# Roof Condensation, Shop Insulation question



## SteveKorz (Mar 25, 2008)

Hey All,

I am in the middle of redesigning my shop. I'm drawing out the electric and studs, planning for insulation, etc. The shop will be heated and cooled. I'm writing this post in hopes to solve a potential condensation issue.

The issue that I'm running into is the roof. I want a vaulted ceiling in my shop. I would like to insulate between the rafter perlins that hold the metal roof on, but I'm afraid of putting fiberglass batting right against the roof. There will be a metal roof, 2×4 perlins with insulation, and then alluminum sheeting on the inside of the shop.

I am afraid that this method will not be adequate enough, and it will eventually draw condensation on the inside of the shop, under the metal roof, and soak the insulation (then drawing more condensation due to lack of effective or wet insulation, etc).

Would it be better for me to drop below the bottom of the perlins, and stud in 2×6 rafters that will hold the 2×6 fiberglass batting? It would be the metal roofing, 6" of air void, 6" of insulation, then the alluminium sheeting for the interior finish.

Will this be enough for air to move if I vent it on the ridge, and on the gables through vented sofit to avoid any condensation? The drip edge has no vent.

I am really perplexed by this… I would really like to plan and begin rough design of the interior, but I don't know where to begin with this roof.

I have thought about having a professional use spray foam, but I still don't know if it would be enough (and I'm having trouble finding someone in my area). The last thing that I want is to have a moisture issue in the shop after I've worked so hard to prevent it. Since the ceiling is vaulted, I don't think that loose, blown in insulation won't be an option because I think it would settle to the bottom over the years, creating condensation at the peak.

Putting a flat ceiling in my shop isn't really an option for me, I need the working height that the vaulted ceiling provides, or that would be an obvious answer for me. A flat ceiling would give me less than 7'6".

Thoughts?

Thank you all in advance for ANY help or thoughts that you can provide….

Steve.


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## canadianchips (Mar 12, 2010)

1st choice would be spray foam for this roof.
2nd choice is what you are planning with the 2×6 under the rafter, insulating ,soffit and ridge vents.
Condensation is going to happen when the heat from inside touches the cool metal,
If you feel you want extra roof insulation you may use rigid styro "blue" in between purlins. The blue is denser plus mold and mildew resistant.


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## JoeLyddon (Apr 22, 2007)

No Thoughts here… except look at other shops, etc.

Sounds like a GREAT NEW Adventure ahead!

Wonderful!

I trust you will keep us up to date… step by step… as you go?

Thank you…


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## Grumpy (Nov 9, 2007)

Different problem in my neck of the woods Steve. Good luck with whatever you decide.


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## AlaskaGuy (Jan 29, 2012)

http://www.airtightinsulationpa.com/Applications/NonVentedHotRoofSystem/tabid/185/Default.aspx


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## patron (Apr 2, 2009)

metal directly over wood 
will do as you state

metal will condense on the bottom
and run down the rafters and rot the top plate

this is what happens to many sheds and hay barns
in variable weather conditions

perlins make it hard to walk and work on the roof
being 'spongy' and prone to bending under foot
why a solid sheeting (full mill lumber or ply/osb)
is laid down directly on the rafters
and a 30# felt (tar paper) is laid down on it
before the metal roofing
the moisture under the metal runs down the tar paper
to the edge of the roof over a metal drip cap
and away from all the wood and insulation

the insulation can then be in the rafter space
(whether spray or fluff)
and covered underneath by the inner ceiling
so the overall thickness is not greater than the rafters 
soffit vents are to let the space 'breath' not air flow
which would defeat the insulation properties
(and in a cathedral ceiling there really is no 'attic' space
so venting it is not really a problem)

this is the way most roofs are built and insulated
and is cheaper than the cost of a separate
inner ceiling just for insulation


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## patron (Apr 2, 2009)

depending on the size of your span
cathedral trusses may be cheaper to use
give the truss maker the pitch and span
and the bearing points on the walls
he will engineer them (they are licensed)
and deliver them ready
and save you money and time

the advantage of this style
is no center posts or sagging ridge
(and no cables or ties straight across the shop
under your cathedral ceiling)
and bulging sides as the roof settles
i know we have all seen old barns 
that are sagging and spread out


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## SteveKorz (Mar 25, 2008)

Thanks all…. I'm going to explore the spray foam further… If I can find someone locally that does it. My options are somewhat limited since the structure is already built.


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## lewis62 (Mar 6, 2011)

If you use the aluminium siding on the inside it will always condensate. Why using metal inside, dont.
From what you said about structure , it is a great application for spray foam inside, sealing every thing and creating the insulation for your heat and cool rention.But most foams need to be protected from uv or will degrade.there should be no condensating on the bottom of it ,since it will be at room temp.

If you use fiberglass ,then you should , have ventalation above insul ,below metal roof. The soffit has to allow air in at bottom to vent at continous ridge vent at top, moisture will travel through insul, and condense on metal roof with temp change.
Air flow above insul is for attic to breath, allow moisture and hot air to excape, or all attics would be mold breading grounds. Insulation is for slowing down thermal transmittance, or in your metal roof to purge moisture in and above insul to stop condensing on bottom of metal roofing. The air space is also to stop sun heat and winter cold from transfering down through conducting,your heating, cooling would have hard time keeping up.


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## rkober (Feb 15, 2012)

From a theory standpoint condensation occurs when air (which includes water in gaseous form) drops below the dewpoint. So as interior air (higher temp which can hold more moisture) passes through the interior sheeting, insulation, air space, etc. may likely drop temp. below the dew point and condense. The solution it to stop the travel of air with a vapor barrier in the interior side. My impression is that spray foam (as suggested above) does this well inherently (with other benefits). Alternatively with more "conventional" construction I would look at visqueen type vapor barrier, insulation, and an active air sweep (ie vented at the eave and peak). I've used dry wall, vapor, r19 ins, air, and metal with a 2×8 truss successfully but I'd seriously consider spray if I was doing it again. As others suggested it's always nice to confirm the theory if you can find someone that's done it.


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## RichardNorris (Feb 25, 2012)

I have a couple of questions before providing advice. Is the shop a free-standing building? I assume so. Will it be heated to a minimum temperature (50 or 55 degrees F, for example) all the time in winter, or only when you are working in it? What kind of heat source do you plan to use (electric, gas, wood)? (If using gas heat, be sure to exhaust the combustion gasses to prevent CO poisoning.) How well insulated will the walls and floor be? Do you have an exhaust fan? Are you planning to make the building air tight, with vapor retarders and air barriers in wall and ceiling assemblies?

If the shop building is free standing and if you plan to heat it only when you are using it, water could condense and frost could form inside the building when it is not heated during the nights (or weekdays, if you are in the shop only infrequently during the week). An air exchange, using the exhaust fan, to remove the moist air from the shop building would solve this problem and significantly reduce the potential for condensation on the ceiling. There are heat exchange vents available to allow you to capture a lot of the heat for both the exhaust fan and the make-up air.

The dew point temperature is mentioned by others. You will want to design the building envelope systems (walls, windows, doors, and roof) to assure that the inside surfaces and any vapor retarders remain above the dew point at all times, including when you are working in the shop. Your climate conditions and the moisture load inside the building will dictate how much insulation to use. You correctly identified the issue that the 2×4s will not provide sufficient depth for fiberglass batt insulation and an airway (1", minimum recommended). Adding 2×6s may be enough, or you may need deeper ceiling joists.

The type of insulation to use is one consideration (fiberglass batts, foam boards, sprayed on foam). Where to put it is another. If you use sprayed on foam, and if you do not prevent it from blocking the airway, you could have unintended consequences. Excessive heat build up during sunny days is one. The absence of a path for any moisture that gets into the roof assembly to get out is another. Both conditions could damage the wood framing. Heat build up would cause the metal roof panels to expand and contract more than they would otherwise, possibly more than the fastening system is designed to absorb.

Good luck with the project and I hope you have a comfortable shop in which to work upon completion.


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## millzit (Feb 5, 2012)

rafter vents….......
http://www.amazon.com/ADO-PRODUCTS-UDV2248-Durovent-Rafter/dp/B001FDHTOG


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## renners (Apr 9, 2010)

Why not felt over the rafters before putting on the tin roof. Put 3" sheet foam between 2×6" pressure treated rafters to leave an air gap and cover up underside of rafters and insulation with 1/4 ply. You won't get a really cold surface for condensation to form on.


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