Adding attic insulation


Old 11-02-03, 08:06 AM
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Question Adding attic insulation

I am investigating the better means to add insulation to my attic.

- Kansas City metro area
- 8 year old home - 2 story
- Cedar shake roof shingles
- Attic rafters are different spaced....varies from 16" center up to 24" (most likely because cedar roofs don't require decking like composite and builder can cut costs).
- 2nd floor ceiling joists are at least 2x8 and have yellow blown fiberglass insulation. Varies in depth, but for the most part everything is covered.
- Attic floor is not flat straight across due to 2nd floor master shower having vaulted ceiling
- 2nd floor square footage is roughly 1000 as per appraisal

Now, I am looking at adding the blown cellulose (paper type stuff) from Lowe's or Home Depot. How much should I add? Is it ok to add this on top of the fibeglass? Will it benefit our 2nd floor temp much?

Is it beneficial from both a temperature/insulation standpoint and a $$ cost payback standpoint to do something with the rafters? Meaning, should I get Owens Corning rolled pink stuff that is sealed inside of red paper and staple it to the slats? Or should I get the styrofoam stuff and cut it (I am sure I will have to cut it because the rafter spacing is not standard dimensions)?

What about a moisture barrier? The attic gets blistering hot in the summer and freezing cold in winter?

Thanks for your input.
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Old 11-02-03, 08:38 AM
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Also, the attic is unfinished and will remain as such.
Old 11-02-03, 10:46 AM
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Never, ever cover the underside of a cedar roof. The reason for the use of wood slats instead of decking is to allow for the cedar shingles to dry out uniformily. If you were to insulate the underside of cedar shingles, you will be replacing your roof in about two years.

Cedar shingled roofs are a perfect example of a water shedding roof. What that means is that it is not water proof. Moderate and steep sloped roofs area all water shedding. Flat and low sloped roofs are water proof. Since cedar roofs are water shedding and yours does not use decking, your roof can be described as anything but air tight. That means the ventilation of your attic far exceeds minimum ventilation requirements. This explains why your attic is so cold during the winter, which by the way is very good.The probability of a moisture problem occurring in your attic during the winter is very low as a result of the cold temperature.

The extreme heat you are experiencing in the attic during the summer is a different story. The reason for the extreme temperature happens to be due to the insulation in the attic. Insulation restricts heat flow or conductive heat transfer by retaining heat. It is the retention of this heat that is causing the extreme temperature in the attic during the summer. Even though you have far more attic ventilation than average types of attics have because of the type of roofing system you have, the radiant heat transfer or heat gain from the sun is far greater than the convective heat transfer from your ventilation.

Convective heat transfer is when air at a lower temperature than other air or objects (insulation) in the attic will extract heat from the air and objects in the attic until they are all equal in temperature. This also applies to the humidity in the air and/or objects in the attic, however, it is applied differently during the winter and the summer. Since the high temperatures in your attic are a result of the insulation in the attic, adding more insulation will only make it hotter in the attic. This is what causes discomfort in the home and the higher cooling costs. Even though the additional insulation reduces conductive heat transfer, which can be measured and calculated for energy savings in both heating and cooling of the home, the insulation does not address radiant heat transfer. Basically all that is, the warmer an objects gets, the more heat it will radiate.

Before I go any further here, there are things you must keep in mind. First, anything you apply to the underside of a cedar roofing systems that utilizes wood slats will dramatically reduce the life of the roof. Secondly, though there is a low probability that installing cellulose over fiberglass insulation will cause a moisture problem, it is even lower if you use similar materials. In my opinion, go with the lowest PRACTICAL application. That implies cost and availability of materials. Thirdly, your entire roof structure is a vent, so adding more vents is not a solution. Fourth, the colder the temperture in your attic during the winter, the better, so adding more insulation is not an issue.

This leaves us with the problem of adding more insulation in the attic and the rising temperature it will create as a result. To properly address this problem is to prohibit the heat from being stored in the insulation and/or extracting the heat better. Without question the best solution is tree shading, however, it is probably not practical. However, you can plan for the future by strategically planting trees around your home to shade the house from the sun.

While there are those who disagree with me on addressing this situation, in my opinion, the best solution, other than tree shading, is induced forced ventilation. An electric thermostatically controlled fan in the attic forces air into the attic. This initiates the convective heat transfer and dramatically reduces the amount of heat stored in the insulation. The attic temperature may still be high to your liking but the affects of the radiant heat transfer will be immediately noticed in the comfort downstairs and your energy cooling bills. I base my opinion on this solely on that wind and solar are intermittant power sources and electric is a firm relaible power source.
Old 11-02-03, 03:02 PM
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Hi resercon.
Thanks for the details. I appreciate and understand what you have typed.
One question I do have....sice adding insulation (blown fiberglass)and a thermostatically controlled vent fan will assist in the summer time, is insulation alone enough in the winter time to eliminate that drafty feeling on the 2nd floor?
As info, the registers are in the ceilings on the 2nd floor and if the furnace is not on, you feel a cool draft coming out of them. The duct work in the attic leading to the registers is not sheet metal, they are the flexible black plastic type (looks cheap to me). Also, we had vinyl replacement windows installed in the entire house this summer and all the trim was removed and insulation added around these new windows. It is better this year than in the past, but I am trying to make it still a bit more comfortable.

Thanks for your thoughts/tips.
Old 11-02-03, 09:09 PM
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Natural Draft and/or Air Leakage

A natural draft is when warm air cools on or against a cooler surface. The cooling of the warm air makes this air heavier than the warm air surrounding it and it drops as a result. It has the opposite effect to cooler that is heated, this air rises. In fact within the structure convective loops are formed. In most cases these are not noticable to the occupants. Unless there is a large enough degree difference. This causes the air to move faster, an example is in homes where they have baseboard heaters. When the unit comes on and if drapes are directly above the baseboards, the drapes sway in the convective loop.

In your situation it is they relatively poor insulation usually found on ducts that creates the natural draft or convective loop. For example, the insulation in the attic maybe R-19, but the ducts that lay above the insulation is probably R-5. There is almost 4 times more thermal insulating value to the insulation in the attic verses the insulation on the ductwork. The result is there is a 4 times greater heat loss through your ductwork insulation than the insulation in the attic. This will create the cooler surface inside the ductwork when the furnace is not on, that will initiate the convective loop. This is the cool air you feel coming out of the vent/registers in the ceiling when the unit is not on. It is for this reason that I usually advise people who intend to add more insulation in the attic where ductwork is present, to bury the ductwork in the insulation. This will usually aleviate the problem you are experiencing. As you increase the thermal insulating value around the ductwork, you dramatically reduce the heat loss which in turns reduces the temperature difference. The convective loop will still occur but it will be so slight, you will not be able to notice it.

The other concern is air leakage and in your case, duct leakage too. The volume of air within a structure remains constant. That means air cannot leave the home without the same volume of air entering it and vice-versa. This is known as a pressure induced draft. Where warm air is always seeking ways to get out of the structure. When it does, it creates negative pressure in the home. This creates the vacuum that draws air from outside the structure into the home. This actually increases the air exchange within the structure which some people may argue is good. Air sealing, duct sealing is included in this, seeks to control where the air exchange occurs. In fact, air sealing primary purpose is moisture control which is directly related to Indoor Air Quality (IAQ), Mold and Mildew growth.

This is a very complicated subject, which makes it a highly debatable topic and I find it difficult to explain it. This is why I use examples, because it makes it easier for me to support my opinion. The reason I am telling you this is the possible solutions I give in my examples may not resolve your problem, there maybe other factors. In my field of work, I do not look at a particular aspect, rather, I inspect and evaluate the entire house, inside and out. Then I interview the occupants and afterwards I will make a determination on the appropriate action to take to reduce the energy bills and increase comfort in the home. The following examples I have extrapolated from what you have told me.

Take the register off from the ceiling. Inspect the seal between the sheet rock and the duct. If there is a gap between the two, then take a cotton swab and push it up into the gap. Remove the swab and feel for moisture. If the swab is moist, go into the attic and inspect the insulation around the duct. Wear gloves and a face mask. If mold is present, bag it and throw it away. Inspect the rest of the ducts for leakage by tugging slightly on them. Duct seal them with either mastic or duct tape. Then seal the gap between the duct and the sheet rock with either spackle or mastic and re-insulate the area. You may also want to do the same to recess lighting if present in this area.
Old 11-03-03, 08:14 AM
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Thumbs up

Thanks for the information.
I will check all the registers/ducts and see if there is any mold in the gaps and seal accordingly. Also, I will investigate sealing the ducts themselves before I add insulation.
The vent fan will be added next spring.

Thanks again.
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