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Community and Q&A

Another WRB question: Need help

CuriousBuilding | Posted in GBA Pro Help on

Thanks for any thoughts.  I’m building a 12×16 Shed on concrete slab in my backyard in Houston.  Just got the slab in last week.  However, I want to build it so I never have to rebuild, so I am willing to overspend on a shed (within reason), so it’s not about the money, I want to do a killer job.   I don’t care to change the design.  It will be a shed for my tools and extra garage stuff.  I’m going to have a workbench out there too for occasional projects.

It is Not going to have AC.  So, I don’t really want to insulate or cover the interior walls.  The goal is to build it very strong, waterproof, and good looking so that I hopefully never have to rebuild or replace rotten wood, etc.   I’m good with my design of 2×4 walls, 2×6 rafters wrapped in plywood or OSB.

MY QUESTIONS are about the WRB, vapor moisture etc.   I don’t want all my Dewalt tools and hand tools rusting, and the building rotting from moisture issues.  Remember, HOUSTON climate.

It is an interesting shed roof design draining one way to the back with a short parapet wall on the front 3 sides.  planning to rain screen it with vertical furring and horizontal NewTechWood.  Since I am not finishing the inside or insulating, I thought a Tech shield type product would help keep the temp down a little in the Houston summers.

Not too concerned about winter, its not much here.  (Siding wrapped in pvc, pretty cool stuff.)  I was thinking solar board whole building from Lowes facing foil inside, then wrap 2x with 15lb felt with the usual window/door details.    –OR–  Wrap in plywood or regular OSB, then wrap with Prosoco Alumaflash Plus the whole building from the outside to get the reflection and help keep temp down a little and make it watertight.   I know the alumaflash will cost much more but I’m ok with it IF it really is as great as MattR claims.

Thoughts from the collective that is “in the know” would be helpfull please.   I’m just concerned about locking moisture IN the building, but I also want this to last the rest of my life and not rot out.

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  1. Expert Member
    Peter Yost | | #1

    Hi CuriousBuilding (be great to have a real name to share with the GBA community) -

    Open-cavity walls dry out great. In Houston, I would make sure you have your shed designed to move roof load away from the structure, do what you can to protect against flood waters, and make sure you have capillary breaks between all porous materials: slab and bottom plates, butt ends of all wood elements, etc. Your WRB should be vapor open so you get drying potential in both directions.

    Radiant barriers--even in your climate--make sense for roofs but not usually walls. And of course, for radiant barriers to work they must face an open space.


  2. GBA Editor
    Martin Holladay | | #2

    Here's how tools rust: You have a cold spell in winter or spring, and all of the contents of the shed (including your tools) get cold. Then you have a spell of warm, humid weather. When that happens, moisture from the air condenses on your cold steel tools (just like on a cold can of beer), and the tools begin to rust.

    So a little bit of heat during cold weather is a good thing.

  3. CuriousBuilding | | #3

    Thanks for the reply gents. You are correct sir, Tony from Houston. Yes, the walls will be open on the inside, not sure what you mean by "move roof load away from the structure." I'm pretty sure my engineering side of the structure strength is fine. I'm asking more about being sure I won't have moisture problems inside. My backyard is high and dry so no flood issues and this is being built on a slab that will be 12" above grass grade due to making it the same elevation of the pool patio. So, no water worry. Yes, the slab has a vapor barrier (plastic) under the rebar. SO hopefully not much water vapor coming through. I was figuring on using the foam type seal under the treated sill plate. I also plan to wrap the bottom plate with an asphalt impregnated peel n stick flashing, probably 6" wide, from about an inch down on the slab moving up covering the sill to create a full lower end seal from concrete slab up over the sill and onto the sheathing. I hope that is making sense. Then I can wrap 15 lb felt 2x around the building, overlapping the flash. WILL FELT work as a WRB with tech shielding sheathing on the (solarsheild or whatever the Lowes version is called) on the inside of the sheathing? Will all this work OK for breathability, drying out, and as a WRB behind the Rainscreen siding plan?

  4. CuriousBuilding | | #4

    I'm asking more about being sure I won't have moisture problems inside with this type of construction. AND, if I didn't go with felt, is alumaflash plus an option or will I be trapping moisture. The building will be fairly tight, ie: real door with weatherstrip, etc. WILL FELT work as a WRB with tech shielding sheathing on the (solarsheild or whatever the Lowes version is called) on the inside of the sheathing? Will all this work OK for breathability, drying out, and as a WRB behind the Rainscreen siding plan? any thoughts?

  5. CuriousBuilding | | #5

    So I've decided my WRB is going to be Aluma Flash Plus behind rain screen siding. This shed will be UNconditioned in Houston, -think hot and humid for 9 months a year. The Alumaflash WRB is definitely NOT vapor/air permeable. So my barrier is on the exterior basically stopping everything. The building envelope will be tight as even the shed door is a commercial front door assembly from Lowes for a house so its weatherstripped and all. QUESTION: With a snug envelope like this will be, and considering that I'm not concerned about heat retention of winter, and that the shed is NOT being insulated so everything should dry from the inside. Should I or should I not add venting? It's a shed roof on 1/12 pitch so I'm either keeping it tight with no vents, OR, maybe add 2 large louvered vents on the low wall about 12" off the floor and 2 vents up high on the high wall. What do you think, needed or not?

    1. GBA Editor
      Martin Holladay | | #6

      You're building a sealed-up building. There will be some air leakage, in spite of your best efforts, but very little drying. Without heat in winter or dehumidification in summer, the conditions inside this shed are unpredictable.

      During certain weather conditions, it's going to be very humid in your shed. During certain weather conditions, you'll get condensation on your tools. It's hard to avoid these conditions unless you heat, cool, and dehumidify the building.

  6. CuriousBuilding | | #7

    It may be, but the question is- install low and high venting to keep air slowly moving and more closely regulated to the outside temps, - or no vents at all considering the wrb I am using and the intended tightness of this building. Thoughts on the vents?

    1. CuriousBuilding | | #8

      I know you are a busy man and I apologize for so many questions. Construction begins this weekend, as I said above, with the Alumaflash plus on the outside, with a 3/4 gap rainscreen outside of it, the building should be fairly snug. yes, leakage, but I don't think that much with the type of roof I am doing, there are no soffits or soffit vents since this is a low pitch shed roof sandwiched between 3 sided parapet wall with the low end of the roof hanging off the backfire water to drain. Much like many of todays strip malls. Anyway, I know many would not choose this construction style, but my question is about the air circulation and venting. Tn this non insulated, non conditioned space, the question is- install low and high venting to keep air slowly moving and more closely regulated to the outside temps, - or no vents at all considering the wrb I am using and the intended tightness of this building. Thoughts on the vents? I am grateful for your knowledge sharing.

  7. CuriousBuilding | | #9

    FYI: forgot to mention that traditional calculators on venting suggest 192 sq ft should have about 96 sq in of venting in and 92 out. (as I read it). I was thinking two of the large soffit type vents with the louvers and screening on the back (low) end of the building mounted low about 12" off the slab in the wall assembly, I believe I have the detail worked out fine. THEN, 2 of the same mounted on the front behind the rain screen, mounted high -as in inches- from the top plate on the front of the building behind the rain screen. Hot air in the shed should come in low from the back, rise through the building, following the pitch of the open ceiling to the front of the building, making its way out the two vents front and high. The rain screen gap of almost 1" should let all hot air slip out, as that toe detail gap will only be a few inches away. SO, Vent or No VENT and keep it closed?

  8. Expert Member
    Peter Engle | | #10

    Martin has already given you the correct answers. If you want the shed to remain dry inside, you need to condition it. That means some heat in winter, some A/C and/or dehumidification in summer.

    If you vent the garage, this will happen, especially in the swing seasons: It will cool off at night. The tools will cool off at night. During the warm and humid Houston daytime, that warm/humid air will come inside through the vents and the humidity will condense on the cool tools, causing rust.

    If you don't vent the garage, this will happen: With no airflow and exterior vapor barriers, the little bit of warm/humid air that gets inside can't get out. Any moisture in the walls or the slab also can't get out. The humidity will go up. A little bit of cool weather will chill the building and it will begin raining inside. Or if not rain, at least some condensation on the tools, leading to rust.

    If you want a building to stay dry in Houston, you've got to provide some sort of conditioning. It won't take much, but that's the only solution.

  9. CuriousBuilding | | #11

    So, here it is a few months later and almost done. Its a concrete slap with plastic under it, sealed at the sill plate, weatherstripped door, solid glass tinted corner window, rainscreened with alumaflash plus (non permeable) and a flat roof with 5/8 plywood, 1" foam insulation board and commercial white membrane roof. I'm putting up the pvc wrapped siding with little metal clips and a 3/4 rain screen gap. OK, so where I am now is to finish the siding and put the metal cap on the 3 parapet walls to finish it off. The front and sides are parapet walls and the flat roof has an 18" pitch over 12 feet. I've got a 20" overhang on the backside to keep water off the back outside wall. QUESTION: If I put in a small window a/c unit on the back wall (there is a small window on the back) I am thinking thanks to this forum, that will be enough to keep it cool and dehumidified, but should I do a small a/c OR just buy a dehumidifier instead plumbed to the outside? Also, since the WRB is air/vapor Impermeable.. will standard pink bat insulation and 1/2" drywall be OK to let it dry to the interior? I don't want mold or problems. thoughts? and Thank you, Tony

  10. Expert Member
    Peter Engle | | #12

    A dehumidifier would work at a wider range of temperatures. Many these days operate well down to 50 degrees or below, maintaining your RH setpoint throughout. A/C works only on room temperature, so during the swing seasons you won't have much humidity management. With A/C, you get the least dehumidification in cool/wet weather when you need it most, but you do get the benefit of a cooler space for working in.

    My garage/shop has a dehumidifier and no A/C. But I'm in NJ, not TX. I'm more worried about heating the space than cooling it.

  11. GBA Editor
    Brian Pontolilo | | #13

    Hey CuriousBuilding,

    You can use unfaced batts and drywall and your walls will be able to dry inward. When it comes to the AC vs. dehumidifier, my first thought is: get both. The AC will help with humidity in hot weather and keep you comfortable, and you'll have the dehumidifier for the swing seasons. They're both relatively small and affordable appliances and will alleviate your concerns about your tools and make the shed a place where you can work year round.

  12. Jon_R | | #14

    I would keep perms below about 5 perms - less and you are unnecessarily letting moisture in where you then need to remove it.

  13. CuriousBuilding | | #15

    I'm not sure about what the perms will be -but- I can tell you I took extensive measures, more and better, than the spec house builder that built my house. So, although some air and moisture will squeak in, most will not. Except obviously when I open the door. So, if we assume generally hot humid Houston climate, I put a small dehumidifier in the corner it should keep the moisture situation mostly under control and at least make it feel a little cooler in there also. I really don't think the building will get very hot anyway. My roof membrane is thick commercial and white with foam insulation board under it on top of 5/8 plywood and the walls are covered with rainscreen siding with 3/4 inch gap and alumaflash plus as a WRB giving me some general impermeability and tightness and radiant barrier to some level. I am also sealed exceptionally tight at the sill plate and the roof to wall joint. *** I thought about leaving exposed wood and stud walls inside and just spraying it all a general off white to lighten it up and make it more pleasing to work in OR the route we just discussed with unfazed bats and drywall. It does not need to be pretty in there, so maybe drywall is overkill, but is the insul and drywall enough of a benefit for temp control with the building being already so well sealed and wrapped in the alumaflash? I have real concerns about trapping moisture in the walls since there will be no drying to the exterior. If I just leave it open inside and paint white, can I just use a primer like bullseye white from Lowes (I have a full gallon)? Am I over thinking?

  14. CuriousBuilding | | #16

    Oh, and thank you! You guys here are incredibly responsive, helpful and knowledgeable. Thanks! Tony

  15. CuriousBuilding | | #17

    #1 - To be clear... with impermeable barrier on the exterior, even with the unfazed bats and 1/2 sheetrock and white primer and paint... the walls will still dry to the inside with only window unit a/c in this shed? Right?

    #2 - IF I DON'T insulate and drywall... I just paint the inside of the plywood sheathing and studs-- can I just use a primer like bullseye white from Lowes (I have a full gallon)? Can it dry though this?

    1. Expert Member
      Akos | | #18

      I would generally not drywall a shed. Drywall just doesn't stand up to abuse as well as plywood.

      Since it is a shed without an active source of moisture (ie people and cooking), there is very little issues with moisture in the walls, you only issue diurnal temperature variation and condensation that might cause without the AC/dehumidifier.

      Pretty much whatever you build will work well as long as you have some way of active moisture removal.

      Most water based paints are moisture open, they won't effect drying of your plywood.

  16. CuriousBuilding | | #19

    So I've decided, I am not going to insulate and drywall. With the IMpermeable alumaflash plus as my rainscreen, and other extra measures I have taken to seal up the structure, I have decided to just paint the inside of the plywood and exposed studs flat white. I figure a flat white primer will stick better to the exposed wood, no? Then I can skip the topcoat and keep it as permeable as possible so the walls will dry to the inside. Sound right? I have a gallon of bullseye 123, I think I am going to call Zinsler to learn if they have a perm rating. Thoughts on flat white primer with no paint in the shed? Drying issues?

  17. GBA Editor
    Brian Pontolilo | | #20

    Some primers and paints qualify as vapor retarders other do not. I'm not sure about Bullseye 123. If inward drying is your goal, it would be a good idea to double check with the paint manufacturer. Keep in mind that primer doesn't stand up to abuse very well, and this is a shed. You may want to consider some type of top coat as well. Again, a vapor open finish.

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