48x39x14 shop build

[RayfromTX]

As I understand it - a vapor barrier is usually polyethelene or tyvek - something that prevents the passage of moisture.

I'm guessing they are standard trusses - 24" on center? I don't know, but could measure them.

The peak and eves are steel over wood 2x4 construction with enough light coming through the holes I doubt vents would add/improve anything.

A vapor barrier is defined as having a perm rating of 1US perm or less. Polyethylene and metal are vapor barriers. Tyvek is a brand name for spun polyolefin. It is vapor permeable. Spun polyolefin is available in perm ratings ranging from 7-60. Tyvek is not a vapor barrier. It was developed to freely pass water vapor while shedding liquid water which it does unless it is in contact with a surfactant or penetrated with non-gasketed fasteners.

Also the term moisture is confusing in these discussions because it refers to water vapor, liquid water, absorbed water and adsorbed water. The last two terms are different states of water found in and on solid surfaces. These discussions come up routinely and getting this right can be very important for durability.

In the building shown in this thread there exists the potential for water vapor to build up in the attic space to levels to which cold night time and seasonal temperatures can allow condensation to form on the underside of the metal roof. This may or may not occur at a level that causes problems depending on the climate. I think Missouri is not a place I would be comfortable using this construction without attic ventilation. While the metal ceiling will limit water vapor from passing from below, vapor will be drawn into the space due to several factors. The most important of these is probably the fact that as the temperature of the air in that attic changes it will expand and contract which will have the effect of the attic "breathing". Richard has ridge and eve vents to handle this in his building.

There are other ways to handle interior moisture other than attic ventilation but not in the construction to which this project is already committed.

 

[ovrszd]

Good stuff Ray.

I was concerned and asked about the attic space. Before I blew in the insulation I added barriers between the trusses so I didn't cover the soffit vents. The insulation is allowed to "cap" the wall headers to minimize heat loss there. The ridge is vented the full length of the building. All builders I talked to here told me that was sufficient. We'll see.

I am curious enough about it that I will take a look in the attic on susceptible days.

 

[JohnBuffaloMN]

My shop was built in the 70's or early 80's I think. Building codes and practices have changed significantly since then! I'm also pretty sure the 30x40 structure was put up by contractors, then interior finished by the homeowner. The concrete floor was clearly done by amateurs who didn't know how to finish concrete. The whole building obviously lasted 30 years or so without the ceiling coming down, so again I have to go back to the pooling water under the floor. I doubt you'll have any problems with your building ovrszd.

 

[RayfromTX]

Richard, susceptible days will be anytime you have had dew points that are moderate to high and then a cold snap. For instance, right now the dew points in missouri are around 64degrees. If that air is in your attic and a cold snap hits that drops the temps quickly to 50 degrees then condensation may form. If it is windy as the front passes then it may flush the air out more quickly. If however it is still and the temps drop due to clear skies and a condition known as radiant overcooling then you may get condensation then. If the sun comes out and warms the roof then it will evaporate the liquid back into the air. A north facing roof is more susceptible because it doesn't get the solar gain early if at all depending on slope and time of year and latitude of the building. There are lots of variables but what you are looking for is conditions where warm humid air encounters cold surfaces for extended time. You should be fine but you will find intermittent liquid on the underside of the metal because it has no ability to store any within it's structure. Wood structures can collect a bit before becoming wet. (hygric storage capacity) but are susceptible to decay if things go too far for too long.

I feel this is a subject that should be carefully taught in school. It is one of the least understood things there is considering how much it impacts our buildings and our lives. In 5 one hour classes, most students down to the 5th grade level can understand this in a meaningful and lasting way. I try to impart the information to every early teen that I encounter.

 

[ultrarunner]

It sounds like a metal roof over sheathing with felt in between adds a lot to the safety margin regarding condensation?

 

[MoPops]

Sure looks good Richard. It may seem like it’s taking a while,from your perspective, going up fast from my view point! Where is the future house location? Trying to imagine best spot looking at the drone shots.

 

[ovrszd]

Richard, susceptible days will be anytime you have had dew points that are moderate to high and then a cold snap. For instance, right now the dew points in missouri are around 64degrees. If that air is in your attic and a cold snap hits that drops the temps quickly to 50 degrees then condensation may form. If it is windy as the front passes then it may flush the air out more quickly. If however it is still and the temps drop due to clear skies and a condition known as radiant overcooling then you may get condensation then. If the sun comes out and warms the roof then it will evaporate the liquid back into the air. A north facing roof is more susceptible because it doesn't get the solar gain early if at all depending on slope and time of year and latitude of the building. There are lots of variables but what you are looking for is conditions where warm humid air encounters cold surfaces for extended time. You should be fine but you will find intermittent liquid on the underside of the metal because it has no ability to store any within it's structure. Wood structures can collect a bit before becoming wet. (hygric storage capacity) but are susceptible to decay if things go too far for too long.

I feel this is a subject that should be carefully taught in school. It is one of the least understood things there is considering how much it impacts our buildings and our lives. In 5 one hour classes, most students down to the 5th grade level can understand this in a meaningful and lasting way. I try to impart the information to every early teen that I encounter.

Once again, good stuff Ray.

Next to the shop is an all steel machine shed with single slope roof to the North. Next time it "rains" in there I'll check my attic.

If you were building a similar building what would you do differently?

I ask because these threads offer excellent advice for someone considering a build. I take no offense to any suggestions.

 

[ovrszd]

Sure looks good Richard. It may seem like it’s taking a while,from your perspective, going up fast from my view point! Where is the future house location? Trying to imagine best spot looking at the drone shots.

The shop sits North/South. Overhead doors in South end.

House will be East of the shop turned slightly Southwest at 205 degrees. 40ft gap between the two. House, garage, porch overhang will be covered by a 72x40 roof. Front toward Southwest.

 

[RayfromTX]

Once again, good stuff Ray.

Next to the shop is an all steel machine shed with single slope roof to the North. Next time it "rains" in there I'll check my attic.

If you were building a similar building what would you do differently?

I ask because these threads offer excellent advice for someone considering a build. I take no offense to any suggestions.

Richard
What I might do is something similar to what I do on the homes I build with some simplification. I would have to study on it to work out the engineering questions but I would probably sheath the structure with cdx. Cover that with 2" of polyisocyanurate foam insulation and then install the metal over that. I would perhaps use furring strips over the insulation to mount the metal to without having to screw the metal all the way through the insulation. Kind of a yeti cooler approach. For intermittent heating and cooling in a utility structure I would use 1" foam and just screw through it. This approach controls the temperature of the first condensing surface and is the safest way to build in any climate. It has many advantages. In a living space structure I go to great lengths to make the shell air tight. If you are interested, I can talk you through it prior to building your house.

 

[EddieWalker]

The one thing that you have to remember when insulating the roof, and also insulating the ceiling is that you have created an attic area that will also create condensation that needs venting. This is why you only insulate one or the other. It would be very hard to get decent air flow from the eaves to the peak with insulation under the roof.