house wrap and rigid board

5 years ago

last modified: 5 years ago

We would like to start building our own house in about 5 years. We are on the edge of zone 5a and 6. Our interior insulation will be mineral wool r23, then 1/2 plywood sheathing. Here is where the debut comes in. I would like to put 3/4 rigid board insulation on the outside to help with thermal bridging and to help as an air barrier. Questions on house wrap.
1. If we put it on under the rigid board, it helps get any inside moister away from the sheathing, but you're letting the outside moister further in. If you put it on over the board your stopping the outside moister before it gets to the rigid board keeping it further away from any wood, but could inside moister get trapped between the sheathing and rigid board? Should you do 2 layers? It's relatively cheap right, about $300 to do a house? Why not do one over and one under the rigid board.
2. Last question, we are probably using vinyl siding (least maintenance). Use furring stripes to create an air gap or does vinyl siding have enough airflow to create a good drying situation?

Thank you everyone!
Ryan

Featured Answer

PRO

Springtime Builders

5 years ago

That GBA article has updated advice on the subject, behind the paywall at combining rigid with fluffy insulation. The free 2010 article Worthy links, features Image#3 as the more important table compared to IRC minimums which could lead to moisture problems.

A 2x6 wall needs 2" of EPS in zone 5 and 3" in zone 6. You could fall back to Worthy's suggested thicknesses with a 2x4 wall. Exterior insulation is arguably more important than cavity insulation. I would prefer the 2x4 sheathed with insulation board, compared to 2x6 or even 2x8 with no exterior insulative sheathing.

Polyiso is our current preferred insulation board and the same type used in Zip R-sheathings. The Zip system has a lot of advantages over the many layers you are considering. If not using Zip R-sheathing, there is a strong case for installing a drainable housewrap between the insulation board and structural sheathing per BSC's Mind the Gap and Matt Risinger's informal tests.

I think vinyl siding makes an excellent rainscreen on it's own but thicker foam will challenge the fastening system of the vinyl. Furring strips might be the best answer depending on details. Continue the good research into window and door installation, another area needing industry wide improvement.

Sort by:Oldest

Comments (10)

PRO
PPF.
5 years ago
See the Building Science website for info.
PRO
Summit Studio Architects
5 years ago
PPF is right. The Building Science website can explain how the 3/4" rigid insulation can create conditions for moisture to condense in the wall as the warm, humid indoor air reaches the dew point in the batt insulation. You need more rigid insulation and less batt insulation. The consensus seems to be that 30-50% of the R-value should be rigid insulation.

I have actually gone back to 2x4 construction with R-13 batts and 2" Zip-R (R-9). You may need to go with 2 1/2" Zip-R (R-12) depending on the code version your building department uses. This combination saves a little in framing, has a good balance between permeable and non-permeable insulation, and has excellent thermal performance.
Related Professionals
South Barrington Architects & Building Designers · Carnot-Moon Home Builders · Reedley Home Builders · Pico Rivera General Contractors · Frisco Furniture & Accessories · Lake Zurich Furniture & Accessories · Fallbrook Furniture & Accessories · Superior Home Builders · Brooklyn Park Painters · Cape Coral Painters · Crestwood Painters · Estero Painters · Saint Louis Park Painters · Florida City General Contractors · Valley Stream General Contractors
rjwassink67
Original Author
5 years ago
Thank you. I have been looking on there and can't seem to find anything. Wow, there is a lot of info. Sorry, for needing it simple, but where would I look? Thank you
worthy
5 years ago
last modified: 5 years ago
No need for housewrap--which is virtually never installed 100% properly--if you use tongue and groove XPS (extruded polystyrene) with taped seams.

See here for explanation of minimum thickness of exterior insulation by climate zone.
rjwassink67
Original Author
5 years ago
Wow, might rethink using outside rigid board. that is a lot of insulation
worthy
5 years ago
last modified: 5 years ago
As per the IRC Table, only 1"-1.5" of XPS if you insulate to the minimum requirement.

( Boards less than an inch thick will easily snap in two if caught by a breeze on site.)
ulisdone
5 years ago
You want to use 2 layers of foam board with taped staggered seam, regardless of thickness.

The link that worthy posted above is also a good site for education: Green Building Advisor.
PRO
Charles Ross Homes
5 years ago
My suggestion is to defer the decision until closer to the time you'll be building. In five years, your project will be subject to a new edition of the building code, plus, products like Huber's Zip-R sheathing will have five more years of installed performance history so you can make a more informed choice at that time.
worthy
5 years ago
last modified: 5 years ago
The free 2010 article Worthy links....

Holladay updated it to 2016.

But, as noted above, always best to check the latest information before you specify anything. Unfortunately, if you are really up-to-date, many trades and building inspectors will consider you meshuga!