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Question DetailsAsked on 2/13/2015

Help! Do I need housewrap?!

We're in the middle of having vinyl siding installed on our 1950s house. The contractors had installed foam board on most of the house before I asked why they hadn't installed the Tyvek we asked for (and have in the contract).

I have read mixed things on Tyvek - that its needed, its not, its only for new houses, it should go over foam, under foam, over or under based on climate, it doesn't allow a house to breath, it is breathable etc. etc.

I will be refunded if its not used. I just am looking for a definitive, simple answer: Should I get housewrap?

* Foam = 3/4" foam (Polar-R I believe)
* House = tongue and groove hardwood
* Original siding and tarpaper were removed
* and age of house has very little soffit
* We live in upstate NY
* Some rotted wood from moisture will be replaced

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3 Answers


Dear Guest: Most importantly, yes you want the rotted wood replaced first and foremost.

Then, yes it is recommended to have a "Tyvek" type house wrap installed on your home. The wrap should go behind the foam insulation. The seams should be taped and the windows and doors should be taped. Once this is done, the wrap is designed to make the home water tight in case a storm comes before the siding is finished being installed. The foam on the siding is designed to channel the water down the siding to the bottom and out. This is an insurance policy to keep water out of your home in case it finds a way behind your siding. The house wrap by design is breathable. It is designed to let water out but not in. Our company does not like to do any siding jobs without installing the wrap.

Answered 5 years ago by ExteriorUpgrader


I agree with what - who was it, ExteriorUpgrader ? - said. In new construction (or where doors and windows are also being replaced at same time as siding) the housewrap would usually go on the exposed studs, as other comment said, and be tied into the waterproofing at doors and windows - wrapping into the rough openings to provide positive overlap with the water shield placed under the window frames and J or C strips around the doors/windows, as applicable.

With re-siding jobs like yours, it is tough to get that full interface so commonly you end up with lap joints made with ice and water shield to tie the window and wall watershield together.

Putting it under the foam board (over the newly repaired existing siding, as I understand it) does protect the house from rain during the siding installation period. My personal preference, if weather conditions allow and you are working wall by wall so a given surface is only open to the weather during one day, is to put it OVER the insulation board to keep it dry too - otherwise you can get water percolating through the cracks in the insulation board and freezing or molding there, as well as reducing its R value due to water absorption, which can reduce the R value of foam board by as much as 90% with open cell, and about 50% with some closed cell boards over their normal lifespan. One reason I prefer the Dow Hi-series products.

Either way, the underlying hardwood siding should be quite dry before encapsulation with the foamboard and housewrap.

Check manufacturer's recommendation on the siding AND the housewrap regarding whether there should be firring strips between the foam board and the siding. Many brands, especially with wood or insulated sidings, want an airgap between the siding and the house wrap. This is always a good idea as it prevents surface-to-surface contact with the housewrap (which increases water transmission through it) and also provides ventilation behind the wall to dry it out if it gets wet, but in some configurations especially with thicker insulation board and/or recessed window frames causes a real problem at the windows and doors with the insulation/siding package protruding way beyond the windows. With insulated siding it is somewhat of an oxymoron, as an airgap behind it eliminates most of its insulation value with respect to heat loss from the house, as opposed to insulation against solar heating, which it does still block to a large extent and actually better than if in direct contact.

If using Tyvek brand (which si my specification preference), this is likely the product you are using - page has installation video for use over foamboard, installation instructions PDF, etc on it -

Also, with the Polar-R, it has a metallic coating to act as a radiated heat reflector. Their website does not say anything about this issue but should, as foil layers only work if they face an airspace - they have insignificant effect one way or the either in direct contact with other materials. Normally, in a warm environment where A/C costs are high, the foil layer would face outwards to reflect back heat radiating from the siding to the house (meaning siding would have to be firred out for best effect, to provide radiative airspace plus to vent heat behind the siding- a so-called "double wall" system), whereas in cold areas where most of the fuel bills is in heating and the heating season is long, the foil layer would face inwards to reflect house heat back into the house - and again, have to be firred out but onthe inside face, to provide an airspace. Not knowing exactly where you are, hopefully they installed it the correct direction - most of upstate NY would presumably be in the latter situation.

But yes - I would definitely use the house wrap - just be sure it is the type designed for use under siding - there are roofing water barriers which are very slightly water vapor permeable, siding water barriers which are more water vapor permeable, and essentailly waterproof types - you need the siding type, which is somewhat permeable to water vapor but in a vertical installation sheds water pretty well.

Also - be sure they lap the housewrap correctly - install from bottom up so upper sheets overlaps lower sheets to drain water down the outside of the sheets instead of being able to enter at the laps. Vertical lap joints should also be taped with manufacturer's joint tape, so water cannot wick sideways through the seams. At the bottom of the wall the housewrap has to terminate such that it drains any water outside the foundation - commonly lapped over the wall sill and the first 4 or so inches of foundation, hidden of course by the bottom of the siding. See housewrap manufacturer website for details for different types of house construction. Too many contractors just stop it at the bottom of the house wall, so any water running down it end up entering the house at the sill plate or rim joist, or enters and causes rot at the bottom of the wall at the bottom plate.

Two afterthoughts - as ExteriorUpgrader said, some house foamboard is tongue and groove or lap type, designed to shed water - so obviously it has to be instanned so any water coming down the board is shed to the outside when it hits the sheet below - too easy to install upside down so any water entering the joint can go right into the wall. As for taping insulation board joints - some contractors do that, but I would not with the housewrap - because it makes for two vapor retarders (the housewrap and the essentially impermeable taped foam board) so can trap mosture between the two.

And if siding is firred out to provide an airspace, the housewrap and airspace have to be open at the top to allow the warm air to rise and escape (or cold air to come in, or come in at the top in winter, with uninsulated metal siding), AND that top airgap has to be protected from rain/snow entry if not protected adequately by roof/soffit overhang. And should vent ot open air, not directly into the attic

Answered 5 years ago by LCD


For insulation, energy savings and water protection, Strip existing siding down to substrst, repace rotted wood, house wrap, insulate and side. in that order. get a good sill dam wrap on any open window and doors.

Answered 5 years ago by the new window man

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