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Discussion Starter #1
Here's the situation that I have:

My house was built 5 years ago in New Jersey, everything was done by the book and fully scrutinized including the proper amount of fiberglass insulation being installed (as far as code requires, I am not sure exactly what R-value it is though).

Under the house is a large 4' high crawlspace. The floor is made up of TJI joists. The fiberglass insulation doesn't come to the bottom of the TJI, it's about 1-2" short.

I would like to add more insulation since the floor is always cold. I think we are having thermal bridging problems since the bottom of the TJIs are exposed.

It would be easy for us to just screw styrofoam insulation panels to the bottom of the TJI's across the entire crawlspace, but I have two concerns.

My first concern is that there will be a 1-2" gap between the styrofoam insulation board and the fiberglass, is that OK? Second, the styrofoam will be a second vapor barrier because I assume there is already craftpaper on the top of the fiberglass where it is touching the bottom of the floor. Will that cause issues?
 

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Here's the situation that I have:

My house was built 5 years ago in New Jersey, everything was done by the book and fully scrutinized including the proper amount of fiberglass insulation being installed (as far as code requires, I am not sure exactly what R-value it is though).

Under the house is a large 4' high crawlspace. The floor is made up of TJI joists. The fiberglass insulation doesn't come to the bottom of the TJI, it's about 1-2" short.

I would like to add more insulation since the floor is always cold. I think we are having thermal bridging problems since the bottom of the TJIs are exposed.

It would be easy for us to just screw styrofoam insulation panels to the bottom of the TJI's across the entire crawlspace, but I have two concerns.

My first concern is that there will be a 1-2" gap between the styrofoam insulation board and the fiberglass, is that OK? Second, the styrofoam will be a second vapor barrier because I assume there is already craftpaper on the top of the fiberglass where it is touching the bottom of the floor. Will that cause issues?
First off what type of flooring do you have? Carpet? Hardwood? Tile? R-30 is code and it does not need to fill cavity top to bottom. If you really want to up grade your insulation under your home sprayed closed cell foam would do the job too. But R-30 is all you need and done. Don't compress the isulation. something you can do is put vent blocks in your foundation vents to close off the winter winds that add to the chill.
 

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Discussion Starter #3
First off what type of flooring do you have? Carpet? Hardwood? Tile?
Does it matter? I am curious about installing rigid foam insulation underneath the TJI joists.

R-30 is code and it does not need to fill cavity top to bottom.
It doesn't fill the cavity, it is about 1-2" short.

If you really want to up grade your insulation under your home sprayed closed cell foam would do the job too.
I already have 2,500 sqft of fiberglass in there. You would recommend removing that in order to have closed cell foam sprayed in?? :eek:

Don't compress the isulation.
Did you read my post at all? The insulation is short, it doesn't come all the way to the bottom of the joists. That is my concern. There will be an air gap, there certainly won't be any compression of the fiberglass.

something you can do is put vent blocks in your foundation vents to close off the winter winds that add to the chill.
And lock in the moisture...

:huh:
 

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Does it matter? I am curious about installing rigid foam insulation underneath the TJI joists.
Yes it does matter what type of flooring you have tile and hardwood will aways be colder unless you have radiant heating under them. carpet will be warmer duh!
It doesn't fill the cavity, it is about 1-2" short. you can have high density batts that meet the R-30 and not fill the cavity. pull one down and read the R-value.

I already have 2,500 sqft of fiberglass in there. You would recommend removing that in order to have closed cell foam sprayed in?? :eek: you wanted a idea you asked for advice so I gave it to you.

Did you read my post at all? The insulation is short, it doesn't come all the way to the bottom of the joists. That is my concern. There will be an air gap, there certainly won't be any compression of the fiberglass.
yes i did I think your idea is not good. kinda fool hardy you can add unfaced r13 batt if you want to



And lock in the moisture...
It won't lock moisture in if you have a good vapor barrier on the floor of the crawl space to keep the water vapor from coming up to your home.

:huh:
These are things That one can do to help remedy the problem feel free to get mad when free advice is given.
 

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Discussion Starter #6
That link is interesting, but it doesn't answer my 2 questions. Will there be a problem installing rigid foam insulation underneath the joists if the fiberglass already has a vapor barrier? And is it ok to have an air gap between the rigid foam insulation and the fiberglass (that link says to have the air gap between the fiberglass and the floor, but that isn't the case for me)?
 

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These are things That one can do to help remedy the problem feel free to get mad when free advice is given.
Nothing that you said had anything to do with this topic. Some of what you said was downright wrong (closing the vents?!?!?! :censored:).
 

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That link is interesting, but it doesn't answer my 2 questions. Will there be a problem installing rigid foam insulation underneath the joists if the fiberglass already has a vapor barrier? And is it ok to have an air gap between the rigid foam insulation and the fiberglass (that link says to have the air gap between the fiberglass and the floor, but that isn't the case for me)?
No problem with the paper facing as when dry it is 1 perm. when wet= 5 perms. There will not be ANY moisture to worry about if you air-seal the f.b. properly AND the moisture vapor will not condense because of the insulating sheathing board, same as a wall; http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

The gap should be on the top as in the link with no gap at the bottom. A gap there would lead to air movement, degrading the f.g. batt. It needs direct contact to the foamboard, cold side--- gap at warm side (top) only. Do you have insulation stays holding it up- stiff, sprung-steel wires, or is it just pressure fit?

Gary
 
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Discussion Starter #9 (Edited)
No problem with the paper facing as when dry it is 1 perm. when wet= 5 perms. There will not be ANY moisture to worry about if you air-seal the f.b. properly AND the moisture vapor will not condense because of the insulating sheathing board, same as a wall; http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

The gap should be on the top as in the link with no gap at the bottom. A gap there would lead to air movement, degrading the f.g. batt. It needs direct contact to the foamboard, cold side--- gap at warm side (top) only. Do you have insulation stays holding it up- stiff, sprung-steel wires, or is it just pressure fit?

Gary
Thanks for the reply.

There are those stiff wires holding the insulation up. The wires are on top of the bottom piece of the TJI joist so it's about 1.5" up fron the bottom face of the joist.

I could remove the wires to let the fiberglass drop, but I would first have to add something across the bottom of the joist for the fiberglass to fall on without coming out completely. This would be a lot of added work, would the results be worth it? What if I left the air gap at the bottom (between the rigid foam and fiberglass? I assume it would still yield much better results than I have now with only the fiberglass? It should definitely help with the thermal bridging.

The house is only 5 years old and it's pretty efficient. It's twice the size of my last house and costs half as much to heat. The only issue is that the hardwood floors are always cold.
 

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The reason for a gap on the heated side is that the still, dead air will be warmed by the room above. The heat will try to leave towards the foamboard, slowed by the fiberglass insulation. With a gap on the bottom, convective loops will form in/around the air-permeable fiberglass, feeding it from the air layer closer to the crawlspace. Fiberglass insulates by trapping air in many, many little pockets, with the glass fibers impeding air movement to block convective heat flow. This is why you can have convective loops in (not just around) wall/ceiling f.g. if a low-density insulation. So you want fiberglass closer to the foamboard so there is no air-space layer there to start convective loops. Similar to just having the room air next to the f.g. (ignoring the flooring/sheeting/sub-floor) ; that is where the thermal barrier starts, uninterrupted to the foamboard, all acting as a unit. You cannot have two layers (separated by air-space) of thermal barriers count as one total layer. You don't add them up as one posted (similar but not exactly) about his rafters insulated AND ceiling joists insulated, with a vented attic between them.

Gary
 
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Just was reading about insulating crawlspaces and there is a lot of information that says insulating the crawlspace walls will give you better warmth for the floor than insulating between the joists.
 

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Gary, thanks for the links, you're an outstanding resource on this site, but the line that struck home with me most was this:

This is another one of those engineering approximations that drives energy weenies crazy.
It seems that no matter what you do you run up against something else, the effort in juggling temps, vapor transfer, dew points, perms, insulation types, etc., etc., and so on to infinity, is exhausting. There ought to be some optimal configuration that doesn't involve a second mortgage for spray foaming the entire structure (oh yeah, then you'll need a system of air scrubbers like a nuc 'cuz it's too tight) but apparently I'm wrong. Not complaining, just sayin'............
 
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