I'm building a heated timber frame workshop as an addition to my free-standing garage. Am using Timberpeg's "wrap and strap" wall system. This calls for (going from inside to outside wall):
* EWP frame
* 7/8" pine t&g
* 6 mil poly vapor barrier
* 3" polyiso foam wall insulation, reflective facing on 2 sides(inside perimeter blocking)
* 1x4 strapping to create air space & nailers
* house wrap (Tyvec)
* CVG WRC horizontal siding (which I will back-prime)

Question:
Does it make sense for the Tyvec to go between the strapping and the siding? With rain drainage wall systems, I thought the usual practice was to but it BEHIND the strapping and leave an air space between it and the Tyvec.

It seems to me, this plan might increase the R-value a little (because the faced polyiso performs better with an air space) but
decrease the moisture-management.Having Tyvec directly behind the cedar siding would seem to defeat the purpose of the air space, from a moisture-management point-of-view.

Thoughts?

I like wrap inside the strapping. Or what about use tyvec tape on the foam and forget the wrap, hmmmm but you are not installing foam continuously, so tyvec is good.

I don't think it would matter a great deal. Putting the airspace towards the outside would allow for back-side ventilation for the wood, though, so like Mark says, tape the iso-board, then tyvek, followed by strapping and siding is likely how I would do it.

Cedar will last longer with the back side ventilated.

Hi Guys,

Can you please explain what you mean by strapping ?

Regards

Ken Hume

Hey Ken

Strapping can also be termed furring in the US, perhaps the UK terms firring and batten are equivalent. (Is that correct?) Basically furring or strapping is placed on panels perpendicular to the siding course to acheive an air gap between the back of the siding and the exterior sheathing.

All the best

Roger

In my experience, the reason you put Tyvek (or tar paper) behind the strapping/furring is to allow the siding to breathe. What advantage would the other way around give a building?
Tyvek and cedar are not compatible. The oils in cedar slowly degrade Tyvek. I've seen it on a few tear-aparts. Typar has a much better resistance to oils.
For that matter, I like tar paper. Why? It lasts. It breathes. And in my very wet environment, I've pulled a lot more rot out from behind Tyvek than tar paper.
About the only good I can say for Tyvek is it sure does go up fast.

All the best,

Zach

Hi Roger and Zach,

Now I understand the terminolgy and yes firring or furring strip would be run vertically and might generally placed over a gap e.g. vertical plank siding or panel join and battens run horizontally.

Zach,

Your observations re the interection between cedar and tyvec are very important as are the typar comments. Since you effectively have a 2 layer system would it be possible to dispense with either Tyvec or Typar and just encourage free ventilation to dry out water ingress ?

I currently have a problem with water ingress into our 2 bay oak framed granary (about 1600). Check out a photo of the south west floor taken yesterday morning after experiencing heavy windblown rain.

http://www.kfhume.freeserve.co.uk/pictures/Granary%20Interior.jpg

and also check out the corresponding exterior view of the larch siding on the exterior corner.

http://www.kfhume.freeserve.co.uk/pictures/Granary%20Exterior.jpg

There is no liner paper and the boards are attached by nails direct to the oak frame infill studs. Clearly this problem has been at work for quite a while (100 years +) since a previous attempt has been made to cut out and replace some of the interior elm plank lining planks which in turn have themselves had rat holes knawed through (to help improve ventilation of the void space - nice work rats !).

I suppose that I might need to tear off the lower sideing on this 400 year old historic building in order to try and identify the source of this water but the problem then remains as to how best to prevent water ingress or alternatively should we continue to allow sporadic events like this and then simply ensure that sufficient ventilation is present to quickly dry out the water.

Your thoughts would be much appreciated.

Regards

Ken Hume

Hey Ken

My first impulse on water intrusion is to look higher up for roof, trim and gutter faults. I would guess that somehow water flow has a concentrated path to the corner post and is revealed at the lower level. I would look for building features that can collect and deliver water to a point. For example, I have seen plates decayed into canoes and decayed water table trim at the gable tie level course water in a lateral path to a vertical member.

Roger

Hi Roger,

Good idea I will check that out and report back. There are no gutters on this building but it does have quite a wide overhang.

Regards

Ken Hume

The only thing I can think of is that they are trying to create a dead air space for improved R-value in front of foil-faced polyiso insulation. I've seen ratings expressed for polyiso with the qualification that the rating is for an installation behind an air space.

If that is so, then I would think the small advantage of the air pocket is much outweighed by the value of good ventillation behind the siding. I'd certaily trade +R-1 for +5 years in the life expectency of CVG cedar siding.