Replies

1. 
   DanH | Apr 18, 2016 09:18pm | #1

   A proper "rain screen" should

   A proper "rain screen" should not "trap moisture" in a wall.

   However, it's not clear that a rain screen is needed behind standard fiber-cement siding.

2. 
   renosteinke | Apr 18, 2016 11:08pm | #2

   You have Several Issues

   First, on the interior you should not have covered the paper batt facing with plastic. Moisture might get trapped between the two and let mold form. Oh Well. Fortunately, most DIY insulation installations are poorly done, and the paper is not properly connected.

   As for the outside of the house ...

   House wraps (like Tyvek) are not vapor barriers; they're barriers against LIQUID water. Even 'old fashioned' tar paper is not considered to be a vapor barrier.

   A 'rain screen' is a small gap made between the siding and the housewrap, to allow and moisture that gets past the siding to drain freely off the wall, rather than be trapped between boards. Such a gap need not be large - one commercial product creates a 3/8" gap. There are also commercial products that block insects from getting into the gap, yet still let the moisture drain.

   Hardiepalnk can be installed over such a gap. Their website has directions as to how to properly use their produce. It is not necessary to back-prime, or prime 'all six sides,' of Hardieplank. You may, but you're nor required.

1. 
   workman67 | Apr 19, 2016 10:17pm | #3

   i agree

   I was not the one who installed the plastic and agree with you that it should not have been done. My issue is that if the house cannot dry toward the inside because of the plastic, then it will need to dry to the outside which it must have been doing for the past 20 years or so. But we go ahead and add the fiber cement siding directly over tar paper with no air barrier(aka rain screen) for the t-111 plywood to dry toward the outside?? Could this possibly trap moisture in the t-111 because it is smothered by the fiber cement??

   I have installed fiber cement over tar paper in a few older homes recent years and did not bother with the rain screen method because they either had old fiberglass inside or closed cell spray foam inside so i was not worried about the exterior plywood collecting moisture. I am worried that this plastic could create a moisture issue if i cover the exterior without the air gap between the siding?

1. 
   DanH | Apr 20, 2016 12:33am | #4

   You still do not understand "rain screen", and Reno grossly overstates the issue with plastic against fiberglass facing -- it's really not a problem.

   There is a slight problem with FC over the T1-11, as FC tends to retain moisture from rain, and this will tend keep the T1-11 damp.  (Though note I use "tend" here to mean "a little bit, not a lot".)  But, to the extent that this is a problem, a rain screen of some sort will help -- plain Tyvek will help a little, and something that creates an air gap will help a lot.

2. 
   renosteinke | Apr 20, 2016 06:11pm | #5

   Words Mean Things ...

   ... and Tyvek is NOT an air or vapor barrier. Nor is tar paper.

   Both types of 'house wrap' will block LIQUID water, but not water VAPOR. You're not going to create a problem.

   I understood the description to be of T-111 place indoors, in place of the usual drywall. Even if the house has T-111 on the outside, house wrap and fiber cement siding are not a problem. I'd even encourage using a house wrap, as the rough texture of T-111 won't help water drain. The glue within the T-111 and the paint atop it are more of a moisture barrier than any housewrap.

   In any event, your new siding will not close the vertical gaps in the T-111, so there will always be a way for moisture to drain.

1. 
   DanH | Apr 20, 2016 06:44pm | #6

   Actually Tyvek IS an air barrier, which is one big reason for using it.

1. 
   workman67 | Apr 20, 2016 07:37pm | #7

   i have read

   many times the sketches on building science about all differant methods pending the siding, insulation, type of house wraps and this is why i get so confused when something differant comes up. I usually go old school with the tar paper instead of tyvek for the reason of anytime i have seen it installed, tears are always evident and missed spots as well so i figure it is never really working as expected anyway. Plus tar paper is so easy to install yourself and you can overlap it as much as you want to make it easy to cut around windows etc..

   So after all of the answers you gave me, I am leaning toward making the rain screen so to let the extra air flow allow that t-111(on the exterior) breathe. By the way, I should have stated that he installed T &G cedar on the interior over the plastic. No drywall! Thank God for that because i would think that would have a chance to hold moisture more than the cedar would.

   Thanks again guys!!

2. 
   DanH | Apr 20, 2016 08:19pm | #8

   I understand what you have is, inside to out:

   T&G cedar

   Plastic vapor barrier

   Studwall with fiberglass insulation, with the paper facing towards the inside, adjacent to the plastic

   T1-11 siding

   You plan to install FC siding (I presume standard lap siding) over the T1-11, and you're wondering about any treatment between that and the T1-11.

   First, it needs to be noted that there is no real problem with having the vapor barrier and insulation facing immediately adjacent to each other, in a heating climate, with insulation on the outside.  No significant amount of moisture will get "trapped" between the two layers.

   So the issue is what to put behind the FC.

   I favor Tyvek, since it's an excellent air and liquid water barrier, but allows water vapor to "transpire" quite readily.  As to its durability, I've found it (the standard stuff) to be quite tough, and it's easy to seal (with the appropriate tape) around windows and other penetrations, and where sheets lap.  "Tar paper", on the other hand, is never really completely sealed unless you do the joints with tar or sealant.  Plus, "tar paper" does not allow water vapor to "transpire" nearly as well. (Also note that there are not-Tyvek options such as Typar which are not as good -- they don't reject liquid water as well -- but probably sufficient.)

   The main question is whether you need a physical spacer between the T1-11 and the FC to provide more of a drain path, and to prevent moisture absorbed into the FC from being transported into the T1-11.  This space could be provided by lath, or by using a "rain screen" product that provides extra thickness (eg, in the form of plastic ridges attached to a Tyvek-like product). 

   It depends a lot on how concerned you are that moisture might intrude.  Probably, if you're only concerned with mosture being transported through the FC then a layer of Tyvek (or "tar paper", if you insist) should be sufficient.  If you're concerned that you will not achieve good water-tight joints in the FC around windows and elsewhere, however, or this structure is in a location where it receives hurricane-wind-driven rain, you may want some sort of full-blown "rain screen" treatment.

3. 
   renosteinke | Apr 21, 2016 07:45pm | #9

   DuPont Says....

   A direct quote from Dupont, maker of Tyvek housewarp:

   "Unique material science helps prevent the infiltration of air and water, but lets water vapor escape to prevent rot and mold inside walls."

   In other words, it is NOT a vapor barrier.

4. 
   DanH | Apr 22, 2016 12:25am | #10

   Right -- Tyvek is not a vapor barrier.  It is an air barrier, however.