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Thread: Floor Insulation/vapor barrier Question

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    Member akhunter3's Avatar
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    Question Floor Insulation/vapor barrier Question

    Hi all,

    We bought a house this spring with detached garage, 20x23 with plywood floor. It's off the ground ~8-10" on one end and about 3' at the other end. Entire thing is insulated R13 walls/ceiling and a NG garage heater is in there for heat (previous owner used it as a shop).

    We have turned it into a gym, and in the process of bolting down the squat rack found out that the floor is not insulated. I'm trying to find the best way to go about insulating the floor so that we can workout throughout the winter without freezing our rears off. Floor is currently primed (Kilz) plywood covered with 8mm rubber flooring. Shed has older windows that let in a fair amount of draft, painted plywood walls and open kraft faced insulation under metal roof.


    I had thought about doing something similar to kraft faced R13 (probably warmer) under the floor, then foam board as a thermal barrier over the floor studs and cover with hardware cloth to keep the rodents out. Where/do I need a vapor barrier? If needed, would it need to go below the insulation between the insulation and the ground, or above the insulation, attached to the bottom of the floor studs?


    I don't know much/anything about this so any/all info helps!

    Thanks all,
    Jon
    Nurse by night, Alaska adventurer by day!

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    I think the first thing you need is a vapor barrier on the ground. Seams sealed with tape.
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    You didnt say what size the floor joists are, assume more than 2 X 4.... if so you should put in the proper thickness fiberglass batt, ie, 2 X 6 R-21, 2 X 8 is R 28 i think... then cover with rigid foam and plywood or screen. VB should be on the warm side, so right under the plywood.... but I wouldn't worry about it for a floor in a occasional use building that you are not cooking, bathing, living in (creating moisture). Moisture tends to migrate more towards the ceiling/roof.... mostly due to the fact that it will be warmer off the floor and warm air can hold more moisture. Big thing with insulation is take your time and cut to fit. 3% voids in fiberglass insulation can lower the R value by as much as 30%...
    “We have digressed from a Nation of Revolutionaries to a country of entitlements"


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    The last I heard, about 25 years ago, 3/4 plywood was rated as a vapor barrier, all by itself. Dont know about other thicknesses.

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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by PtMac View Post
    The last I heard, about 25 years ago, 3/4 plywood was rated as a vapor barrier, all by itself. Dont know about other thicknesses.
    Not say'n you didn't hear that. But it's absolutely false.
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    Quote Originally Posted by Akgramps View Post
    You didnt say what size the floor joists are, assume more than 2 X 4.... if so you should put in the proper thickness fiberglass batt, ie, 2 X 6 R-21, 2 X 8 is R 28 i think... then cover with rigid foam and plywood or screen. VB should be on the warm side, so right under the plywood.... but I wouldn't worry about it for a floor in a occasional use building that you are not cooking, bathing, living in (creating moisture). Moisture tends to migrate more towards the ceiling/roof.... mostly due to the fact that it will be warmer off the floor and warm air can hold more moisture. Big thing with insulation is take your time and cut to fit. 3% voids in fiberglass insulation can lower the R value by as much as 30%...
    I went out and looked, it’s 2x6 joists. There won’t be much moisture aside from sweating, although I do sweat enough for 3-4 people at a time.


    Thanks for all the info guys, I may just forego the vapor barrier as it’s sounding like that may not be essential? You now have me debating between a the hardware cloth or just doing some plywood on the bottom



    Jon
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    Hardware cloth is a great solution as it allows for some air flow.
    “We have digressed from a Nation of Revolutionaries to a country of entitlements"


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    Supporting Member iofthetaiga's Avatar
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    Your 8mm rubber floor covering will serve as a pretty good vapor barrier for most of the field. You'll only leak moisture around the edges and unsealed seams. Forego the VB and put your money/effort into doing a good job on insulation and keeping the ****ed rodents out of it. Can't overstate the importance of keeping the rodents out of your insulation cavities! Hardware cloth would be my choice too...just be sure to inspect it occasionally, especially if you have squirrels in your area. They'll chew right through hardware cloth if motivated. (obviously, they'll get through/around/behind/underneath plywood too, but it's easier to monitor for and repair any damage with hardware cloth. If they get inside plywood you'll have a hell of a time opening things up to fix the insulation damage).
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    Quote Originally Posted by akhunter3 View Post
    Hi all,

    We bought a house this spring with detached garage, 20x23 with plywood floor. It's off the ground ~8-10" on one end and about 3' at the other end. Entire thing is insulated R13 walls/ceiling and a NG garage heater is in there for heat (previous owner used it as a shop).

    We have turned it into a gym, and in the process of bolting down the squat rack found out that the floor is not insulated. I'm trying to find the best way to go about insulating the floor so that we can workout throughout the winter without freezing our rears off. Floor is currently primed (Kilz) plywood covered with 8mm rubber flooring. Shed has older windows that let in a fair amount of draft, painted plywood walls and open kraft faced insulation under metal roof.


    I had thought about doing something similar to kraft faced R13 (probably warmer) under the floor, then foam board as a thermal barrier over the floor studs and cover with hardware cloth to keep the rodents out. Where/do I need a vapor barrier? If needed, would it need to go below the insulation between the insulation and the ground, or above the insulation, attached to the bottom of the floor studs?


    I don't know much/anything about this so any/all info helps!

    Thanks all,
    Jon

    You didn't say what the building is sitting on? Is it a post foundation, concrete walls? Or is it all open under the floor to the outside? If it's all open, you can always lay down a good vapor barrier on the ground, skirt all around the perimeter, and insulate the skirting walls with blue board.
    Sheep hunting...... the pain goes away, but the stupidity remains...!!!

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    Tongue and groove subflooring would generally qualify as a Class II vapor retarder....
    "– Gas boats are bad enough, autos are an invention of the devil, and airplanes are worse." ~Allen Hasselborg

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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by cdubbin View Post
    Tongue and groove subflooring would generally qualify as a Class II vapor retarder....
    I've in the past heard that proposed to be the case. I call BS on such based on personal experience. I can say definitively that high quality 1 1/8" T&G subfloor, glued and screwed as meticulously as humanly possible, with the best available adhesive, under optimal conditions, will NOT subsequently prevent liquid water from passing through like a sieve. And if it doesn't retard passage of liquid water it sure as hell doesn't retard passage of gaseous water vapor.
    ...he who knows nothing is nearer to truth than he whose mind is filled with falsehoods & errors. ~Thomas Jefferson
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    Quote Originally Posted by iofthetaiga View Post
    I've in the past heard that proposed to be the case. I call BS on such based on personal experience. I can say definitively that high quality 1 1/8" T&G subfloor, glued and screwed as meticulously as humanly possible, with the best available adhesive, under optimal conditions, will NOT subsequently prevent liquid water from passing through like a sieve. And if it doesn't retard passage of liquid water it sure as hell doesn't retard passage of gaseous water vapor.
    The goal is rarely to prevent vapor from passing through the building envelope entirely (which is an exercise in futility), merely to slow it down enough that it STAYS gaseous....there are constant, and measurable, anabatic/katabatic type forces at play in the thermal life of a structure over the year...it's best to be prepared for all of them. For example, during these hot summer days we're having, I can guarantee that many houses will have moisture accumulating on the INNER side of the walls during the day, especially around the top plate on the south side...moisture always flees the heat....
    "– Gas boats are bad enough, autos are an invention of the devil, and airplanes are worse." ~Allen Hasselborg

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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by cdubbin View Post
    For example, during these hot summer days we're having, I can guarantee that many houses will have moisture accumulating on the INNER side of the walls during the day, especially around the top plate on the south side...moisture always flees the heat....
    If such was indeed the case one would have to ask: from where did said moisture originate, and why in our climate is the temperature of the area of the wall you describe below the dew point on a hot day?
    ...he who knows nothing is nearer to truth than he whose mind is filled with falsehoods & errors. ~Thomas Jefferson
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    Quote Originally Posted by iofthetaiga View Post
    If such was indeed the case one would have to ask: from where did said moisture originate, and why in our climate is the temperature of the area of the wall you describe below the dew point on a hot day?
    Ok, back to the fundamentals...water vapor, for all intents and purposes, originates in THE AIR...lol...
    ...also important to remember, "dew point" can be in the 70s Fahrenheit....and warm air holds much more moisture than cold. Fun fact: Alaska has the highest average relative humidity of any state in the nation...we're #1!

    https://www.forbes.com/sites/brianbr.../#5e1c9773330c
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    Supporting Member iofthetaiga's Avatar
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    Yeah, fundamentals. So again, I would ask: why on a "hot day" (what's the temp?) in (where are we, Anchorage?): why is there an accumulation of moisture inside a wall cavity? From where did that moisture originate, and why is the inner surface of the wall cavity below dew point? A demonstrable example (showing the math) of how such a condition commonly exists in a properly constructed structure would be interesting.

    Nothing personal, mind you. But you put it out there, so it would be interesting if you were willing to show actual data/math to support the assertion.
    ...he who knows nothing is nearer to truth than he whose mind is filled with falsehoods & errors. ~Thomas Jefferson
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    Member cdubbin's Avatar
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    It's elementary, my dear Taiga:

    Water vapor moves from areas of high vapor pressure to areas of less pressure. As in the case of heat transfer, water vapor moves from areas that are warm to those areas that are cold. Therefore, in the case of a wall, the vapor pressure will move from the hot, humid air found in the summer time on the outside to the cooler, dryer temperature on the inside of the building. This means that water vapor is being driven though the wall cavity.
    http://www.awcitechnologycenter.org/...ture-migration

    P.S: "dew point" calculators are available online....plug in your local conditions to see what the moisture is doin'....
    "– Gas boats are bad enough, autos are an invention of the devil, and airplanes are worse." ~Allen Hasselborg

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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by cdubbin View Post
    P.S: "dew point" calculators are available online....plug in your local conditions to see what the moisture is doin'....
    Yeah, the internet's pretty cool, huh? Handy for folks who don't have the DP charts just sitting on their desk. So let's take a quick look at some Anchorage stats. In the past week, the highest dew point temp was 66ºF., which occurred only briefly on the 13th. The average dew point temp was 55.57ºF. Your hottest day was July 4th when the temp hit 89ºF with a RH of 35% and a dew point of 58ºF.... If we scrutinize every day of this summer of 2019, we can find occasional instances when the dew point was impressively high, but again, those instances occurred only very briefly. So, if the inside of your stick-frame wall is acting as an active condenser of external ambient moisture during these hot summer temps in Anchorage, either your wall construction and/or insulation R-value is total crap, and/or you're keeping the inside of your structure really, REALLY cold....or the moisture in your wall is coming from somewhere other than the external ambient airmass. You're correct, it is elementary.
    ...he who knows nothing is nearer to truth than he whose mind is filled with falsehoods & errors. ~Thomas Jefferson
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    Sounds like ANC was a real peach this summer alright!...lol

    ....just a couple final Thoughts to Ponder:
    ...when the summer sun shines directly on a wall, it can become significantly hotter than the ambient air temperature....also, building materials like wood, concrete, and insulation are terrific natural sponges, so relative humidity is only a starting point for the amount of moisture contained within a wall assembly. Finally , a "total crap" R-value would actually MITIGATE condensation in this scenario, as there would be less differentiation between indoor and outdoor temps....voila! ...no moisture migration...
    "– Gas boats are bad enough, autos are an invention of the devil, and airplanes are worse." ~Allen Hasselborg

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    Buildings are built for people, people exhale moisture with each and every breath, our bodies exude it as well. Doesn't take much science to out-science those who just want to argue a ridiculous point that has nothing to do with the original post.

    If you want to know how to build a building that is vapor tight, AOD is apparently the place to go. From everything I've ever read and experienced, buildings need to "breathe". The way I understand it floors are not the place to install a vapor barrier, walls and roofs are...but then I don't build buildings for a living, maybe some of those commenting here do?
    “I would rather have questions that can't be answered than answers that can't be questioned.” Physicist ― Richard Feynman


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    Quote Originally Posted by Patsfan54 View Post
    The way I understand it floors are not the place to install a vapor barrier, walls and roofs are...but then I don't build buildings for a living, maybe some of those commenting here do?
    Unfortunately, there are LOTS of people in Alaska who "build buildings for a living", but don't understand basic physics.

    No web forum is a good place to go to learn about anything, unless those involved are actually interested in understanding what they're discussing. Science, by definition, is unemotional. Unfortunately, most people are unable to discuss science without injecting emotion. Typically it devolves into individuals with delicate egos standing round wagging their d**** at each other. ...And then the trolls arrive to snipe from the sidelines.

    Regardless, when you hear phrases such as: "moisture always flees heat"; "heat rises"; warm weather following a cold spell "draws the frost out of the pavement"; "you don't need vapor barrier in floors"; etc....you know it's likely pointless to try to engage a conversation, and you'll have a long row to hoe if you do. So it goes.
    ...he who knows nothing is nearer to truth than he whose mind is filled with falsehoods & errors. ~Thomas Jefferson
    I would rather have a mind opened by wonder than one closed by belief. ~Gerry Spence
    The last thing Alaska needs is another bigot. ~member Catch It

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