Hi all! Been a while since I have asked any questions on here. I have used much of the valuable information to build my home that you were all so gracious to provide. Here is an update on my build; I built my home with ICF from the basement to the rafters-except for 2 walls in my master bedroom over my garage. These were built on 2x6 plates with staggered 2x4 studs filled with spray foam. All windows are a triple glazed, low-e & argon filled crank out casement style. Doors are high-efficiency model. Gable ends are typical construction and roof trusses were purchased as a package. I plan to use spray foam in my attic. I had hoped to create a warm attic and have sealed the space completely. The plan is to spray Polyurethane spray foam against the roof decking in between the rafters and to lightly coat each rafter with the foam for moisture repulsion. I was going to spray the attic in completely-no air movement up there as is usually done. My HVAC would offer 2 changes per hour and has been designed by a friend that is fully aware of the design of my home and used all the factors in his design. BEFORE I spray the entire space in next weekend, am I doing this incorrectly? As there isn't the same hot to cold on the roof decking, the condensation shouldn't be present. There was mention to me that I may need to actually heat the space with my furnace, which would also provide a return air. As always-ALL COMMENTS AND SUGGESTIONS ARE WELCOME!! Lastly, just for code, best practices, etc my home is being built in Mid-Northern Alberta Canada. (north of Edmonton). Starting tomorrow I am taking the course to become a certified and registered spray foam applicator (as I purchased a plural spray machine) and plan to apply the foam myself. Please let me know if plans, pictures etc would be helpful, and I'll figure out how to get them all to you (I could email everything).

Also, how do I write these topics on this forum with proper paragraphs etc? I had spaces in there but they don't translate for me?

Posted By eli98 on 04 Dec 2011 02:00 PM
... I plan to use spray foam in my attic. I had hoped to create a warm attic and have sealed the space completely. The plan is to spray Polyurethane spray foam against the roof decking in between the rafters and to lightly coat each rafter with the foam for moisture repulsion. I was going to spray the attic in completely-no air movement up there as is usually done...

What size are the top chords of your roof trusses?  2x4, 2x6, or ?  If only 2x4 (3.5" deep), how will you get enough R value up there for your climate if you are just spraying between the rafters?  I would think you would need to completely cover the 2x4 and then some to get sufficient R value.

Assuming you are using 2# closed cell foam which has an aged R value of ~6.0 per inch, at 3.5" thickness that is only R 21 neglecting the thermal short provide by the wood (actual whole wall/roof R value will be substantially less than R 21).  If the top chord is a 2x6 @ 5.5" deep, you are looking at R 33 not counting the thermal short circuit provided by the wood which reduces the effective R value of the assembly.  Even R 33 is below code minimum here of R 38 in a much milder climate than you have.

Since you mentioned conditioned air may be provided to the attic area, will you be required to provide an ignition barrier over that foam?

Posted By eli98 on 04 Dec 2011 02:00 PM
Hi all! Been a while since I have asked any questions on here. I have used much of the valuable information to build my home that you were all so gracious to provide. Here is an update on my build; I built my home with ICF from the basement to the rafters-except for 2 walls in my master bedroom over my garage. These were built on 2x6 plates with staggered 2x4 studs filled with spray foam.

All windows are a triple glazed, low-e & argon filled crank out casement style. Doors are high-efficiency model.

Gable ends are typical construction and roof trusses were purchased as a package.

I plan to use spray foam in my attic. I had hoped to create a warm attic and have sealed the space completely. The plan is to spray Polyurethane spray foam against the roof decking in between the rafters and to lightly coat each rafter with the foam for moisture repulsion. I was going to spray the attic in completely-no air movement up there as is usually done.

My HVAC would offer 2 changes per hour and has been designed by a friend that is fully aware of the design of my home and used all the factors in his design.

BEFORE I spray the entire space in next weekend, am I doing this incorrectly? As there isn't the same hot to cold on the roof decking, the condensation shouldn't be present.

There was mention to me that I may need to actually heat the space with my furnace, which would also provide a return air. As always-ALL COMMENTS AND SUGGESTIONS ARE WELCOME!!

Lastly, just for code, best practices, etc my home is being built in Mid-Northern Alberta Canada. (north of Edmonton).

Starting tomorrow I am taking the course to become a certified and registered spray foam applicator (as I purchased a plural spray machine) and plan to apply the foam myself. Please let me know if plans, pictures etc would be helpful, and I'll figure out how to get them all to you (I could email everything).

"Also, how do I write these topics on this forum with proper paragraphs etc? I had spaces in there but they don't translate for me?"

I just quoted your post and hit enter a few times to insert paragraphs.  Have you tried to edit your post to see if you can insert paragraphs?

Eli98,
If you are using Internet Explorer as the browser, then get into the GBT forum, then click on Tools on the menu and then Compatiblity View.  This should add this forum to your Compatibility View Settings.  Then go to the testing area in this forum to see if the change works for you when you post there.

+1 to all of arkie6's advice.

Where are you located (climate zone), what is the truss top chord depth, etc.

At ACH = 2, I am guessing that is at 50 Pa?

"At ACH = 2, I am guessing that is at 50 Pa?"

I read that as the ventilation rate as provided by the HRV, not a blower-door test number.

An all-closed-cell foam solution at code-min RSI-6/R34 is going to be expensive, about $60 per square meter. If the plan is to go with a sealed attic with sufficent foam to protect the roof deck and rafters, then doing the rest as fiber at the attic floor it might work. There's reason to believe that if you did 5cm (R12) of ccSPF at the roof deck and did the rest as dense-packed wet-spray cellulose or 1.8lb wet sprayed JM Spider (which would qualify as the ignition barrier) directly in contact with the foam you would do OK from a moisture control point of view up to ~R50 or so (50% over code-min), and it would likely be less cash outlay. See:

http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

Your climate would be similar to US zone 6A to 7A, depending on just how far north of Edmonton you are. Look at table 3, p14, as long as you don't have a low-E metal roof 2" of foam against the roof deck would do it, with the rest done as high-density fiber.

Sorry for the delay in response. The trusses are 2x6 against the deck. I am guessing that I need 6" of spray foam (as that would give me a R36 rating) for code. Can anyone elaborate on teh JM spyder insulation?



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.

http://www.specjm.com/player/index.php?src=spiderusa&speed=hi&player=mov

Posted By eli98 on 05 Dec 2011 09:38 PM
Sorry for the delay in response. The trusses are 2x6 against the deck. I am guessing that I need 6" of spray foam (as that would give me a R36 rating) for code. Can anyone elaborate on teh JM spyder insulation?



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.

The thermal bridging is also a function of the depth, which makes ccSPF cavity fill a poor value. A 3.5" shot of cc foam in a 2x6 cavity may hit R21, but will thermally underperform a full 5.5" cavity fill of 3lb cellulose that has only R19-R20.  In the SPF case the thermal bridge is less than R4, whereas with the cellulose case the thermal bridge is R5.5.  With a 20% framing factor and allowing a modest amount for the sheathing & siding that works out to ~R14 whole-wall for the cellulose but only ~R12 whole-wall for the ccSPF.

But putting the same amount of foam on the exterior of the sheathing thermally breaks the studs, studwall plates, band joist, foundation sills, etc and you'd get a whole-wall R of R21- nearly 2x the performance for about the same amount of material.  (Doing it with spray foam requires pre-installing furring on XPS standoffs at the required foam depth, through screwed to the studs. Doing it with rigid foam one can add the furring as you go.)

The moral of the story: Save the foam budget for the exterior, put the cheap stuff in framing cavities.

At 3lbs+ cellulose (or Spider or Optima at 1.8lbs+ ) is sufficiently air-retardent to prevent loss of  thermal performance, but being air & vapor permeable isn't sufficient protection against wintertime moisture accumulation in the sheathing on it's own.  In US zones 6 & 7 putting R25-30 as foam above the roof deck would be sufficient to allow high-density fb or cellulose on the interior without a vapor barrier (other than latex paint on gypsum), but an interior shot of 2" of ccSPF makes the foam the condensing surface, which is doesn't wick the moisture toward the roof deck, yet is still thin enough that the roof deck can seasonally dry toward the interior.  But an interior shot doesn't deal with the thermal bridging issue. 

Short of puttting rigid foam above the roof deck, putting lateral 2x4s across the  2x6 rafter elements on which to hang the blowing mesh deepens the cavity to 9", and puts 3.5" of fiber (~ R10-R14, depending on material) between the rafters & interior.   Without the 2x4s to deepen & thermally break the rafters, with 2" of foam and 3.5" of cellulose between the rafter bays, assuming a 10% framing fraction (simple roof lines, no skylights, etc.) you end up with a center-cavity R of ~ R25, and a whole-assembly cost (decking & gypsum included) of ~R19.  By adding the 2x4 laterals it boosts that to  a center-cavity R of ~ R37-R38 (meets code), but boosts performance to ~R30 whole-assembly.

By contrast a code-min R34  (low density cellulose between a 2x10 floor joists, no thermal break) yields a whole-assembly performance of only ~R27.

Thermally breaking the roof with 2" of iso on the exterior (~R11, derated for climate) would have about the same performance as adding the lateral 2x4s and deeper fiber, but since it leaves the roof deck warmer, it's more protective.

Question - if this is a truss package i,e, little usable space, why would you go with a "warm" attic?

Posted By FBBP on 06 Dec 2011 09:22 PM
Question - if this is a truss package i,e, little usable space, why would you go with a "warm" attic?

Seal attics, when done properly, can be more efficient and especially if there is ductwork in the attic.

Ducts & mechanicals in attics aren't standard fare in Alberta the way it is in the southern US, but who knows?

Those that like the "sea of potlights" look have can have difficulites making a reliable air-seal at the attic floor/upper ceiling level, which could tip the balance toward a conditioned attic, even with trusses. There are many reasons to go that route, but it's usually at a significant uptick in cost.

FBBP The plan is to build a "tight" house. Air infiltration/exfiltration is the greatest enemy a home can have. A 3/4"x3/4" hole in a wall can leak 30L/6gal of water per year. That is due to the air passing through carrying it's relative humidity with it. As the humid air reaches a cold spot in the building it condenses and water is left behind.

Another issue with air in/ex-filtration is convection heat loss. A 1/8" gap between the substrate and the insulation will result in a 40% reduction in the RSI value of your insulation. Blowing in more isulation on top of a leaking building effectively filters your lost heat better before it leaves your home completly. Again, properly sealing the home via Polyurethane insulation sprayed directly on the substrate will prevent convection currents, and ensure that my insulation is actually working to its expected value.

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.

eli98 - re airseal. Your preaching to the choir!!
Windows - I won't say that warm attics are not effective as I am not use to them. BUT if you take a 30 x 40 house you have 1200 sq. ft. of upper building envelope. If you put a modest 5/12 pitch roof on it and seal/insulate at the roof you have 1720 sq.ft. plus 332 sq.ft. for gables plus about 100sq.ft. for truss heal for a total of 2152 square feet of building envelope!!! As well you add around 6680 cubic feet of conditioned airspace to the building. (Hope all my math is right ;-) Further I suspect the shadow effect and wind protection of a cold roof provide some benefit in both heating and cooling climes. Dana makes a valid point about potlites (guilty!) but with due care and attention a vapour hat can be sealed to either the drywall or v.b.
In any event spraying the foam on the topside of the attic ceiling would seal these and all other leaks for half the price. To attempt to spray the truss members would be ••• (well what can I say!!) Dana is right that we don't usually put ductwork in our attics in Alberta but even if you did, would it not be more cost effective to encapsulate them with foam while you are sealing the rest and then bury them in cellulose. Just my gut reaction to the warm attic thing. There may be a number of things I have not taken into consideration. Bob

The reason for the warm attic is not to save money on construction but to prevent moisture issues in the future. If using cold air drafting is the solution to moisture against the roof decking what happens at -45•C? The relative humidity is terribly low resulting in air either leaving moisture on my deck as the air is already saturated or depositing water for the same reasons. With that thought, does a vented attic make any sense?? Or does it make more sense to prevent air movement in the space completely? Air not moving isn't the problem, it's warm air (and moist) contacting a cold surface. Then convection currents start and condensation is possible depending on R/H.

Also, PSF (polyurethane spray foam) cannot be within 3" of pot lights due to be heat. They need more than a poly hat to spray over them.
And yes spraying the truss is going to be a pain, but I am the one spraying them.

Posted By eli98 on 07 Dec 2011 03:09 PM
The reason for the warm attic is not to save money on construction but to prevent moisture issues in the future. If using cold air drafting is the solution to moisture against the roof decking what happens at -45•C? The relative humidity is terribly low resulting in air either leaving moisture on my deck as the air is already saturated or depositing water for the same reasons. With that thought, does a vented attic make any sense?? Or does it make more sense to prevent air movement in the space completely? Air not moving isn't the problem, it's warm air (and moist) contacting a cold surface. Then convection currents start and condensation is possible depending on R/H.

The Code requires all no conditioned areas to be vented!! Low relative humidity is NOT the cause of moisture on the deck (but thats Dana's department) If the attic is vented to code and insulated to code you will not have condensate.
Pot lites in insulation must be certified for such ie an insulated can which is vented to the inside and has thermal protection.
If you apply the foam to the top of ceiling below you will not get warm moist air in the attic space. If some should sneak by it will be vented out the attics vents without causing a problem.

Posted By eli98 on 07 Dec 2011 03:09 PM
The reason for the warm attic is not to save money on construction but to prevent moisture issues in the future. If using cold air drafting is the solution to moisture against the roof decking what happens at -45•C? The relative humidity is terribly low resulting in air either leaving moisture on my deck as the air is already saturated or depositing water for the same reasons. With that thought, does a vented attic make any sense?? Or does it make more sense to prevent air movement in the space completely? Air not moving isn't the problem, it's warm air (and moist) contacting a cold surface. Then convection currents start and condensation is possible depending on R/H.

Dilution happens, that's what.

RH is RELATIVE humidity, and without the temperature to which it is relative, is a meaningless number. The dew point is a measure of the absolute humidity and a determinant of when condensation will occur.  At 20C/35% RH the dew point of that air is ~4-5C, so any material at or below that temp will take on moisture via direct adsorption or condensation/frost. 

With a vented attic the RH in the attic stays pretty much at the outdoor RH, which is quite low when it's -45C outside.  Minor air leaks get sufficiently diluted by the ventilation air in the, which saves the rafters & roof decks from taking on moisture. With insufficient venting or big air leaks you get frost.  But even with minor air leaks, along the exfilration path there is some point where it reaches a material below the dew point of the interior air.  If that material is tolerant of the moisture, no big deal. But if it's susceptible....

If the "condensing plane" within an assembly adjacent to an air-permeable layer is impervious to water (such as SPF or polystyrene foams), a small amount of condensation is of little consequence, so as long as the AVERAGE temp at the condensing plane doesn't remain below the dew point of the interior air for weeks/months on end the assembly is relatively resiliant.  If the condensing plane is a wood material that will absorb water when below the dew point of the cavity air the safety margins on minor air leaks into the cavity are considerably more stringent.  If the fiber material has the capacity to store moisture with damage (such as cellulose), it can be somewhat protective of a wood condensing plane, but not enough to tolerate larger air leaks, or very long sustained temps below the dew point of the interior air through highly vapor-open interiors.

I was not arguing that a sealed attic was the only way to go.

Vented attics, when done properly are a time tested design.