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Question DetailsAsked on 5/17/2016

Should the insulation be taped under vinyl siding

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Answering this question is treading on dangerous ground - because some will say yes, some not. Huber and DuPont and ZipSystem say yes do tape, Dow and Owens Corning say do not though their products do mention that as a use for their housewrap tapes - and Hardi (or hardiBoard or HardiPlank) have said both in different documentation, so that gives you an example of how fragmented the opinions are.

If I had to give a generic yes or no, not relating to specific house design, I would favor YES over NO as a general preference, though of course not best for all cases. And generally, tape manufacturers say minimum nominal (plus or minus 1/4 inch) 4" wide tape so it has better chance of accomodating the expenasion and contraction of the foam board.

There are code provisions and manufacturer recommendations both ways - generally building codes do not require it, energy efficiency codes do - if concerned you would have to check with local building department on whether they have a specific requirement for it. I can't imagine any would prohibit it, but maybe.

Reasons people say not to:

1) the foam insulation board (assuming that is what you are talking about) is a fairly good vapor barrier, so taping the joints closes up the only remaining place any moisture in the walls can escape through if it gets in there, assuming you have an interior vapor barrier right under the interior drywall as most houses and most areas do.

2) many will say that if there is a water barrier/moisture barrier (housewrap) under the siding that will stop the water before it gets to the foam board so no need to tape the joints

3) if the foam board is outside the water barrier (done in some cases, especially with the thin accordian folding sheet foam panels used under some siding to give it some thermal insulating value, no matter how little), then the water barrier sheeting is counted on to stop the water and airflow so not needed

Reason people say to do it:

1) you want to stop any water that makes it past the water barrier elements (siding plus moisture-water barrier sheeting/housewrap) so it can't get into the insulation in the wall

2) taping joints cuts air exfiltration from the house, improving energy efficiency - in fact, if left open and the wall is drafty because surfaces and wiring penetrations and such were not sealed in construction, their open presence with the open joints can almost negate the insulation benefit of the foam board

3) not taping, so leaving the joints open mean any moisture in the wall will be exiting at a narrow distinct location - hence form frost or mold stripes on the back side of the housewrap or siding because it is concentrated, rather than diffused through the foam board. This argument had distinct weaknesses, because it basically says leave the moisture in the wall rather than have concentrated stripes of dissipation through the insulation. Also assumes moisture can diffuse through the foamboard, which is not really valid with the higher thermal resistance closed-cell foam boards usually used for this purpose.

My personal criteria in lieu of specific code requirements in a given area -

1) if the house has external vapor barrier (true vapor barrier near outside of wall layer, usually done only in very wet locales like Pacific Northwest rain forests and some of far Northeast and some hurricane/typhoon areas) and has NO interior-face vapor barrier, then do not tape because you are putting two vapor barriers in the outer wall, which can trap moisture. If no internal vapor barrier OR exterior one, then the foam board is the only vapor barrier so DO tape joints.

2) if a "tight" house, with good energy efficiency (basically fairly new construction with good Energy Star rating) with expected very low moisture gain and moist air transmission through the wall from inside the house, then do not tape it if there is a direct contact moisture barrier sheeting over the inside or outside of the foam, do tape it to limit through-wall airflow if there is an airgap outside the foam - either due to no moisture barrier under the siding, or an intentional "double-wall" air gap under the siding and moisture barrier/housewrap.

Some contractors hedge their bets and tape horizontal joints so any water running down the face of the insulation does not enter at horizontal joints (thereby hopefully keeping most but not all of the water out), but not tape at vertical joints, to let airflow occur. Others tape with permeable housewrap tape - is moisture permeable, not waterproof. A few manufacturers recommend this - but to my knowledge the Dow Housewrap vapor permeable tape is not out yet, so foreign products (at typically $10-20/roll !) from Siga and Contega and such are your only choice. Domestic products, to my knowledge, are all waterproof and air-blocking - designed to seal seams on permeable housewraps and air gaps and such, but not vapor permeable themselves. This in one main reason why a building with insulation board on the outside of the framing should have a vapor permeable (in almost all cases) water shedding housewrap between it and the siding material, to stop infiltrating water before it gets to the insulation. This also keeps the cheaper insulation boards from soaking up moisture from the wet housewrap, as all too many of them do. (Reason I recommend only Dow Hi-series foam boards - Dow HI-40 or other Hi-series products as applicable to the application).


If by some chance you were talking batt insulation, then it should only be faced if it is supposed to be acting as a vapor barrier, in which the joints should be taped to maintain vapor barrier integrity (though batt facing is generally not a very effective vapor barrier in any case in the as-installed wall).

Answered 4 years ago by LCD

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