Spider insulation

[azgreg]

My initial plan was to use spray in cellulose insulation, but after looking up thermal bridging, high 'R' walls, super insulated walls, double stud walls, staggered stud walls, my head is spinning. However, in my studying I came across Spider insulation and it looks like a good alternate to cellulose.

http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/getting-know-spider-insulation

I still believe that if it will fit in the budget (a long ways off by the way) closed cell foam insulation looks like the ticket.

Has anybody here have any info on this stuff?

[MushCreek]

I have no experience with it, but it looks promising. The key to any insulation is to avoid thermal bridging, and pay close attention to air sealing. Air leakage can have a huge impact on energy loss. An easy way to eliminate thermal bridging is to apply rigid foam sheets over the outside of the sheathing, under the siding. Sites such as greenbuildingtalk dot com can help you determine the optimum wall build-up for your climate.

[NathanS]

I am going with exterior rigid foam. Like MushCreek says, it is a great way to avoid thermal bridging.

Personally probably going with fiberglass batts, they are cheap and install pretty cleanly. Might even layer batts in the attic so that when I need to do any service up there I can just move them out of the way instead of being worried about messing up the cellulose.

The problem with some of these new products is the availability. That 70k equipment the article mentions... probably not too many contractors invested in that expense, so it's not available in too many areas.

I got a couple quotes that shocked me. 3" of exterior rock wool was about 4-5k on my house. About the same price as spray foaming the whole house 5.5" thick.

[azgreg]

Spray in cellulose with a layer of XPS on the outside is the direction I'm leaning. However, there is a lot of learning I need to do on the subject.

[MushCreek]

Your local climate has to be considered when planning a wall structure. I'm no expert, but I do know that the wrong thickness of foam can lead to moisture problems within the wall due to condensation. In other words, I know just enough to be dangerous! It has to be the right thickness, and have the moisture barriers in the right place. As I said, I've had good results in the forums on greenbuildingtalk, and there are other similar sites that will help you plan your structure.

[flyingvan]

      How well does it breathe?  What does it off-gas?  I always remember EVERYTHING has a vapor pressure (though for most solids is pretty low), but I'd still like to know how much of what volatile it kicks off and for how long, and how will it poison firefighters when the house burns down. 
     

[NathanS]

Certainly have to consider vapor permanence. The main thing with using tons of insulation, or even R-19 batts, compared to old uninsulated houses is that they a robbed of energy - drying potential.

Stuff gets wet, then stays wet for a lot longer. 150 year old houses with little or no rot would rot in a few years if you put R-19 batts in the walls and cladded the exterior with R-13 polyiso, and didn't make sure you are doing a good job managing rain.

You definitely don't want an interior vapor barrier on the inside of the wall if you're doing exterior insulation. Or really, I think the truth is that you don't want any vapor barriers at all, if you can help it. But once you mix OSB with ANY exterior insulation, you have something pretty dang close to vapor closed. Any water stuck in between the foam and the OSB isn't going to dry out faster than the next rain wets it again.

This is why rain screens are so important now. If water gets stuck behind the siding, it can only dry outward. No way is it drying inward, even if it's a little vapor open. If air can flow behind the siding it will dry in no time.

There is a lot of stuff written on condensing surfaces, and you don't want a vapor barrier on a cold surface. That is absolutely true with something like polythelyene, which can't absorb any water what-so-ever. I don't think poly has any place in a wall or roof. The vast vast vast majority of moisture laden air is transported by air, not diffusion. If you have leaky drywall then the poly is only there to potentially collect water. If you must have an interior vapor retarder I would go with kraft faced batt insulation.

At this time of the year if you leave a scrap of poly on the ground, the next morning it will be wet. If you do the same thing with wood, it generally won't be wet. Because wood can absorb the moisture. You could even put a block of wood in the fridge and then sit it on your desk and it likely won't act as condensing surface. If you did the same thing with a piece of glasses you'd have water forming on it almost immediately.

[flyingvan]

Around here, there's more of a water source inside the structure than out.  Showering, cooking, burning wood, breathing----all put a lot of moisture in a house.  I think we're in a kind of bad time, building-wise, where you have to meet energy compliance regulations, without much regard for air exchange.  Hence the epidemic of mold issues.

[NathanS]

You're definitely right about that. There is a lot to consider and you definitely have to do your homework.

A lot of homes now have heat recovery ventilators to control stale air and humidity levels. I am planning to forgo that in my house because I will be aware of humidity since I built the place. Hoping I can crack a window instead of relying on expensive machinery. One thing to consider would be to install the ducts (which I think are only 4") and then install the HRV later if humidity is a problem.

In Florida I believe that dehumidification is going to be required in the code soon. New houses are so much more efficient the AC does not run long enough to control humidity levels.

I think there are lots of other problems that are still arising from all the extra insulation and air tightness. I try to follow the buildingscience.com Insights and even the leading engineers will be surprised by new problems sometimes.

Energy calcs for the house I am building vary hugely with changes in air tightness.

With R-21 cavity, R-13 exterior foam, R-20 under the slab, R-60 in the attic @ 0F design temperature:

.5 air exchanges per hour = 17k BTU loss per hour
1.0 air exchanges per hour = 24k BTU loss per hour

If I remove the exterior foam (just R-21 walls now) and retain .5 air exchanges per hour I am only at 20k BTU loss per hour. So it's significantly more engergy efficient with 40% less insulation in the walls if I can make the house very air tight.

[MushCreek]

With a small, tight, well insulated house, sizing HVAC is tricky. Even in hot SC, our total cooling load is only 9,000 BTU, or 3/4 ton. They don't make central systems anywhere near that small, so we went with a mini-split, and couldn't be happier. It also has a dehumidify mode, which is handy because every HVAC system is too big during the shoulder seasons when very little heat/AC is needed.