Posted By eli98 on 07 Dec 2011 02:09 PM

...My goal is not to use as much insulation, instead seal the house properly first. In doing so I will get a better return on my dollar as the air is not allowed to leave except where I designate i.e. HRV, hot water tank exhaust, dryer vent, etc.

Where does the make-up air come in for that hot water heater exhaust and dryer vent?  When that dryer vent is blowing outside, the same amount of outside air has to come in somewhere.

Greetings,

Quote> It is unfortunate that the underwriting labs haven't come to realize that R value is over-rated. Anyone that doesn't see that R value is based on a board foot, and studs are 16" apart is too busy being a lemming. There are sheeples out there that believe that R20 fiberglass and R20 spray foam and R20 ICF are all the same. They are correct on a measured board foot, but not on the performance of the entire wall. Enough rant, sorry.

Please, rant on. As I have mentioned before the best That OC could get with their 3.5" batt in their million dollar lab, in situ test, was about 1/2 of their advertized "R" value.

You're right it does come from the o/s. You have to heat and cool that air.
Two tube high eff furnaces get part of their eff by drawing in combustion air thru a pvc tube.
Not only do furnaces pull out side air the stack draws air for it's operation.

You can retrofit existing gas furnaces, water heaters and dryers to help eliminate this problem.

Regarding insulation ceiling deck and roof deck. I have never seen such an app that did not eventually result in deck rot.

The hot water will have its own fresh air intake. The dryer however I haven't figured out. I had thought of building a HRV for that where it would heat its own incoming air. I can't lie and say I've thought of or solved everything!

"regarding insulating the roof deck I've never seen such an app that didn't result in deck rot"

Does your experience with this include SPF? I am looking for suggestions here and only attempting to defend my ideas. If I'm wrong I'll admit it (eventually when no one is listening;)

The HRV becomes the air intake for any appliances that don't provide their own makeup or combustion air. When the appliance is operating the HRV is unbalanced and doesn't recover the exhaust heat from the appliance, but that's not usually a big penalty.

The gain in net system efficiency of furnaces that are sealed combustion drawing their own combustion air is small- the very low single digits percentage wise. The far greater gain is from the condensation that occurs in the heat exchanger, not the flue. But in a very tight house there's also a modest low single-digit gain in envelope efficiency by not using conditioned space air for combustion.

Using SPF on the underside of the roof deck will trap roof-leak moisture in the roof deck when it inevitably occurs. Using closed cell SPF on the outside of the roof deck will protect it from leakage, since it's waterproof to liquid water.

Using rigid foam above the roof deck held in place with furring through-screwed to the structural deck, with a nailer-deck for the roofing attached to the furring leaves a ventilation space between the nailer and the foam, and gives you the best of both worlds. The nailer deck then dries to the vent cavity. Using somewhat permeable foam (EPS, or fiber-faced iso) then allows any moisture that finds it's way to the structural roof deck to dry into the vent space. With 2-4" of exterior rigid foam and a vented nailer deck, and 1-2" of interior closed cell spray foam as an insulating air-seal & moisture-resiliant condensing surface you can have the best of all worlds. EPS & fiber faced iso is typically $0.9-1.10 per R per square meter (f.o.b. the distributors yard) whereas SPF runs ~$1.70-1.80 per R per square meter (installed.) The delta in price leaves a lot of room for installation cost & scrap rates, and on simpler roof lines it cheaper/better to apply most of the foam budget on the exterior, since you also get the full benefit of the R, since the only thermal bridging would be the furring screws every 3/4 meter or so.

A code-min stackup that would not require (and should not have) an interior poly vapor barrier would be:

roofing/nailer / vent space / 75mm EPS (R12) / ~50mmSPF(R12) /~75mm wet-spray cellulose (R10) as the interior ignition barrier.

But it would thermally outperform any code-min R34 between rafters or joists since you would have R10 of thermal break over the rafter elements.

But because cellulose is so CHEAP relative to foam, at "typical" utility rates there's a 25 year NPV financial argument for going with 150mm or even 250mm on the interior. If you go thicker than 75mm it's advisable to use a semi-permeable vapor retarder on the interior of the fiber layer in that climate (eg. Certainteed MemBrain), or go higher-R on the exterior of the roof deck.

Greetings,

Condensation is not the result of interior moisture only.
When the dew point is reached it will also pull moisture from the out side.

Foam also becomes more permeable to moisture as it ages. You also have the problem of moisture passing thru the rafters and condensing under the sheathing. This is probably why you see chunks of foam. cellulose too, falling off ceilings.

It isn't so much a matter of a little water as it is the prolong saturation after the vapor is condensed.

Old frame, uninsulated buildings, endure condensation because moisture is not held to the wood and evaporates quickly enough.

The most efficient energy saver is to insulate directly above the ceiling. You always want the insulator to be as close as possible to the conditioned space. The reason you have to consider resorting to these expensive add ons is because you realize that the prods that you are considering are not as efficient as promoted.

Consider the old broken record, " insulate with RI". Maximum eff., no moisture problems, permanent.
I insulated my parent's house in 1974 with 3 layer RI after vacuuming the rock wool out. Works today just as good as it did when I installed it.

Summertime dew points in N. Alberta aren't the problem the way it can be in the eastern & southeastern US. (Do a weatherspark.com dew point graph of Edmonton.) Neither the roof deck nor the ceiling gypsum are ever going to be below the outdoor dew point except on the once in a century few hours of extreme humidity. The mid-summer average outdoor dew point for Edmonton is about 52F/11C. Unless you like to wear your heavy sweaters indoors on the hottest days of the year you won't be cooling the interior to anywhere near 11C, eh? ;-)

Stack effect pressures mid-winter are huge there, air-tightness is critical to performance.

There may be an argument that 3 layer RI might beat the performance of 6-8" of poorly installed rock wool, but so what? (First show me the test data on the 3-layer RI though.)

Posted By Dana1 on 09 Dec 2011 01:51 PM
Summertime dew points in N. Alberta aren't the problem the way it can be in the eastern & southeastern US. (Do a weatherspark.com dew point graph of Edmonton.) Neither the roof deck nor the ceiling gypsum are ever going to be below the outdoor dew point except on the once in a century few hours of extreme humidity. The mid-summer average outdoor dew point for Edmonton is about 52F/11C. Unless you like to wear your heavy sweaters indoors on the hottest days of the year you won't be cooling the interior to anywhere near 11C, eh? ;-)

Stack effect pressures mid-winter are huge there, air-tightness is critical to performance.

There may be an argument that 3 layer RI might beat the performance of 6-8" of poorly installed rock wool, but so what? (First show me the test data on the 3-layer RI though.)

Thank you for an Alberta perspective! :) BTW, my roof is shingled, so that can't be changed cheaply. I have received approval from the local inspector to spray against the roof deck, essentially sealing ALL aspects of my house from the outside elements. (yes I realize ALL means everything, but I say it knowing that the house can't be a vacuum sealed container) Not only does the dew point change here in balmy norther Canada ;) but the requirements change as well. There are emerging studies that indicate the [i]benefits[/i] of a sealed attic space. I realize that there [i]can[/i] be issues if the shingles leak. But please tell me there has been one instance where a leaking roofing substrate (shingles, tin, shake, etc) had no effect on the substrate below it thanks to a wonderful draft of air bringing in the same moist air from below? I say the same moist air as if the roof is leaking-there is a source of moisture! It is either raining or snowing, or dew point etc, but that is the same air we hope is [i]drying[/i] our substrates?!? Please read the following article regarding RSI value, and the myths it debunks: http://www.cufca.ca/foam/R-Value%20Fairy%20Tale.pdf This article is supplied on the CUFCA webpage(Canadain Urethane Foam Contractor's Association) (which btw is now being used by many American QC groups due to it's daunting requirements for near perfection.)

Greetings,

QUOTE> There may be an argument that 3 layer RI might beat the performance of 6-8" of poorly installed rock wool, but so what? (First show me the test data on the 3-layer RI though.)

Since you don't understand how RI works, it would be a waste of time.