putting foam under roof deck

[ITE]

has anyone installed foam insulation under their roof deck essentially making the whole house, (including attic), one envelope? Its my understanding that you can do this with an R19 value and it will be acceptable.


thanks,

[Cole]

What material roof?

Metal, I have seen it done.

[kadoka]

If you have your attic vented to the outside as most are, what good will it do? You are still allowing outside air into the attic. It will make no difference in insulation value of the house.

[GreenBuilder007]

From my understanding, the purpose of insulting the roof deck is so that your hvac unit will run in conditioned space, as opposed to 120+ degree temperature. This should put a lighter load on the unit, allowing you to save energy. Also, it should help on the amount of heat/cool loss of the home (heat rises, escapes through roof...etc).

[Cole]

Is biobased your company?

[GreenBuilder007]

No, not at all.

[ITE]

kadoka, in this situation you do not vent the attic to the outside.

GreenBuilder007 Posted: Fri Nov 18, 2005 1:20 pm Post subject:

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From my understanding, the purpose of insulting the roof deck is so that your hvac unit will run in conditioned space, as opposed to 120+ degree temperature. This should put a lighter load on the unit, allowing you to save energy. Also, it should help on the amount of heat/cool loss of the home (heat rises, escapes through roof...etc).

This is one of the things I was thinking about as well as completely, as much as possible, eliminating air infiltration into the house. I have heard that if the roof pitch is much more than 6/12 that it doesn't pay off much due to the amount you pay for the extra foam.

Cole, I was thinking under asphalt shingles. Is there any problems with this under a metal roof?

Thanks for the replys guys,
Jason

[homebild]

It is not an economical use of materials in regards to payback on energy savings.

Foam sprayed under the roof assembly will only help a bit by keeping your attic cooler in summer and do nothing to keep the house warmer in winter.

Not only is foam extremely costly, so too is the code requirement to separate all foam plastic insulations in an attic with a thermal barrier.

Since all foams ignite when sufficiently heated, and since foam when ignited produces noxious fumes and rapidly spreads flame, building codes require that the foam be isolated from the attic by using either 1 1/2" mineral fiber insulation, 1/4" wood structural panels, 3/8" particleboard, 1/4" hardboard, 3/8" gypsum or .016 inch stainless steel.

This increases the cost to use foam in such a manner and renders its use even less economical. The payback period in energy savings for such a plan can often be decades before you stop paying for the foam installation and actually see a penny in real energy reduction costs.

Not a wise move for energy savings in most cases.

[danh]

Definitely it can't be very economical to insulate the underside of your roof in terms of energy savings.

Unless you are in a region where duct leakage rates are on the high side.

And the house has at least 2 storeys (so that the aggregate volume of the attic does not add a significant volume to the existing conditioned volume).

And you have to be in a hot / humid climate, as latent loads make up the bulk of energy consumption in air conditioning systems.


More valuable is probably the prevention of condensation & mold potential generated by the exclusion of exterior humidity from contact with the A/C system, and the reduction of wind blown rain under hurricane conditions. These mold generators are where the real cost savings come from.

The worst thing to have happen is to be evacuated from your home, only to be barred from returning from it long enough for the wind-blown rain that enters your attic to have caused a full outbreak of mold in the ceiling.


Thanks for pointing that out homebild - those are important issues.

...the purpose of insulting the roof deck is so that your hvac unit will run in conditioned space...
The last time I insulted a roof deck, that wasn't the reason. It was because it was a celebrity roast. That's where the term "hot" roof comes from; everyone was throwing insults at, or "roasting", that roof.

[GreenBuilder007]

homebuild, it is not necessary to have a thermal barrier with foam (in most circumstances). Foam does not have a low flash point, does not spread flames quickly, and does not produce noxious flames. If you were to hold a flame up to foam, in most cases, the foam would just blacken or char. Here are a couple links I think you'll find usefull:

Product specs:
http://www.biobased.net/products/index.php

Roof Deck Ventilation Discussion:
http://www.buildingscience.com/resources/roofs/unvented_roof_summary_article.pdf

[danh]

homebuild, it is not necessary to have a thermal barrier with foam (in most circumstances). Foam does not have a low flash point, does not spread flames quickly, and does not produce noxious flames. If you were to hold a flame up to foam, in most cases, the foam would just blacken or char.[/url]

The real answer is that the building code has a list of exemptions from thermal barrier requirements for foams, but thermal barriers ARE a requirement in MOST circumstances.

Just like the building code has a list of exemptions from thermal barrier requirements for FIBERGLASS. Yup, since it's a combustible, it has to be protected from occupied space. But fiberglass earned a special exemption in basements ;)

AND SO BEGINS PAGE TWO ! ! ! ! ! !

[Jimmy]

If you would like to use foam in an un-vented attic the best thing to do is to go to the ICC AC-12 and look what it says about this practice you will see that there are many assembly options. I am sure you can find one that will work for you. This is an accepted practice and has been found to help with the overall comfort of a home. Don't listen to people with other motives just learn what is acceptable according to ICC and enjoy living in a better home.;)

[rabadger]

Just my opinion....

If you want to insulate the deck so the HVAC equipment will run in a contioned area you have to include the attic in the load calculations and install supply restisters. Thats the only way to call it a conditioned area. Now you have just increased the needed tonage by about 2.5 tons. It's just an idea to sell more insulation.

Mechanical equipment stuck in an attic is a bad idea in the first place. You can't find a HVAC tech anywhere that likes attic work. It's just drives up the cost of service.

If I am wrong correct me.

[danh]

Just my opinion....
If I am wrong correct me.
You are wrong. :)

Somewhere in the United States, there are places where:
- duct leakage is SO HIGH,
- attics are SO SMALL compared to the rest of the house, and
- outdoor humidity is SO ASTRONOMICALLY HUGE

that converting the attic into a conditioned space actually results in a load that is LOWER than if the attic was left at outdoor conditions.

That place is NOT Indiana.

But it IS Florida.
And Louisiana.

Think 2-story house with 50% sensible heat ratio on the cooling loads, with 80% humidity at 80oF, and 35-50%+ duct leakage. If you have some free time, play with some numbers in Manual J and it will probably start to make sense.

Then again, even without the energy consumption balanced, it might be worth the additional load just to prevent mold in the attic.

And who said anything about installing supply registers?

[David]

Dan - How would you condition the attick if you don't have a register in there?

[danh]

Dave - you put in a ceiling that doesn't block heat transfer, or moisture flow. Like sheetrock ;)

[David]

OK. I would never have thought of that.
Now, what about the 35-50%+ duct leakage? Is that really possible? You are talking about new construction and new equipment, right? 50% leakage sounds aweful. Is that realistic?

[danh]

OK. I would never have thought of that.
Now, what about the 35-50%+ duct leakage? Is that really possible? You are talking about new construction and new equipment, right? 50% leakage sounds aweful. Is that realistic?
Probably not in new houses. But in retrofits it isn't that hard to find homes with duct leakage taking up half the flow volume. Isn't that 100% duct leakage ;)

And apparently they've been having a problem with the new high-efficiency HEPA filters for furnaces / air conditioning systems. Filters are put on the return side just before the coil, to prevent dust from getting into the equipment. But when they start getting dirty, they cause such a huge drop in static pressure that it becomes easier for the system to pull return air through cracks & gaps than it is to pull air through the filter. So you create a 35 - 50% leakage condition where one did not previously exist.

That's why many expert HVAC contractors recommend cheap WalMart filters - even when they get dirty, the minimal static pressure drop does not allow the system to circumvent the filter.

Houses are tricky. We can't win, can we?

[David]

Houses are tricky. We can't win, can we?

Nope. If we did, what would we spend our time on? Cars? :rolleyes:

I've followed the advice to use the cheapos since I bought my first house. Never really knew the reason for using them, until now!