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Foam board in OLD Garage - air gap ?

Hi all,

Newb... I have an old (1960s) SEPERATE Garage, that got flooded in Hurricane Sandy and i had to gut it out, the Framing and Plates and Ply are fine (its on a Slab). Has NEW:
-Elect BB Heat
-Air Cond for summer (mid atlantic)

I wish to leave it open (no drywall) in case of flooding again BUT wish to INSULATE it with 2" Foam (R10 i think) board between the studs. I do NOT want to use FGlass or Dow fiber Insulate.

The Plywood wall has Vinyl siding on it but no Vapor Barrier under it (it was added in the 1980s), however the ply is mounted Vertically so there are NO seams in it (gaps). I know it should prob be corrected (VB added on Ext) however, due to the Financial LOSS of the Hurricane, i am on a restricted budget, to say the least . Please understand i will not be able to make it 100% correct due to $$$ loss, but want to get it as close to right as possible.

**** My main question is :
-Do i put the Foam board directly against the plywood (on the interior of bldg, the ply is the only wall) ...OR..
-Do i leave an Air GAP between the Foam and the Plywood wall (again there is NO Interior wall/drywall) ?
-Should any type of Vapor Retarder be added on the inside?

I am looking to keep it basically Climated (not 100% - 50% is fine----at c. 80 in Summer and c. 55 in winter ...Mid Atlantic) - it has Dehumidification (now).

I have been told both by various people , but they didnt know for CERTAIN.

Thanks !


Re: Foam board in OLD Garage - air gap ?

The truth is for this application only, it probably won't make any difference. The temperature difference between the interior won't be that great and unless you install a shower and a kitchen in the garage, you won't generate that much interior moisture. I am also guessing that the garage won't be sealed up tight like a house either, expect a little air infiltration around the garage door and windows.

But if it were mine, I would leave a small gap between the foam board and the plywood so that the inside of the plywood can breath. Unless you seal up the garage like a house, I would not bother with any vapor barriers.

Re: Foam board in OLD Garage - air gap ?

Thanks.... so you mean it doesnt matter if i put it flush against ply or leave gap?

But to leave a small gap for ply to breathe...i should note that the ply was applied vertical on studs and tight (this was in 1962 so they may not have done the expansion spacing then?) - anyway there isnt much air gaps in it. Yes, the main "leaky" airflow is from the garage door and the old window framing, as you mentioned (no kitch or shower or running water).

How is Foam board (if against the ply) diff than SIPs' or Spray in foam which is against the ply i think?

Thanks much !!:D

Re: Foam board in OLD Garage - air gap ?

Installing the foam against the sheathing will be easier. I don't believe leaving an air space will do anything other than creating a space for condensation to occur or a place for mice and bugs to live.
Most building codes don't allow foam insulation to be left exposed unless rated for exposure.

Re: Foam board in OLD Garage - air gap ?

With SIPs and spray in foam, the foam is bonded to the plywood or OSB. No matter how close you stuff the foam boards in, there will be small gaps from the irregular surfaces.

I cannot find a good argument against what ed21 said, but if it were me. I would leave a small gap, but again, I don't think it will really make any difference for this case only.

Re: Foam board in OLD Garage - air gap ?

What Ed said about exposed foam on the interior is very valid check with your insurance co.
They most likely will not allow it to be exposed.

Re: Foam board in OLD Garage - air gap ?

Thanks all !!

VERY Helpful.

OK, so i will end up closing it in and go to FGLASS, not Foam. Due to flood potential, is there any INSURANCE reasons i against the following:

-building a 2 foot compartment between all studs [a 2x4 is horizontal between studs to keep Insulation seperate , so any flooding damage to Insulation/Drywall will be "contained"]...ie: IF it floods (usually 2-6" deep if a major Hurricane) i can simply remove the lower 2 feet of DWall and Fglass (screwed on , No Glue or Mud for removability), and Dwall is seperated from upper part by 1/4" gap,(prob i had before was moisture wicked up the Ins and DWall) ?

My Insurance covers Fire, Wind, etc....NOT Flood, so i am trying to have some BASIC Flood mitigation method with this (cant stop it, so trying to make it Easier to repair (able to quickly screw off and OPEN lower walls, VS having to SAW them out and ruining whole wall possibly) and Contain any "wicking". Flood Ins is a Gamble here , many people in this area didnt break even with it and paid out more than recieved.

Also, is an Open , Exposed Garage (just studs and Sheathing) OK to Insurance companies IF NO Insulation (you see this often)..?

To Close in, does it HAVE to be Drywall or is Pegboard valid (to Insurance) ?

Thanks again!!

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