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Fiberglass after Icynene


PZimniewicz's Avatar
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03-12-06, 05:43 PM   #1  
Fiberglass after Icynene

Hello all. I own a one and one half story Cape Cod in which I am turning the attic space into an attic master suite. I recently had icynene sprayed to fill all studs off all external walls upstairs. Job came out great. One question though, is it worth the money for me to fiberglass the knee wall sections and ceiling sections even though the Icynene is around the cavity behind them? Just curious for opinions on a cost benefit equation. Any thought would be appreciated.

 
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resercon's Avatar
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03-15-06, 06:26 AM   #2  
You do not want to insulate those areas due to the icynene insulation. The reason for this is this spray foam insulation qualifies as a vapor barrier.

 
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04-04-06, 03:14 PM   #3  
Insulation

Thanks for the reply. Just want to clarify my statement a bit as it may have not been totally clear. I have the Icynene on the outside walls and all the way up the roof line. However, I have collar ties running across the room around 8 feet up as an interior ceiling. The ceiling at peak aboce that is about 12 feet. I was thinking if I did not insulate the at the interior ceiling area, the heat/cold would just wastefully move through the drywall and into the area above the ceiling. It might be kept in over that by the Icynene, but would not be in the "usable" room cavity. I conjectured that if i did the ceiling area to say R30 or over, I would keep this heat/cold in the "usable" room area. Same argument for behind the knee wall area. If this is how you understood me, and the answer is still the same, I appreciate it. Thanks for any replies.

 
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04-05-06, 08:44 AM   #4  
This is quite a difficult subject to explain because it is literally impossible to reply without getting technical. The Icynene quailfies as a vapor barrier, which means it has a low Perm Rating. Or if you prefer, it impedes moisture flow A good note here is that vapor barrier materials DO NOT stop moisture flow, they slow it down considerably.

The reason for this has to do with Relative Humidity (RH%). Insulation also does not stop heat flow through it, it SLOWS it down. An object that drops in temperature will increase in RH% and as it increases in temperature will drop in RH%. Since insulation slows down heat flow and most do not slow down moisture flow, like fiberglass insulation, a vapor barrier must be applied to slow down moisture flow.

For example, as the heat traverses unfaced fiberglass insulation, its temperature begin to drop. Since the moisture was not slowed down due to a lack of a vapor barrier, as the temperature drops the RH% increases and will eventually will condense once "Dew Point Temperature" is reached. By applying a vapor barrier the probability of condensation forming inside the insulation is dropped dramatically.

I am assuming by your description is that the Icynene is NOT sprayed on your knee walls or ceiling. The problem here is that the spaces behind them, even unused, will drop in temperature if you insulate the knee walls and ceiling. What compounds this is the Icynene qualifies as a vapor barrier and will impede the moisture flow through it. Thereby trapping the moisture in these unused spaces. The probability of condensation if you insulate the knee walls and ceiling is extremely high.

 
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04-05-06, 01:19 PM   #5  
Response

Thanks very much for the detailed response. You are correct in your assumptions regarding the knee walls and ceiling portions. The only point to add from my perspective is that from what I understood, Icynene would release vapor and not allow the condensation to be trapped (or at least the website says that). It also mentions it will not conduct the heat or cold through to the airspace behind it. (However the wood rafters certainly will) I am mainly concerned with the moisture issue more than the cost of new insulation. If getting a bit more efficient in terms of R value on the ceiling or knee wall portions causes me a moisture problem that will mold/etc, it is not worth it to me. If it would be relatively safe, I would spend the extra money to add a bit to the efficiency.

If you have any addition comments, I certainly welcome them. Your response is very much appreciated.

 
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04-09-06, 10:40 AM   #6  
What is important here is that you decide for yourself on what to do based on the information you receive. Icynene claims that their product does not need a vapor barrier and will not trap moisture inside it. Materials that qualify as a Vapor Barrier do not stop moisture flow, they slow it down. This is why Icynene can make that claim. Let's take both senario's.

ICYNENE ONLY ON RAFTERS and no insulation on ceiling and knee walls. As heat and moisture approaches the Icynene in these unused spaces, condensation is unlikely to occur. The reason for this is the lack of insulation in the ceiling and knee walls will make the temperature in these unused spaces approximately the same as in the house. Yes, you will be heating these unused spaces to avoid condensation.

ICYNENE ON RAFTERS and the ceiling and knee walls are insulated with or without a vapor barrier. What the insulation on the ceiling and knee walls do is drop the temperature in these unused spaces. As stated earlier, as the temperature drops, the Relative Humidity (RH%) of the object increases.

Even if the insulation on the ceiling and knee walls have an excellent vapor barrier, it will not STOP the heat and moisture flow through it. It will only SLOW IT DOWN. The Icynene will do the same with the heat and moisture in these unused spaces. Unfortunately the temperature inside these unused spaces would have dropped due to the ceiling and knee walls being insulated. The probability of condensation under these conditions is literally at 100%.

To illustrate how most people misinterpret applications is that they do not identify the source of the problem or what the application attempts to address. For example ATTIC VENTILATION became needed in homes in cold climates when insulation was installed. In other words, homes built in the years before insulation was installed did not have attic ventilation. If you insulated these vintage homes you have to install attic ventilation, but the walls don't have to be ventilated.

The reasons for this is really quite simple. when there was no insulation in the house, there was very little temperature drop between the house and attic. Walls do not have to be ventilated because outer wall sheathing and siding materials do not qualify as vapor barriers. However, most roofing materials do qualify as vapor barriers. So the primary purpose of attic ventilation in cold climates is to bypass the low moisture vapor permability of roofing materials.

While we hear a lot about the purposes of attic ventilation like avoiding moisture problems, many don't see the source of the problem or its primary purpose. The insulation causes a drop in temperature in the confined attic space which increases the RH% and the low moisture vapor permability of roofing materials.

 
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