In a few days I’ll need to make a final decision on house-wrap and I have a few questions. I read an article that talked about how house-wrap seams should be taped to block air infiltration. My framer said he would tape it, if I really wanted it. But it was easy to see he was not keen on it.

When a house-wrap is generously overlapped and stapled, can it block air infiltration as good as if it was taped?

Do staples act (either immediately or over time) as little penetrations in the house-wrap that let rain water get behind it? If yes, would same be true for the nails that will be used to affix the horizontal boards as prep for metal siding? I notice that all of the home builder in our area, when they use house wrap, they staple and do not bother to tape. 

If/When water does get behind a modern house-wrap (i.e. not felt paper), I’ve read that it can not get back out. Is that true? Is this is high risk for problems down the line?

Thanks!

Figures I have show house-wrap to be 3x better at letting moisture vapor out (or in). Definitely tape it.

Another thing that has been shown to be a factor is that interior and exterior pressure differences cause nail and staple holes to expand. You don't want to have a good wind storm and then find out that your ACH50 suddenly increased.

Housewraps like Tyvek et al tend to be highly vapor permeable (~25perms+ )- they can't trap water. But they won't block liquid-water trapped up against it by siding, nor can it absorb & re-release water the way felts can. 15lbs felt runs ~5 perms when bone-dry, but becomes highly permeable when the humidity rises, and will even take on some amount of moisture itself.

With either, building in a 3/8" gap or "rainscreen" between the siding and the housewrap/felt (aka "drain plane") enhances the drying capacity of the drain plane, and limits the amount of initial exposure to wind-driven rain that gets behind the siding. (It's now required by code in Canada, and it's a good idea anywhere that gets more than 20" of rain/year- which would be most of the US.)

I'm with jonr-tape it, if you're looking for it to a be an air-barrier, and not merely a drain-plane. But long term integrity issues would seem to be against housewrap-as-air-barrier. Air-tight sheathing with primered & taped seams & foam-sealed edges seems a better bet. (The Huber Zip airtight sheathing methods work using standard OSB/ply, as long as you apply primer to the wood on either side of the seams for the tape to adhere to.)

Jonr and Dana1, thanks for your input. So tape it is and not stables.

I'm thinking of buying Tyvek just because I read a few forums where some of the builders really seemed to prefer that brand. I only need three or four rolls, so I can handle the cost. But the cost of the tape seems really quite high. I'm wondering if there is a good substitue for the Tyvek brand tape. Any suggestions?

Above the 14' walls will be an unconditioned, vented attic space and we plan to insulate above the 'ceiling' in the shop. Should the gable end walls get house wrap too? Reason I ask is because I may opt to tape the walls if the builder doesn't want to do it, but I won't be able to reach the gable ends (and besides I'm not keen on high ladders!). If the attic is vented, do we care if the gable ends aren't 'wrapped'?

Dana1 said:"...building in a 3/8" gap or "rainscreen" between the siding and the housewrap/felt (aka "drain plane") enhances the drying capacity of the drain plane, and limits the amount of initial exposure to wind-driven rain that gets behind the siding."

I think I understand the concept of the gap to reduce wind driven rain problems. In our building, we are sheathing in OSB, then house wrap, then agricultural type metal siding (siding affixed to purlins, over house-wrap, over OSB). This use of horizontal purlins is not something I can change - it is apparently how this siding is done. So I'm wondering if:
- our larger than 3/8" gap (purlins are 2x4) presents any problems in terms of water or air infiltraion that I could head off in some way
- our purlins (horizontal boards affixed to housewrapped OSB) will create problems by preventing water from 'draining' down the plane of the house wrap, and if there is anything I can do to reduce the potential problem

Re: the Zip product. I've seen it in magazines and am interested. If we do a frame house (likely) in the spring 2011, I will definately consider this instead of the OSB/house-wrap combo.

Thanks!

There might be some perforated steel purlins/girts that would let water flow downward (and resist rotting).

There's no down side to using 1.5" depth for the depth of the rainscreen cavity. Some builders in the US use as little as 1/4" to good effect, but Canadian building codes spells out a minimum of 10mm (a hair deeper than 3/8"). If anything more depth would perform better, but not so much better that it's enshrined in code.

Off the top of my head, adding a small amount of pitch to the purlins (not dead-level) and cutting them 1/4" short on the low end of the slope would allow any bulk water to drain. If you used standard minimums for low-slope roofs a reasonable pitch would be an inch of drop for every four feet of length. If that proves to be too much for the full length of wall, it can be segmented with 1/4" of weep space between the offset sections. Most of the time any leakwater that made it to the drain plane & down to a purlin would evaporate into the cavity space air before it dribbled off the weeps, but in driving gale/hurricane rains it could make a difference.

To make standard OSB into an air-barrier, paint the exterior seams with a decent PVA primer ~2" either side of the split, and use Huber Zip tape to seal it. The primer is necessary to ensure good long-term adhesion of the tape. Huber Zip system paints the entire surface (so that cut sections still supply the adhesion surface), but if you're doing it yourself, painting just the seams will suffice. Use spray foam sealant at the top/bottom edges to make them air-tight.

Thanks Jonr and Dana1 for your help. We ended up taping the tyvek ourselves. We were able to get most of the structure done, but a portion of the east (gable) wall we couldn't get done before they sided it. If taping the tyvek has to do with preventing air infiltration into the building, then I wonder if we have a chance to verify/improve our situation as we turn to the interior work which we'll be doing at our own pace.

First off, I would like to observe the interior of the building in various bad weather conditions to see if we can determine if/where air leaks are happening. All we have on the inside is the OSB and 2x6 studs. Can you just hold up your hand and feel for leaks?

If there are leaks through the OSB joints, could I paint the OSB edges and tape them as Dana1 mentioned in reference to the exterior Zip system. Would that work on an interior? I don't think we can afford to foam the interior (insulation) and will likely be going with fiberglass batts, carefully installed. Also, for reference, what is "PVA" in reference to paint?

Any comments appreciated.



PS. On your idea (Dana1) for slanting the girts a smidge with a 1/4" break between them to allow rain to run down and out ... in the end I didn't bring it up to the framer. I think his head would have exploded, because I think they considered some of our measures (even if we did them ourselves) extreme ... for a shop afterall. We had insulated the foundation wall and taped the tyvek ... they don't even do that on houses! We provided a rubber gasket for under the sill for him to use. And for the windows, we installed them oursleves to get the flashing done according to the best practices I had read about. So I let the girts go in as per his usual practice. Sometimes I think you need to pick your battles and I appreciated that he was doing a good job on the framing, siding and trim and working hard to stay on schedule. I don't think the focus is on the energy efficiency details and you can't blame him really, as I'm sure most customers aren't going to pay for the extra time it would take to get those details right.

You probably won't be able to "feel for leaks" unless there is a really strong wind. This is really where you need a "blower door" which will blow enough air out of the building so you will be able to feel the drafts coming in. I'd suggest calling energy auditors in your area & asking them to set up the door & let it run for an hour or so to find the infiltration problem areas.
A theater type fog machine also works but I don't know where you'd find one short of a theater company or local college with a theater program.
Either way will enable you to detect leaks that will rob you of heated air.
Taping the inside will help. Vycor tape may stick to unprimed OSB, but it isn't inexpensive. GreenBuildingAdvisor.com just did an article about tapes - check that out.
http://www.greenbuildingadvisor.com/blogs/dept/musings/air-sealing-tapes-and-gaskets
Most fiberglass is installed poorly, so a careful installation will help a lot. Cellulose is a better bet, but if the budget only allows fibrglass, do it well. I've had several clients tell me that houses I built in years past, insulated like all the houses in the neighborhood with fiberglass, has been much easier to heat & more comfortable their neighbors. The only reason it would be different from their neighbors is I was probably more careful about the installation & maybve the otehr builder hired the low bid.

Painting & taping the exposed seams on the interior of the sheathing works fine, and I'd do it as a matter of course- especially when insulating with batts.

Using high-density "cathedral ceiling" batts will yield a higher (& stabler with temperature) R value, but with any batt system attention to detail is EVERYTHING for getting the rated performance out of it. Attention to the interior-side air-barrier as well as the exterior is far more important with batts than with foams, high-density fiberglass, or blown/sprayed cellulose, all of which have at least an order of magnitude lower convection/infiltration potential. With no gaps or compressions and good air barriers on all sides batts do OK. It's often as cheap or cheaper (installed price) to use wet-sprayed cellulose though, which is more forgiving of faults in air barriers.

PVA= PolyVinylAcetate (used in many high quality fast-drying "water-based" polymer pains. It bonds well to tape adhesives, but other paints such as acrylics work tool.)

Don't sweat the slope issues- the steel siding purlin rainscreens are well designed as-is, and have a good track record.