Options for adding additional insulation over vinyl metal building insulation?
mcurrier
| Posted in General Questions on
I’m a small organic farmer who tries to do things myself located in zone 5, ~2 hrs. north of Springfield, IL. I’ve read over a lot of articles on this wonderful cite which kind of touches on the topic, in a round-a-bout way… I have a pole shed that my dad had built in 2017 and there is vinyl face insulation pinched between the outer girts and steel siding. I want to add insulation to the inside to make it more comfortable for winter work; probably won’t AC it in summer. Am I good to put foam board against that and seal up or would it be best to remove it altogether? Any advice or references would be appreciated. Matthew
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Replies
Hi Matthew,
Is the insulation's vinyl only facing the inside with exposed fiberglass against the steel? Or does the vinyl facer entirely wrap the insulation? My research into the product suggests it is a vapor retarder. In your climate, this is on the wrong side of the wall (a vapor retarder should be on the warm in winter side of an assembly.) I'm wondering if the vinyl is also trying to act as a water resistive barrier (WRB), to keep bulk water leaks out of the wall. If you cut the MBI out, will there be any product acting as the WRB?
If you leave the MBI in place, and add insulation, I'd recommend using a krafted faced batt or a class II vapor retarder (smart vapor retarder like Membrane), this will allow at least some drying potential inward. I'm assuming this is an accessory building, not residential, ag buildings have much different codes than residential structures but building science principles still apply.
Randy
Hey Randy, thanks for the reply. The vinyl is just on the inside (not against the steel) and isn't taped to seal the vertical edges (I can slip my hand behind the 4" overlap, which is every ~3 ft). I am going back and forth between: #1.) leave/remove blanket insulation-----add foamboard & seal w/ canned foam around that--------add osb or batts/osb over that
and
#2.)leave vinyl blanket insulation in---------add bats over that----- ------add osb board In (#1) I didn't know whether to put foam board against vinyl or if that would be problem sandwiching 2x6 girts. In (#2) I didn't know how much to allow to breath towards inside (you recommended faced bats?). I prefer not to do big spray foam cause doing this as time & money allow for. thanks, Matthew
No WRB if I were to cut it out.
Is something like the attached photograph what you are talking about?
I would think so long as you have a vapor retarder (like taped closed cell foam, painted drywall, taped faced batts, etc) on the inside that you would be ok, no?
Yes, that is precisely what it is (our walls are 8' on center). I have read on other articles on "greenbuildingadvisor.com" that it's preferable to put the foam board on in side of the girts vs in between the girts. Would having this gap between the two layers be and issue, where I would be better off butting it against vinyl? Placing foam in between girts wouldn't seal as much but there is that extra layer on the outside so... Thanks again for you time today and all good ideas. Matthew
So, in wall and roof assemblies you want to look at condensation. If there is no heating and cooling there isn't as much to worry about but when there is heating or cooling we have to look at the condensation point. Luckily it sounds like you just want to heat the structure so the air will likely be drier in the winter but many heat sources like propane or natural gas add humidity to the air so what you don't want is the warm moist air reaching a cold surface that is lower than the dew point of the air or if condensation does occur the location of the condensation is able to dry to one side or the other. This point may or may not be the vinyl backing of the insulation that is already installed. Do you know the r-value of the existing insulation? When you say "foam board" do you mean EPS or XPS? EPS will breath to some degree but XPS not so much. EPS is also more environmentally friendly than XPS apparently. From the two or three different approaches you've mentioned I don't know that it would matter much either way. If you use XPS and seal it well then that should be a vapor retarder and keep moisture from entering the wall but on the flip side if moisture did enter the wall it might have a hard time finding its way out. If you use EPS that assembly might dry to the inside since it is an open cell foam. If you use batts with osb that should also dry to the inside. What kind of batts? Fiberglass is damaged by moisture but generally mineral wool is not. Would you paint the OSB as that could act as a vapor retarder but should also allow the assembly to dry to the inside. I'm more of a mechanical guy so maybe some of the other building experts on here can help more than I. I was just interested to see possible solutions.