Re: ? The Science Of Condensation ?

[Guest guest]

We've had a nice wooden shed since 1994 and just replaced the roof this

Fall. We always keep the windows cracked all year long without any problems.

Hi, I have a storage shed built on piers with only two walls. The roof is

plywood, bituthene & metal. Today I see water droplets on the 'ceiling' but

not near the edges of the opened sides. The air here is beyond damp - it's

wet. There's a lot of wood in the shed - which went in ranging from

slightly damp to wet. I have to stress that the air is water-logged. I'm

thinking

that the droplets on the 'ceiling' is condensation rather than an all-over

leak...that avoids the opened edges. Any thoughts? Thank you.

[Guest guest]

Jeff,

Good response to . I have question about the insulation on

the underside of a roof in an open structure. The insulation could

reduce the radiation of heat through the roof, but wouldn't the

surface temp of the insulation also reach dew point, producing

condensation?

BTW, this is a recent issue in the Colorado mountains with the

weatherization program recommendations for open attics.

Carl Grimes

Healthy Habitats LLC

-----

,

The condensation is due to radiational heat loss from the roof.

You see this phenomenon frequently on parked cars. Water condenses on the roof,

trunk and hood but not on the sides. There can be as much as a 10 degree

difference between the roof and sides of a car, even on a warm, clean night.

On a clear night, surfaces facing the sky loose heat to the universe which is

very cold. Surfaces not facing the sky absorsb heat from the surroundings and so

do not cool as fast.

If the temperature of a surface is below the dew point of the air, water will

condense on the surface.

The surfaces does not even have to have condensation to develop mold. If the

temperature of the surface is low enough so that the relative humidity of the

air just above the surface reaches more than about 80%, mold will grow.

I have seen many moldy ceilings on gazebos and porches that were open on 3 or

four sides.

This is why it is a good idea to use some insulation (such as sheet foam behind

the wood) at a porch ceiling even if the roof is " outdoors. "

May

May Indoor Air Investigations LLC

www.mayindoorair.com

>

> Hi, I have a storage shed built on piers with only two walls. The roof is

plywood, bituthene & metal. Today I see water droplets on the 'ceiling' but not

near the edges of the opened sides. The air here is beyond damp - it's wet.

There's a lot of wood in the shed - which went in ranging from slightly damp to

wet. I have to stress that the air is water-logged. I'm thinking that the

droplets on the 'ceiling' is condensation rather than an all-over leak...that

avoids the opened edges. Any thoughts? Thank you.

>

>

----------

The following section of this message contains a file attachment

prepared for transmission using the Internet MIME message format.

If you are using Pegasus Mail, or any other MIME-compliant system,

you should be able to save it or view it from within your mailer.

If you cannot, please ask your system administrator for assistance.

---- File information -----------

File: DEFAULT.BMP

Date: 15 Jun 2009, 23:10

Size: 358 bytes.

Type: Unknown

[Guest guest]

,

The condensation is due to radiational heat loss from the roof.

You see this phenomenon frequently on parked cars. Water condenses on the roof,

trunk and hood but not on the sides. There can be as much as a 10 degree

difference between the roof and sides of a car, even on a warm, clean night.

On a clear night, surfaces facing the sky loose heat to the universe which is

very cold. Surfaces not facing the sky absorsb heat from the surroundings and so

do not cool as fast.

If the temperature of a surface is below the dew point of the air, water will

condense on the surface.

The surfaces does not even have to have condensation to develop mold. If the

temperature of the surface is low enough so that the relative humidity of the

air just above the surface reaches more than about 80%, mold will grow.

I have seen many moldy ceilings on gazebos and porches that were open on 3 or

four sides.

This is why it is a good idea to use some insulation (such as sheet foam behind

the wood) at a porch ceiling even if the roof is " outdoors. "

May

May Indoor Air Investigations LLC

www.mayindoorair.com

>

> Hi, I have a storage shed built on piers with only two walls. The roof is

plywood, bituthene & metal. Today I see water droplets on the 'ceiling' but not

near the edges of the opened sides. The air here is beyond damp - it's wet.

There's a lot of wood in the shed - which went in ranging from slightly damp to

wet. I have to stress that the air is water-logged. I'm thinking that the

droplets on the 'ceiling' is condensation rather than an all-over leak...that

avoids the opened edges. Any thoughts? Thank you.

>

>

[Guest guest]

Mine is 6 months old ! How discouraging.

>

>

> We've had a nice wooden shed since 1994 and just replaced the roof this

> Fall. We always keep the windows cracked all year long without any problems.

[Guest guest]

Thanks a bunch Jeff. I DID suspect it wasn't an actual leak. When the weather is

dry I'll add rigid insulation. Should I put it under the bituthene?

>

> ,

>

> The condensation is due to radiational heat loss from the roof.

[Guest guest]

Good question Carl. Now I'm thinking that insulation wouldn't fix the problem

either.

>

> Jeff,

>

> ...but wouldn't the surface temp of the insulation also reach dew point,

producing condensation?

> Carl Grimes

> Healthy Habitats LLC

[Guest guest]

A lot of the newer apartment buildings in my frigid northern region have this

exact situation with the windows. The windows are generally metal and with low

insulation value, and so end up with a lot of condensation. The situation is

made worse by bad installation, so the frame joints allow air to leak from the

outside and onto the frames. The apartments seem to use two different styles, a

black-framed window and a silver-framed one, that come from the same one or two

manufacturers.

From: jmhiaq <jeff@...>

Subject: [] Re: ? The Science Of Condensation ?

Date: Wednesday, December 8, 2010, 11:08 PM

,

The condensation is due to radiational heat loss from the roof.

You see this phenomenon frequently on parked cars. Water condenses on the roof,

trunk and hood but not on the sides. There can be as much as a 10 degree

difference between the roof and sides of a car, even on a warm, clean night.

On a clear night, surfaces facing the sky loose heat to the universe which is

very cold. Surfaces not facing the sky absorsb heat from the surroundings and so

do not cool as fast.

If the temperature of a surface is below the dew point of the air, water will

condense on the surface.

The surfaces does not even have to have condensation to develop mold. If the

temperature of the surface is low enough so that the relative humidity of the

air just above the surface reaches more than about 80%, mold will grow.

I have seen many moldy ceilings on gazebos and porches that were open on 3 or

four sides.

This is why it is a good idea to use some insulation (such as sheet foam behind

the wood) at a porch ceiling even if the roof is " outdoors. "

May

May Indoor Air Investigations LLC

www.mayindoorair.com

[Guest guest]

Carl,

Sorry if I was not clear. The insulation is not on the underside of a roof in

the open structure.

The sheet foam should be sandwiched between the wooden porch ceiling and the

support joists into which the ceiling boards are nailed, not under the roof

sheathing above (into which the shingles are nailed).

This way, the " attic space " still gets cold but the ceiling stays warmer due to

heat gain from the ambient air.

May

May Indoor Air Investigations LLC

www.mayindoorair.com

>

> Jeff,

>

> Good response to . I have question about the insulation on

> the underside of a roof in an open structure. The insulation could

> reduce the radiation of heat through the roof, but wouldn't the

> surface temp of the insulation also reach dew point, producing

> condensation?

>

> BTW, this is a recent issue in the Colorado mountains with the

> weatherization program recommendations for open attics.

>

> Carl Grimes

> Healthy Habitats LLC

>

[Guest guest]

Carl, there are probably a couple things. The insulation prevents a steep temp

gradient from occuring - measuring from underside surface to the top of the roof

surface. The layer of insulation allows the coolest on the top (what Jeff said

due to cooling upward), the bottom surface remains (for a while) at the original

warmer temp, similar to the side facing surfaces, so by the time the low surface

reaches dew point, the air under it ('in' the gazebo) is actuall a new lower

night time temp and RH, so you get a new DP, and viola, no condensation.

I would also suggest that in these types of conditions (open sides with roof)

you get trapped warm moist air. That wet air does not move out, and will likely

abosorb into the soft wood, regardless of DP. Since there is no air movement,

i.e., ventilation, to evaporate the water from the surface, you get wet wood

long term enough to support growth.

>

> Jeff,

>

> Good response to . I have question about the insulation on

> the underside of a roof in an open structure. The insulation could

> reduce the radiation of heat through the roof, but wouldn't the

> surface temp of the insulation also reach dew point, producing

> condensation?

>

> BTW, this is a recent issue in the Colorado mountains with the

> weatherization program recommendations for open attics.

>

> Carl Grimes

> Healthy Habitats LLC

>

[Guest guest]

Hi Jeff, I'm sorry. My brain just hasn't grasped where the layers should be

positioned. In the order in which it was built, mine has the support beams, then

plywood, then bituthene & then metal roofing. I feel so retarded but could you

please explain the order I would put the layers on when I add insulation. Seems

to me you told Carl it would be the support beams, then the rigid insulation,

then the plywood, then the bituthene & finally the metal roofing. That doesn't

seem right to me though. Sorry again, I get confused with the way the layers are

explained - in walls too.

[Guest guest]

,

I was referring to a porch that has a horizontal ceiling under the roof. I now

understand that your " ceiling " is just the underside of the roof sheathing and

the rafters are exposed.

You would have to install sheet foam between the rafters (attached to the

underside of the roof sheathing) and then plywood (or planks) over that if you

did not want to look at the insulation.

May

May Indoor Air Investigations LLC

www.mayindoorair.com

>

> Hi Jeff, I'm sorry. My brain just hasn't grasped where the layers should be

positioned. In the order in which it was built, mine has the support beams, then

plywood, then bituthene & then metal roofing. I feel so retarded but could you

please explain the order I would put the layers on when I add insulation. Seems

to me you told Carl it would be the support beams, then the rigid insulation,

then the plywood, then the bituthene & finally the metal roofing. That doesn't

seem right to me though. Sorry again, I get confused with the way the layers are

explained - in walls too.

>

>

[Guest guest]

Thanks again Jeff.

>

> ,

>

> I was referring to a porch that has a horizontal ceiling under the roof. I now

understand that your " ceiling " is just the underside of the roof sheathing and

the rafters are exposed.

>

> You would have to install sheet foam between the rafters (attached to the

underside of the roof sheathing) and then plywood (or planks) over that if you

did not want to look at the insulation.

[Guest guest]

Make sure you don't have an air space between the wood and the insul; this may

end up trapping moisture and causing rot/mold.

Especially if there is any water coming top-down, like thru old shingles or old

wood sheating.

Same with the sides against the rafters.

That is one reason for choosing spray foam sometimes, it prevents air /moisture

from entering those areas.

The alternative is foam the gaps around the rigid insul.

> >

> > ,

> >

> > I was referring to a porch that has a horizontal ceiling under the roof. I

now understand that your " ceiling " is just the underside of the roof sheathing

and the rafters are exposed.

> >

> > You would have to install sheet foam between the rafters (attached to the

underside of the roof sheathing) and then plywood (or planks) over that if you

did not want to look at the insulation.

>