Ask Your Question

Angie's List Answers is the trusted spot to ask home improvement and health questions and get answers from service companies, health providers and consumers. For ratings and reviews on companies in your area, search Angie's List.

Top 30 Days Experts
Rank Leader Points*
1 kstreett 240
2 Guest_9020487 110
3 Guest_9190926 105
4 GoldenKid 100
5 ahowell 95
6 KnowledgeBase 95
7 skbloom 80
8 Guest_98024861 70
9 Guest_9311297 70
10 Guest_9400529 70

*Updates every 4 hours

Browse Projects By Category

Question DetailsAsked on 8/6/2017

Do you put on house wrap THEN fan fold or do you put the house wrap on and fan fold over it???

My house currently has cedar shake which I'm removing because it is in poor condition. I plan on putting vinyl siding. My question is do I install fanfold foam insulation over the substrate and then put housewrap over the fanfold or do I put the housewrap on and then fanfold over that and then vinyl siding any help with this would be greatly appreciated

Do you have the same question? Follow this Question

1 Answer


In my opinion and that of many other building pros (lots of blogs on the subject if you google "fanfold siding insulation" - forget the fan fold. It acts as an unwanted partial vapor barrier from the inside, with the holes punched in it (and generally with most brands due to thinness also) does not act as a water barrier because significant water leaks through the holes, and the insulation value is negligable and incredibly pricey per R value unit. if you need wall insulation go with insulation board with a meaningful R value, though that does commonly cause issues with door and window flashing and trim, and commonly means they should be removed and reset in the process.

DuPont has said on their website and product flyers that if using both fanfold and housewrap and committed to both, put the Tyvek housewrap OVER the fanfold, but they actually recommend (assuming the insulation is totally closed-cell with very low absorption rate) putting the insulation directly under the siding, overlying DuPont DrainWrap - which is designed to provide a better drainage path between the two than regular housewrap.

But for my money and that of many contractors, the two are not considered a good fit - better to use insulation board rated for that application, or a siding with integral sealed-in insulation in the siding, though the latter is largely wasted insulation because of the airflow behind the siding which allows airflow to defeat the insulation to a large extent, and unless a real thick product does not have a very meaningful insulation rating anyway.

Note if you use fanfold insulation - most of them are a lot more absorbent than their ads imply, so if you don't get a top-quality closed cell foam product it can become basically a sponge. I have seen several brands being removed in tearoffs just a couple of years or less after installation because of wall rot, where you could actually wring substantial amounts of water out of the thin insulation (roll or fanfold installed, either one).

My personal recommendation, assuming you are in a locale with winters where the practice is to put a full vapor barrier under the interior drywall, is to use a closed cell foam board rated for use under siding if additional insulation is needed, covered with DrainWrap - or better yet, foamboard then vertical treated or cedar standoff strip firring then DrainWrap or Housewrap with appropriate outer-wall use moisture permeability, then the siding. That firring out provides an airspace to evaporate moisture (requires bottom and top-of-wall screened venting in many cases and with some products to work right) and provides a wicking break to minimize moisture transfer into the wall from the sometimes substantial blow-in from the lap vinyl siding.

Note that vinyl (and metal) lap siding is NOT a water barrier, it is a cladding - protects against impact and solar damage and sheds a lot of the precipitation, but is NOT waterproof because of the blow-through and wicking between boards and at joints. This can be mitigated with proper installation, but look at pretty much any vinyl siding tearoff job within a week or so of a blowing rain (and commonly after any normal to hard rain) and you will see wet areas or total wetness behind it.

Answered 3 years ago by LCD

Related Questions

Terms Of Use
Privacy Policy