foam insulation question

Wayco Wayne_2

who is going nuts. He is a home improvement contractor who specializes in kitchens. He starts working on his own house and before you know it, he has moved his family into a rental house and has torn down all of his walls and stripped the drywal so the house is down to the shell. His wife must be a saint. He plans to foam all of the walls and the plywood that holds up the roof. The attic is the subject of my question. It has a shallow pitch and is made up of scissor trusses. The scissor trusses are made out of 2 x 4's and he plans to spray 12 inches of foam onto the back of the roof plywood which will encase the top of the trusses and make the attic a lot smaller. When he puts the drywall back on the ceiling it will be it's own space. He thinks I should pipe in conditioned air into the attic since the envelope of the house will soon be the roof. I don't think so but I don't know. Will there be condensation problems. No one lives in the attic. I would rather ventilate. Anyone have experience in such a case. WW

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Mark Hunt

Hmmmmmmm

So he is going to make the thermal boundry the roof?

Is he doing this to the entire house Wayne?

You will need an HRV for sure, but not just for the attic, you will need it for the entire house.

Check out Lifebreath

Mark H

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Constantin

Interesting Project...

As you know, we're going through something similar here in Cambridge. Since I used corbond in the rafter bays, it had to be covered by sheetrock. I understand that Icynene is not as flammable and hence may not require a fire-rated cover like Corbond does... but I would inquire.

If the HO is going to Icynene the whole house, then an HRV/ERV is a must. I bought the Lifebreath 195TRV and the 300TRV but we have yet to have experience with them. Jerry Scharf alerted me to the Stirling 200 DX, which looks like it's an even better HX than the Lifebreath dual-core stuff. From looking at the engineering spec, the stirling unit will probably filter air better and be less expensive to maintain.

Wayco Wayne_2

Yes

we are installing a ventilation system with heat recovery. I'm unsure about what to do about the spacein the attic. He doesn't plan to have any gable vents. Looks like a solar collector to me. He says the Lycene foam will give him R 40 between the roof and attic. He sugges5ed me running an A/C duct into the attic but that makes no sense to me.

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Mark Hunt

Wayne


I have never heard of putting hrv or A/C into an attic space.

Once the home is sealed, there should be little or no air movement into that space.

You keep mentioning A/C. Is the builder concerned with the attic getting too hot in the summer? How many projects has he done like this?

I would not worry about the attic area getting too hot, I would worry about the moisture issues in the house itself during the winter months. A/C will handle humidity in the summer, but the whole point of Icynene is to stop air movement as well as insulate so the stack effect will be almost nil.

Mark H

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Constantin

I agree with Mark...

... once the attic is part of the whole-house system, you shouldn't treat it any different than as part of the conditioned space. That is, no rafter vents and the like... with an R45 roof insulation, I doubt that the space will ever get that warm in the summer... Besides, you can solve part of the summer problem simply by installing a non-dark roof cover.

The thing to watch out for are the transition spaces between building floors and the like, some opportunity for voids in those areas, with the attendant lower insulative value and/or air movement.

You can get even more creative with the HRV if you like. For example, HRV's are easily oversized WRT the actual CFM requirements for so and so many AC/h. In our home the HRV/ERV's will be running at the lowest speed continuously to get the ASHRAE 62.2 recommended AC/h.

However, you can use an oversized HRV to your advantage by wiring a Leviton room occupancy sensor to the HRV... the ceiling-mount leviton unit can be adjusted to give you a set timer function, so every time someone goes to use the loo, the HRV goes into high speed mode.

This means fewer penetrations for the house envelope... and less infiltration potential for untreated exterior air. The power pack that comes with the Leviton (usually used for overhead flourescents) can power up to three occupancy sensors. Hence, you get your bathroom vents, clean air, etc. all in a tidy package that does not require occupants to remember to hit a timer button or to install a huge dehumidstat.

jerry scharf_2

roof envelope for house

I'm doing the same thing, insulation to the roof deck, no vents in the attic. I am running all the A/C ducts in the attic space but have no air in or out of it. Some air will migrate through the access hole (the local AHJ made me put in a vapor barrier in the ceiling to prevent condensation, go figure.)

I promise you it will not be a solar collector. With R30 foam, I would figure with an outside to inside delta T of 60F, the face of the foam to be about 2-3 degrees off from the inside temperature.

jerry

jerry scharf_2

oops

server bug

jerry scharf_2

oops

server bug

jerry scharf_2

yet another

server bug

Bob_42

Attic as conditioned space...

Wayne,

Here's the icynene link from the dealer in the DC area ... scroll down to 3(a) 3(b) for attic links.

http://www.insulatewithfoam.com/prod02.htm

During the summer you eliminate that mass of warm humid air that gets pulled into the home when the system is on and during the winter you eliminate that "sinking" mass of cold air as soon as the system shuts off. I didn't have time to do this in my home this fall but had the foam sprayed into the renovated basement and brink/block on the first floor. I'll tackle the attic in the spring.

Bob

Dale

Las Vegas

Gets pretty hot there and the AC Condenser is often ON the roof, they had so much trouble keeping the house air out of the attics and attic ducts ( slab construct.) they have gone to making the attic part of the conditioned space, much easier to insulate the bottom of the roof and sidewalls. So, sure go ahead and insulate the roof and attic sidewalls but, DO NOT try to seal this from the rest of the house unless you can assure not moisture will collect there.

eleft_4

WW

Roofing will cook regardless of color. The Idea the roof can cool with out a temperature differential is imposable.
The material gets so hot it will curl crack and even ooze. Venting is not only for moisture control it removes the heat build up to allow the roofing material to transfer the solar effect.


Constantin, We discussed this at wetstock. I don't want to confuse you with facts, when you asked your mind was already made up.
"Besides, you can solve part of the summer problem simply by installing a non-dark roof cover".
When was the last time you were roofing at 90 plus degrees and had to stop by 2pm because the material became so unstable it was starting to separate under knee and foot?

Theory is one thing practical or hands on is another.

al

jerry scharf_2

hmmm

Al,

Maybe your experience isn't universal. If they are doing this regularly in Las Vegas, something must work. It does get a tad hotter there than Boston.

I agree that other than polished metal, color doesn't matter near as much as people think. Most white paint has less than 5% less emissivity than black paint in the spectrum in question (.5-40 micron.) The better the paint, the lower the difference. Now if you covered your roof with solar panels (www.powerlight.com,) you wouldn't have much problem.

jerry

eleft_4

uh hu

If you went with Cedar you still must vent between shingle and sheathing.

Tile, concrete, epoxy or metal are not implied in this post.

Quoted from:

Roofing Materials Resource

Composition shingles
"Composition shingles are used on the majority of homes in the United States. These shingles are made of a base (organic or fiberglass) that is saturated with asphalt and coated with minerals on one side to resist weathering. The fiberglass shingles are more flexible and stronger than organic shingles. Shingles come in a wide variety of colors. Composition roofs can be overlaid with a new roof if the initial roof is a single layer and in good condition. The life expectancy of composition shingles depends on the rating (e.g., quality) and ranges from 20 to 30 years. Most manufacturers will cover a composition roof under warranty, but only if it has been installed by a certified roofer as the most common problems and maintenance issues occur when the installation did not include adequate venting and/or flashing. As with most types of roof, moss buildup needs to be controlled to prevent damage to the shingles."

Merry Christmas

al

Weezbo

the insulation serves as a vapor barrier and insulation,

making that space part of the building envelope. , adequate roof ventilation even though it is a hot roof still matters. crown vents work best when the vent is designed to travel Up the roof under the sheating attached to the roofing materials so, the sleepers or nailers for that sheathing should run Up the roof with an opening along the entire ridge under the crown vent....that will set a convection current eliminating heat to the best of its ability. you wouldnt think Alaska got 120+ in the shade:)

eleft_4

Correct Weez,


BUT.
Foam is being applied to existing roofs with out this consideration.
Where is the heat transfer supposed to take place?

Merry Christmas

al

Constantin

How much heat transfer takes place?

I decided to stay with the foamed roof truss sections for a simple reason: The Building Science Corporation published a neat guide to a Boston home that had its roof trusses foamed in their entirety. I happen to trust their judgement because their recommendations appear to be based on research and real-life experience.

Here is why I happen to think that the roofing material will be fine if the roof underneath it has been insulated: The thermal transfer you think is taking place is probably not very powerful.

On any sunny day, the sun is dumping around a kW of energy for every meter² of area or about 300BTU per ft². That's going to cook your roof in short order. How much of that energy is rejected back to the outside air vs. being conducted to the inside of the home? I would argue that most energy is rejected to the outside, which is one reason that working on roofs in the summertime is as miserable as it is.

A thermal equilibrium will establish itself that balances the heat input and the heat loss/rejection in your attic space. So, the temperatures will go up to a certain level inside the attic, then stabilize. If you have an attic fan, the temperatures are likely to be lower than if you rely on ridge vents, other passive means, or have no venting at all.

Next, consider that the heat has to move through the roofing material, the felt, the sheathing, etc. before it can warm your attic. Let's assume that an uninsulated roofing system has a R-value of about 1.

Furthermore, the air that flows past the underside of the sheathing has to be the same temperature as the outside air... or higher, depending on how much heat it has picked up by the time it reaches a certain area. Therefore, the thermal transfer is not going to be as great as on the outside, where there are plenty of convective currents (even in a dead calm) to help carry the heat away.

Incidentally, the installation instructions for DuraSlate roofs require built-in venting. On page 8, we find the following justification: "The importance of good attic ventilation beneath the roof cannot be overstated. Such movement of air will prevent or inhibit condensation of moisture on the undersurface of the Royal Dura Slate Tiles, or on the roof decks". (emphasis theirs).

The justification appears to be geared towards minimizing the potential for a roof deck that has a low temperature to be drenched via condensation by warm, moist air rising beneath it. However, with an impermeable insulation layer between the roof and the conditioned space on the inside, the moist, warm air never has a chance to get to the roof sheathing.

So while a cold roof is certainly a proven building method, there appear to be alternatives that work just as well with an impermeable insulation material like Corbond. However, if I and others turn out to be wrong, I'll be happy to report that too. Cheers!

eleft_4

Good Luck

Constantin

One PS...

Have a look at the results from the Building Sciences Corp research in Las Vegas.

The maximum ΔT they measured on the deck between an unvented roof vs. an vented roof in August was 20°F.

Allegedly, the temperature levels stayed well below the safe limits of the roofing material for either roof insulation option. Meanwhile, the unvented homes had lower energy consumption both during the heating and cooling seasons.

So far, I'm still sleeping well at night.

Bob Morrison_3

Ventilation, warm roofs, etc

Insulating the rafter spaces (or equivalent) and incorporating the attic into the conditioned space of the building is a valid insulation approach. However, despite the R-40, the approach described will result in a warm attic on some days, especially if the roof has continuous solar radiation, since the heat will be transmitted to the space eventually. I believe that venting above the insulation to remove heat is a better approach. Installing "propavents" before spraying would be better.

Mechanical ventilation may be a good idea, but this insulation system will not by itself cause moisture problems. If the foam serves as a vapor retarder, then there is no issue with condensation within the insulation itself. If not, then a vapor retarder should be applied to its warm side, assuming that the climate is cold.

Locating a HVAC fan coil unit in this attic is a good idea. Ductwork within this space will not be need to be sealed as carefully and freezing pipes will not be a problem.
I have done all of the above in my own well insulated, very tight house, except that I vented above the insulation in the rafter bays. I installed a Fantech 2000SHR HRV, also, but only because I've carefully air-sealed the exterior and can not rely on infiltration for outside air.

Bob Morrison

Constantin

You're right...

... if there is nothing up there to remove the solar heat, some thermal energy will eventually propagate into the thermal envelope of the house. However, what percentage of the heat actually makes it through vs. getting rejected to atmosphere?

The studies that Building Sciences did of houses with and without vented attics seemed to indicate that the unvented attics were cooler precisely because of duct sealing issues in the AC system. Unless you are 100% sure that your AC duct system has no leaks, blowing that cool air via vents to the outside seems like a waste.

Bob Morrison_3

Roof surface temperature may be an issue

Another concern with the unvented approach is excessive roof temperature which might cause premature deterioration of roofing materials. Some roofing materials may not be warrenteed without attic venting.

The research on these issues is ongoing. The Builders Guides issued by EEBA and written by Building Science seem to be the best references. However, would you build walls or roofs without vapor barriers on the warm side, as they recommend for "cold" climate? I wouldn't...

Gordy

When the roofing material prematurely fails, don't even think you will collect on any warranty with out proper ventilation fact.

I don't care what reading material is out there disproving the need of roof ventilation,in the end when the shingles fail it is the manufacture that you will deal with and they do not disprove the need for proper ventilation.

Remember hot flows to cold, so any delta t you create on the backside of the roof surface will enhance the dissipation of the heat build up, some is better then none at all.

When I replaced my old roof you could tell exactly where the hot spots were. The original attic ventilation had no soffit vents and two gable vents, because original roofing was cedar it did not make to much difference.The reroof was fiberglass shingles and ventilation was not enhanced roof failed in 15 years.

Since I have moved in and installed a new roof this summer and changed the ventilation. It makes a difference.

In the case of spraying foam on the backside of the roof, fine but you should have a 2" gap between the foam and the plywood with continuous soffit and ridge vents.

Gordy

When the roofing material prematurely fails, don't even think you will collect on any warranty from the manufacture with out proper ventilation fact. Catch 22 You won't find data from the manufacture indicating a "maximum" acceptable attic temperature either.

I don't care what reading material is out there disproving the need of roof ventilation,in the end when the shingles fail it is the manufacture that you will deal with and they do not disprove the need for proper ventilation.

Remember hot flows to cold, so any delta t you create on the backside of the roof surface will enhance the dissipation of the heat build up, some is better then none at all.

When I replaced my old roof you could tell exactly where the hot spots were. The original attic ventilation had no soffit vents and two gable vents, because original roofing was cedar it did not make to much difference.The reroof was fiberglass shingles and ventilation was not enhanced roof failed in 15 years.

Since I have moved in and installed a new roof this summer and changed the ventilation. It makes a difference.

In the case of spraying foam on the backside of the roof, fine but you should have a 2" gap between the foam and the plywood with continuous soffit and ridge vents. Air circulation in each rafter bay is key.

Gordy

J Matthers_2

There is a lot of discussion on this subject posted on sipweb.com if you want to take a look. The sip industry has been dealing with this for some time.

Weezbo

sips...hmmmm....

i Told the doctor not to use them he no wanna listen.the joints are a definite problem area he no belive....NOW HE BELIVE:))smart alec.

Constantin

PS #2

I also think that it depends on the building materials that were selected for the exterior. A metal, concrete, slate, tile, etc. roof isn't going to care about high deck temperaratures. The felt/poly underlayer shouldn't suffer much either.

On the other hand, asphalt roofing could be problematic simply because asphalt melts at very low temperatures. Time will tell how well DuraSlate will hold up.

Lastly, I don't quite understand why anyone gets worked up about roofing warranties. Usually, it's years before a problem becomes apparent and by then most roofing material warranties cover something like 10% of the material costs. Read the policies, I found them an entertaining excercise in limiting liability.

Friends of mine have a roof covered with premium cedar shingles but a bad underlayer and no copper drains in the valley's etc. Allegedly, they have a cold roof design, though the pink panther crew obviously was doing other things than a thorough insulation job. Ice dams, leaks, etc. are the result. Now they'll have to rip out the whole thing and start over, only 15 years after the roof was built.

While they are at it, I am going to recommend to them to get the ceiling spaces properly sealed against infiltration, the non-IC recessed lighting cans replaced with IC rated ones, etc. Hopefully, they'll have better contractors in the future...

Gordy

Dura Slate

Dura Slate, I looked into that product, its the same material we use as a bearing pad under precast box beams in the bridge industry, we use to say that the material would make a heck of a shingle and walla. Very durable product I was surprised by the warranty only being 50 years though, should be 100 years. At a starting cost of 200 bucks a square, and only providing an additional 20 years warranty compared to asphalt architectural they were a little out of my budget. You should be quite happy for many years "life time".

You are quite right about asphalt being the heat sensitve roofing material as compared to the other options. Though it should still be stated the importance of ventilation for a roofing system " of ashpalt nature" since I would say a high percentage of homes are indeed asphalt roofs.

Warranty well you are right about the prorating, amusing indeed, goes to show you most manufacturers don't know how long their asphalt roofs will perform. No matter how much you do right by their procedures. Southern exposures always the first to go. Although I have seen some go to hell in 10 years a little money is better then none at all.

Gordy