fiber, or cellulose (old stuff), asbestos (older) or wood chips, (even older). I've even seen shredded newspaper treated with a flame retardant (c. 1930's).

Just so you know, I didn't stay at a Holiday Inn Express, last night, but I spent 12 years as a home inspector.

Wow...
I'll assume that you honestly believe that to be correct, I won't ask where you came up with that monumental clanger, and I won't question your credentials or competence; but since you are disagreeing with building codes and experts all across North America and the world, where would YOU put that vapour barrier?

 

[Beefer]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Wow...
I'll assume that you honestly believe that to be correct, I won't ask where you came up with that monumental clanger, and I won't question your credentials or competence; but since you are disagreeing with building codes and experts all across North America and the world, where would YOU put that vapour barrier?

First of all YOU need to show me where the code requires a moisture barrier be installed for attic floor insulation. Sorry, but don't start telling me about building codes. Want to know what the building code requires? A minimum R value rating. That's it, nothing more, nothing less. So, no, I don't disagree with codes (well, not this one at least). Yes, they sell and install in attic floors lots and lots and lots of barrier backed batt insulation. Does that make it the best practice? Not a chance. They also sold and installed lots of LP (Louisiana-Pacific) siding, Quest (polybutylene) plumbing systems, Chinese drywall, lead paint, asbestos pipe wrap, and hoards of other materials and products that were sold and installed improperly. Doesn't make it right.

My argument against moisture barrier insulation on the attic floor is that the moisture will not be vented properly, will condense, can come back through the ceiling, and can create an excessive moisture issue up there (read as mold). This is why in the better built homes you will find loose fill blown-in insulation, of which uses no moisture barrier (did all those builders/manufacturers/installers violate code???).

A 'Healthy House' is one that readily allows for the removal of moisture, and the prevention of moisture intrusion. Vapor barrier plays a huge role in this when installed correctly in the right areas. The attic floor is not one of them. You want the moisture to be able to escape the building envelope, not get trapped inside of it.

Even though you don't want my credentials, here's a bit - Licensed home inspector in 3 states, licensed GC in 2, Mold certified in 3 states, pest certified in 2 states (3rd didn't have a certification in effect), and a certified roofing inspector. As far as experience, I have personally crawled through over 11,000 attics, so I have seen lots of types of insulation, and lots of problems related to all sorts of insulation problems.

Now, of course, there may be a Canadian Building Code that requires a vapor/moisture barrier in an attic, but I highly doubt that, as I did some of my training in Mississaugua.

 

[Beefer]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Oh, and as to your question of where I would put the moisture barrier - in an attic I wouldn't use it at all. Exterior walls and crawlspace are the only places I would recommend moisture/vapor barrier.

 

[rbh]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Oh, and as to your question of where I would put the moisture barrier - in an attic I wouldn't use it at all. Exterior walls and crawlspace are the only places I would recommend moisture/vapor barrier.

YO BEEFER, hows it going!!


Anyways, as I have always been taught the vapour barrier goes on the warm side of the studs, joists, trusses, you do not want the moisture of a home permiating the insulation as this just sets up an area for mold to grow this is why you tape the polys joints and around outlets.
The cold side of the roof trusses were the insulation goes needs air flow, IE vents to move any trapped warm air/moisture out. An attic can be insulated or non insulated, but if heated it needs a vapour barrier and the insulation needs to breath on the cold side, due to heat that builds up from the roof. (rolling shingles ETC)

 

[Beefer]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Don't have many heated attic spaces here in the US, don't know about in Canada. As for the first half of your statement, then why is there no moisture barrier when using blown loose insulation? I don't mean to rub people the wrong way, but just because it's how it's always been done, doesn't mean it's a best practice. The condensation that can accumulate on the attic side between the insulation and interior living space surface (ie drywall) has to be able to escape. I really think that the whole moisture barrier in the attic floor was started by a company that wanted to sell more paper back batt insulation, and that became the norm. Remember, before batt insulation, it was either loose cellulose, asbestos, or shredded what-not. Then came batt (backed and non-backed), and now we're using loose again, with no barrier.

And yes, if you were to insulate between the rafters, then you'd use the moisture barrier, but again, insulating that isn't the best way to insulate an attic. Yes, there are some instances where there really isn't a choice, but in a typical attic, floor insulation provides the best thermal protection for the living space, and the best overall environment for the attic/roof structure.

Also, just wanted to say that the quote that Tim Frank used, probably should have read: Moisture barrier is not required/needed/necessary in attic floor insulation. Maybe that will clarify a little.

 

[nwcove]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

wow....i live in the frozen north, and the bank wouldnt consider giving a loan for new consruction unless it was r-2000 built. thats vapour barrier with acoustic sealant on every seam of the barrier. heating moisture laden air is a waste, and causes mold and mildew issues. air exchangers are code in this neck of the woods, they dump the humidity of an airtight home, but recover some of the heat. ( most people disable them tho, a humid house feels warmer,.....just like the outside with a humidex value)

 

[rbh]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Don't have many heated attic spaces here in the US, don't know about in Canada. As for the first half of your statement, then why is there no moisture barrier when using blown loose insulation? I don't mean to rub people the wrong way, but just because it's how it's always been done, doesn't mean it's a best practice. The condensation that can accumulate on the attic side between the insulation and interior living space surface (ie drywall) has to be able to escape. I really think that the whole moisture barrier in the attic floor was started by a company that wanted to sell more paper back batt insulation, and that became the norm. Remember, before batt insulation, it was either loose cellulose, asbestos, or shredded what-not. Then came batt (backed and non-backed), and now we're using loose again, with no barrier.

And yes, if you were to insulate between the rafters, then you'd use the moisture barrier, but again, insulating that isn't the best way to insulate an attic. Yes, there are some instances where there really isn't a choice, but in a typical attic, floor insulation provides the best thermal protection for the living space, and the best overall environment for the attic/roof structure.

Also, just wanted to say that the quote that Tim Frank used, probably should have read: Moisture barrier is not required/needed/necessary in attic floor insulation. Maybe that will clarify a little.

Maybe we are just talking code depending were you live (do not know why?)

In the winter time in the northern climates the humidity in a house compared to the outside is huge!
Your doors and windows are not open to get a good air flow moving so in some instences the air is stagnent and if you have a humidifier for your furnace (or you cook alot), or you have a lot of people in your house (normal person exhales over 2 liters of water in their breath a day)

AH, just read wikis take on it bud!!

http://en.wikipedia.org/wiki/Vapor_barrier

 

[hrdwrkingacguy]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Let me say up front that this post was 2 pages long when I started it and I didn't read every response as thoroughly as I would have liked...But as for the original queston "where does moisture go when something is heated"...From an AC perspective no where...Relative humidity is just that relative...If you have a cup of water in a two cup measuring cup you essentially have 50% relative humidity(I know assuming the cup of water was actually a percentage of moisture vapor in a certain amount of air)...When you warm up the air, the airs capacity to hold moisture increases...So essentially if the measuring cup is two cups at 60 degrees it would be more at 70 degrees...So at 80 degrees it might be 4 cups and your relative humidity would be 25%...The amount of water didn't change, but the container that holds it gets bigger and smaller...

Moisture in the air is called latent heat and the only way to get rid of it is by moving it into a medium that is colder(Thats what AC units do)...

If you notice that at 70 degrees your house is 45% Relative humidity and at 80 degrees its its 38% you still have the same amount of moisture in the house...Your house can just hold more moisture at the higher temperature...

BTW one of the byproducts of combustion is water...So the steam or moisture coming out of a flue on a furnace isn't(exactly) the moisture in the house...

 

[Beefer]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

I have no argument with anything that is referenced above, including the external links. However, they are talking about exterior wall insulation, and not about attic structures. One thing that leans towards what I'm saying is

A vapor barrier on the warm side of the envelope must be combined with a venting path on the cold side of the insulation.

So with vapor barrier on the interior ceiling side of the attic floor, a means to eliminate the moisture that accumulates must be present. Isn't that the whole point of what I've been saying? Vapor barrier on the attic floor will allow/promote the accumulation of trapped moisture.

It is quite possible that we are all right, as I'm in the US (live in FL now, but my licenses are also for Oregon and Washington). I'm starting to think there is a difference in the recommendations (not requirements) between US and Canada homes. But I'll bet even in the Canadian application, there is a means for moisture elimination, because moisture accumulation occurs. I don't know. But I do know that any amount of accumulated moisture in the envelope is not a good thing, and by not having the vapor barrier, you reduce the chance of that moisture accumulating, and the related negative effects of such.

Just to reiterate, I whole-heartedly agree that exterior walls not only should have a moisture barrier, but need it. You climate decides how it's installed. But I also firmly stand by what I've said all along about attic floor insulation. Haven't you seen stains in ceilings from moisture that was trapped by the barrier? Or mildew under the insulation? Like I said, it's possible in Canada that this is not an issue, but it is in the (relatively) warmer US.

 

[captmello]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Haven't you seen stains in ceilings from moisture that was trapped by the barrier? Or mildew under the insulation? View attachment 127679

As long as the attic space above the vapor barrier is properly insulated, there should be no consensation accumulating. The ideal situation is the point at which the temp drops to the dew point, the point in which the water vapor will condense into water, is happening somewhere in your attic insulation where there is no water vapor present because of the vapor barrier keeping all the moisture out of the insulation above. The staining on the ceilings are the result of a lack of insulation above the vapor barrier, allowing the temp at the vapor barrier to drop to the dew point.

Someone mentioned all the moisture coming from inside the home. This is why we need to run our bath fans and run your heat exchangers if you have one.

IMHO...

 

[bigdee]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

As long as the attic space above the vapor barrier is properly insulated, there should be no consensation accumulating. The ideal situation is the point at which the temp drops to the dew point, the point in which the water vapor will condense into water, is happening somewhere in your attic insulation where there is no water vapor present because of the vapor barrier keeping all the moisture out of the insulation above. The staining on the ceilings are the result of a lack of insulation above the vapor barrier, allowing the temp at the vapor barrier to drop to the dew point.

Someone mentioned all the moisture coming from inside the home. This is why we need to run our bath fans and run your heat exchangers if you have one.

IMHO...

Agree+ dead-on answer. Also if there is NO vapor barrier and there is a crack or leak in the ceiling moist air can condense on or in the insulation and create serious moisture problems ABOVE the ceiling. But remember, humidity levels are lower during winter so this is usually not an issue except in bathrooms and kitchens.....these areas definetly need a vapor barrier if you are in a cold climate.

 

[Tim Frank]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

First of all YOU need to show me where the code requires a moisture barrier be installed for attic floor insulation.

I am not sure that I "need to show" you anything, but:
From the NBC
9.25.4.3. Installation of Vapour Barriers
1) Vapour barriers shall be installed to
protect the entire surfaces of thermally insulated
wall, ceiling and floor assemblies.
2) Vapour barriers shall be installed
sufficiently close to the warm side of insulation
to prevent condensation

Sorry, but don't start telling me about building codes.

OK.

Want to know what the building code requires? A minimum R value rating.That's it, nothing more, nothing less. So, no, I don't disagree with codes (well, not this one at least). Yes, they sell and install in attic floors lots and lots and lots of barrier backed batt insulation. Does that make it the best practice? Not a chance.

Uhhhh....so why are YOU telling me?...

They also sold and installed lots of LP (Louisiana-Pacific) siding, Quest (polybutylene) plumbing systems, Chinese drywall, lead paint, asbestos pipe wrap, and hoards of other materials and products that were sold and installed improperly. Doesn't make it right.

Not sure why you are bringing this up, and not sure who exactly "THEY " are, but if you are saying bad material and advice can get out there, I certainly agree. You missed one other notable...Urea formaldehyde on the material side was a huge fiasco; now, if we can just jump on this no attic- vapour-barrier craziness on the advice side....

My argument against moisture barrier insulation on the attic floor is that the moisture will not be vented properly, will condense, can come back through the ceiling, and can create an excessive moisture issue up there (read as mold). This is why in the better built homes you will find loose fill blown-in insulation, of which uses no moisture barrier (did all those builders/manufacturers/installers violate code???).

That argument is incorrect and your suggested procedure worsens the problem. That is why it is not recommended practice by qualified experts. Unchecked moisture infiltration into attic insulation, and eventually into the attic space is a far worse problem and causes many millions of dollars in damage annually.

A 'Healthy House' is one that readily allows for the removal of moisture, and the prevention of moisture intrusion. Vapor barrier plays a huge role in this when installed correctly in the right areas.

Agree 100%

The attic floor is not one of them.

Actually, it assuredly is.

You want the moisture to be able to escape the building envelope, not get trapped inside of it.

OK, let me understand this. The attic is NOT within the building envelope so it is OK to allow moisture to build up in that space?

Even though you don't want my credentials, here's a bit - .....

Not relevant, especially on an internet forum where of course EVERYONE is an expert, which is why I did not ask. I am NOT saying that you are unqualified ....just that you are wrong.

Now, of course, there may be a Canadian Building Code that requires a vapor/moisture barrier in an attic, but I highly doubt that, as I did some of my training in Mississaugua.

...and of course learned everything....and stayed at a Holiday Inn....

 

[Tim Frank]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

I have no argument with anything that is referenced above, including the external links. However, they are talking about exterior wall insulation, and not about attic structures. One thing that leans towards what I'm saying is

A vapor barrier on the warm side of the envelope must be combined with a venting path on the cold side of the insulation

.
So with vapor barrier on the interior ceiling side of the attic floor, a means to eliminate the moisture that accumulates must be present. Isn't that the whole point of what I've been saying? Vapor barrier on the attic floor will allow/promote the accumulation of trapped moisture.

No. They mean lots of attic space exterior venting. The current wisdom is that "too much attic ventilation should just about be enough".

It is quite possible that we are all right, as I'm in the US (live in FL now, but my licenses are also for Oregon and Washington). I'm starting to think there is a difference in the recommendations (not requirements) between US and Canada homes. But I'll bet even in the Canadian application, there is a means for moisture elimination, because moisture accumulation occurs. I don't know. But I do know that any amount of accumulated moisture in the envelope is not a good thing, and by not having the vapor barrier, you reduce the chance of that moisture accumulating, and the related negative effects of such.

As discussed, no again. Moisture elimination/reduction is completely separate issue from moisture containment. Heat exchangers and exhaust fans are used there.

Just to reiterate, I whole-heartedly agree that exterior walls not only should have a moisture barrier, but need it. You climate decides how it's installed. But I also firmly stand by what I've said all along about attic floor insulation. Haven't you seen stains in ceilings from moisture that was trapped by the barrier? Or mildew under the insulation? Like I said, it's possible in Canada that this is not an issue, but it is in the (relatively) warmer US. View attachment 127679

The problems you are describing are not caused by the barrier, but by some other problem.
As an additional thought, you keep referring to "attic floor " insulation, by that, do you mean that there is an attic subfloor on top of the ceiling joists, and rather than doing it right and lifting that floor and insulating in the joist spaces, that insulation is added with a vapour barrier directly on that subfloor, which leaves anywhere from a 6"-10" cavity between the finished ceiling below and the attic subfloor?

If so, as they say on our East Coast "Skipper, I think I sees yer problem.... " :facepalm:

 

[dwco5051]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

No. They mean lots of attic space exterior venting. The current wisdom is that "too much attic ventilation should just about be enough".

:facepalm:

Not mentioned is the fact that heat is the biggest enemy of roof shingles. An improperly vented attic space can really decrease the lifespan on a roof.

 

[GTS225]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Heat always rises, it never goes sideways (which is why gable vents are no longer used).

********************************************************************************************
Beefer.....I read through this entire thread before responding to this, so I did see your credentials.

I'd like to take this opportunity to allow you to correct this statement.

Roger

 

[NYBo]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Sorry, Beef, you're busted on this one. Properly done attic insulation has a vapor barrier below it, even blown-in. You need to watch some episodes of Holmes on Holmes and/or Holmes Inspection to see what happens with a poor or missing vapor barrier. This issue isn't nearly as important in Florida due to the relatively warm winters, so there are probably many, many houses in your neck of the woods without a proper attic/ceiling vapor barrier with no big problems. But those of us up north can't get away with it!

 

[sschefer]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

There is much controversy surrounding the use of vapor barriers. Most of that stems from folks not understanding how and where they should be used. I remember when the fad was to wrap an entire house and seal it up tight. Heck, in some of those houses you couldn't even keep the pilot lights lit let alone get enough draft for a fireplace.

Since then we've learned that a house needs to breathe but not too much and not too little. Vapor barriers work great at controlling moisture build up if the home is first evaluted properly. Sometimes, such as in warm winter climates, the cure is not to install vapor barriers but instead to properly vent the exterior walls and use ridge vents on the roofs.

Vapor barriers work to control the movement of air with the intent of slowing it to allow it to warm or cool as the seasons dictate. If properly installed in the right areas of the home they should not trap moisture.


Side note.. They've been termed "Vapor Barriers" but that's really not the right name. Air Flow Control Membrane is probably more appropriate.

 

[joed]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Vapor barriers are supposed go on the warm side. So in Florida the vapor barrier should probably go on the outside since most of the time you are cooling and not heating the house.