Foam Board

Mstrlucky74 said:

So in one of my other threads someone said foam board is unnecessary under vinyl siding. Is this true? Just the house wrap and vinyl is ok. Doesn't the foam board provide some R value of insulation? Every video I watch all the builders install the foam board? Thanks

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Some siding comes with a thin foam that would do little but add stiffness to the vinyl. Foam board is a different application and would add insulation.

Nealtw said:

Some siding comes with a thin foam that would do little but add stiffness to the vinyl. Foam board is a different application and would add insulation.

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Ok so I'm referring to foam board.

Mstrlucky74 said:

Ok so I'm referring to foam board.

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I think there was a little talk about the stuff that came with the siding as not doing much. I would do the foam board.

Nealtw said:

Mstrlucky74 said:

Ok so I'm referring to foam board.

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I think there was a little talk about the stuff that came with the siding as not doing much. I would do the foam board.

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Thanks

The last time I checked the NY code limited extruded foam underlayment to a max of 1/2”.

Old Thomas said:

The last time I checked the NY code limited extruded foam underlayment to a max of 1/2”.

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Hi Thomas, if they did they made a mistake. Foam board on the exterior is a great addition but it needs to be thick enough to keep its inside surface above the dew point and that means the inside cavity fill needs to be considered, it is a balance. But there are no climate regions where 1/2" foam board is recommended.

An exterior application (under the siding) covers all of the framing and reduces the thermal bridging including top plate and rim joist. With proper taping it also reduces a lot of the air leakage. The result is an improvement above just the r-value.

There is guidance for different climate regions and interior cavity insulation.

Bud

My old house is t&g boards and full of air leaking joints. The house was renovated and has r11 fiberglass. I'm adding 1" xps to outside, mainly to air seal, and asked at the building office if those added numbers will be accepted as one r value wall, ie, r16. It was ok. Going further, to 1.5" boards, would be keeping to the standard but added too much work with reframing for window, doors, and impossible cleats for vinyl siding 8" nailing schedules. Also I still am not sure if roofing nails that long won't sag over time. I wish there was a study on it.

carpdad said:

My old house is t&g boards and full of air leaking joints. The house was renovated and has r11 fiberglass. I'm adding 1" xps to outside, mainly to air seal, and asked at the building office if those added numbers will be accepted as one r value wall, ie, r16. It was ok. Going further, to 1.5" boards, would be keeping to the standard but added too much work with reframing for window, doors, and impossible cleats for vinyl siding 8" nailing schedules. Also I still am not sure if roofing nails that long won't sag over time. I wish there was a study on it.

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There have been a lot of homes where vinyl has been installed over rigid foam and i'm certain all of your concerns have been addressed. I like articles by Building Science Corp and Green Building Advisor.

Bud