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Question DetailsAsked on 9/22/2017

My house has foam board sheathing (no OSB) in SC. I am having vinyl siding replaced. Do I need housewrap over foam?

Contractor says No - As existing foam board already gives a barrier and if he uses house wrap he won't be able to find studs to nail in the siding. I want the job done right but don't want siding that comes loose due to not being nailed to the studs. Hate the cheap way they are building with foam. No house wrap on it now. House is 24 yrs old.

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4 Answers


Hello AdviceNeeded,

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Iann M

Answered 3 years ago by Member Services


Here are some previous similar/related questions with answers FYI:

Note that vinyl siding is NOT a waterproof surfacing - it is a water barrier but water commonly blows and evaporate up inside it, so you need a drainage barrier behind it to protect the wall from water. And foam board is not waterproof - even if closed cell ( and a lot of what I see these days is pretty water permeable) the joints are generally far from watertight, not to mention being an acceptable airflow barrier.

As for his argument about not finding studs - has he never heard of running a plumbline or chalkline on the housewrap ? Not to mention a lot of housewrap comes with studline markings on it if you install it spaced correctly so the lines align with the studs. Sounds to me like he is trying to save about $0.25-0.50/SF in materials and labor at your expense - did the contract (if already under contract) call for use of housewrap ? You could also ask local building inspector - housewrap is required by code under ALL siding in most areas since about 2004. The - or what is it called - - VInyl Siding Insitute as I recall, which is the manufacturer association, requires it in your situation. Most siding manufacturer instllation instructions also require it - if you have a chosen brand, check manufacturer website for instruction sheet (which also comes with wach bundle of siding). If not under contract, you can just require firring out and housewrap in the bid - could be this is how he intends to come in as low bidder, by skimping here and there.

My recommendation, especially in a generally warm/hot and probably humid area like yours, is a ventilated siding - standoff vertical battens to hold the siding off the wall, with vapor-permeable housewrap directly under the siding and over the battens to keep the housewrap (which can get damp in rains) off the foam where it can cause mold to grow and moisture to get into the wall.

Answered 3 years ago by LCD


LCD - Thanks for answer - Some further follow up - Contractor is saying he will reduce the bid by ~$800 assuming foam board sheathing is there with no plywood and no OSB .... as he does not think the wrap is needed. He indiacated the foam shaeathing (Styrofoam 1/2 inch R3 blue board) already has a coating on it to prevent moisture from penetrating. He is also concerned with trapping moisture between the wrap and the foam crating issues later.

Answered 3 years ago by AdviceNeeded


To predicate my response on one thing I assumed - perhaps incorrectly. That your vinyl siding is ordinary plank/lap siding, not vertical panel sheet siding, whcih can make a difference in some cases.

I would say read the manufacturer installation instructions on the products - I have never seen a (modern) lap siding installation instruction that did not call for a water barrier (asphaltic felt or housewrap) right under it, a gap between plank siding and a solid foam layer, nor a foam insulation product (presumably fan-fold in your case) which said it acts properly as a housewrap. Most specifically state they are NOT intended to be used as a vapor retarder - though there is at least one closed cell foam board out there which is fused during installation which is intended as a vapor barrier - but you wouldnot want that in that location in a wall unless in a VERY wet area AND do not have a vapor barrier under the interior drywall like most houses have.

The issue about trpping moisture between the housewrap and the insulation board leads me to think the siding guy does not know his vapor barrier rules - because the proper moisture transmission rating on the housewrap for your construction and area is necessary to prevent moisture being trapped under it (be it from indoor moisture working through the wall or from siding leakage or whatever). The gap between the vertical firring strips and the housewrap is specifically intended (other than to also give definitive nailing strips on the invisble studs) acts as a ventilation layer, vertically venting moisture behind the housewrap out a gap at the top of the siding (which has to be properly designed in, taking into account whether you have a roof overhang or not).

Not providing the gap and nailing direct to foam board, which is not recommended by manufacturers, DOES provide the opportunity for wetness (both from inside the wall and from the siding) being trapped and molding, or being transmitted through the foam board to the wall. Consider interior wall misture (coming in through interior drywall, electrical outlets, etc) passing through the wall to the siding plus the moisture that WILL get in there from rain on the siding - when the back of the siding is at a temperature below the dewpoint, that moisture will condense on the back of the siding. Without the firring-out air gap to allow that moisture to dissipate / get vented (usually downward in the wall at night when siding is colder than interior of wall, rising up behind the siding in warmer temperature and when siding is sun-warmed - this being the primary time it dissipates the moisture.

If the housewrap (proper moisture-permeable type) is used in direct contact with the foam board, then it WILL get wet at that interface - the housewrap acts like a fabric (as opposed to vinyl) shower curtain behind lap siding - intercepting and letting the infiltrating water run down it, and letting moisture inside escape through it when it is dry outside. hence, the need for the firring gap.

I would refer back to the manufacturer instructions, check with your local building official (they most likely require housewrap under the siding), plus all the installation recommnedations I have seen on siding and foam board manufacturer associations have called for housewrap under lap siding.

This gap is why insulation-backed siding plank is largely a waste of money.

Push comes to shove, if you have a local Architect on the job, ask him to document his professional judgement on the issue.

On the discount offer - I would pay attention to doing the job right, not taking a discount for taking a shortcut. It sounds like he was going to put the siding directly on the foam board - I wonder how he thought that would not cause the same potential for mold as using the housewrap.

Note - the housewrap needs the proper perm rating (water vapor permeability measure) for your wall construction - we are NOT talking a water and air-tight vapor barrier like visqueen here - which in a typical wall cross-section with interior vapor barrier under the drywall WOULD have the potential for promoting rot in the wall. Your wall need to be able to breathe - typically in one direction, with vapor barrier under the first layer one one side, open of moisture dissipation to the other side, and except in some very wet climates the vapor barrier is on the interior side - visqueen under the drywall in probably 95% or more of instances. you can find a lot more about this subject on the DuPont Tyvek website, with illustrations of how moisture dissipates and how it gets in through lap siding (which is a water shedding surface, NOT a water barrier).

One other thing not addressed - is whether the foam insulation on the house now is properly vapor permeable - some contractors/homeowners put on "fused surface" closed cell foam board with sealed joints which is essentially waterproof - this acts as a vapor barrier and can cause issues, especially if thin like yours probably is. The foam layer has to be designed as a siding underlayment, able to vent interior wall moisture to the outside.

Here are a few more similar questions with answers FYI:

Good Luck

Answered 3 years ago by LCD

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