I have a Cape Cod style with many square feet of knee wall on the second level. There is plastic sheet behind the dry wall and faced fiberglass stuck between the 2x4s that is about 6 inches deep. The paper is facing the wall.

I would like to increase the insulation in this area or at least make it more resistant to air infiltration and thermal heat gain in the summer time. I do not want to install something that might be a fire concern in this location.

I could install house wrap to allow the moisture to escape and reduce air infiltration into the fiberglass. This would not add a great deal of R value it would help protect my R value. Would this material be a fire concern since it would be exposed on the attic side?

I could install radiant barreir material either single or double with air space as part of the barrier. This would not allow the moisture to escape unless it was perforated. It would help with the air infiltration except for the perforations. It would help reduce heat gain. I do not think this material is a fire concern since it can be layed over existing attic insulation in much the same way.

I could add a foam core board with one side foil backed. This would not allow moisture to escape unless perforated which sounds like a bad idea for this material anyhow. It would increase the R value and reduce air infiltration. But since it is foam I beleive it would need to be covered with dry wall on the attic side to reduce its fire hazard. This would add to the cost and difficulty of getting the material into the knee wall area.

These have been my thoughts but have been unable to find a source online that deals with either in any detail.

I believe that each of these products works well in the proper place but maybe there is something better I am not thinking of. Many homes have this type of weak point in their thermal barrier so there must be a better way. Any suggestions on these ideas or what to do?

Troy

 

Cape Cods are notorious for this problem. For example, a house wrap like Tyvek should be installed from inside the kneewall and over the floor in this space. It forms an "L" over the outside of the insulation. The idea is to make the so-called air barrier continuous. The air/vapor barrier is supposed to form the same type of "L" shape over the inside of the insulation and again, it is supposed to be continuous. If you look closely at the insulation, the insulation on the knee wall goes down to the floor and the insulation on the floor goes under the knee wall. There is a clear break in the air/vapor barrier between the bottom of the knee wall and the ceiling below it.

Because of this break in the air/vapor barrier, any attempt to apply a house wrap or anything else will probably cause a moisture problem. Even without doing anything, there is a good chance there is already a moisture problem. The rule that applies to house wraps or anything else you apply on the outside of insulation is 5 to 1. What this means is the product that is applied to the outside of insulation which has a air/vapor barrier, must have a perm rating 5 times greater than the air/vapor barrier. A perm rating of 1 is considered a vapor barrier, most vapor barriers used by professionals have a perm rating of .1 or less. House wraps so as not to qualify as a vapor barrier usually start at 1.1 perms. Clearly more than 10 times that of a normal vapor barrier.

However, since there is a break in the air/vapor barrier in most Cape Cod homes, applying the house wrap in this area or anything else that has a perm rating, no matter how little, will actually trap moisture in the insulation.

Perm stands for permeate, the rating stands for how well moisture diffuses through a specific material. There is a solution to your problem, but as said earlier, easier said than done. If you are interested, I will require more information.

 

What type of addtional information would help? I would appreciate hearing more about your input.

 

In my line of work I use a blower door to measure and verify. This measuement is done primarily for the utility who pays me for the work performed in these homes. The reason I'm telling you this is because this type of service is not readily available to the public and if you want to correct this problem, you are more than likely going to have to do it yourself.

It has been my experience with Cape Cods that this area we are discussing is one of the major concerns, not only with air leakage but also moisture problems. The terms usually associated with it are by-pass phenomea, air leakage and stack effect.

The information I require from you is the floor in the area behind the knee wall floored? Do you have access to the insulation in the floor? Are you willing to do the work or you just want a better understanding of the problem?

 

The area behind the knee wall is not floored. It is the ceiling of the first floor with 12 inches of loose blown in fiberglass. I know that the area under the second floor plywood is not insulated except for the loose fill that is installed up until the plywood begins. I have planned on adding 3/4 inch styrofoam panels into the open joist areas and sealing so that this air space will be sealed from infiltration between the ceiling and the second floor. This is an obvious problem area.

I have access to the area and am willing to do the work needed myself as long as I know what I am doing. I know about air door testing and understand it is very useful if done properly. I don't have access to such equipment so I only have the general knowledge of this problem area to work with.

Am I heading in the right direction?

 

The blower door is an excellent measuring and verification device and has limited diagnosis capabilities towards air sealing. There is a great deal of hype about blower doors. You do not need a blower door to resolve your problem, just common sense. I personally wish the people in the air sealing industry would use more common sense than rely on the blower door to identify the source of air leakage. The truth here is the blower door cannot distinguish between air leakage (high pressure) and air infiltration (low pressure).

Fiberglass insulation has a very low absorbancy rate and a very high explusion rate towards moisture. Because of this it is unwise and highly unlkely to install fiberglass, batts or loose fill without a vapor barrier. So the next things I need to know is if a vapor barrier is installed under the 12 inches of loose fill? Does the loose fill cover the entire first floor ceiling or only where the knee wall space is located?

 

The loose fill only covers the area behind the knee wall and stops where the second floor flooring begins at the knee wall. Therefore there is leakage under the second floor.

I am not at home now (out for the holiday) so I can't look to verify but I do not believe there is vapor barrier under the loose fill. There is definately vapor barrier behind the dry wall on the warm side of the knee wall.

I would be willing to install vapor barrier under the loose fill if needed (if possible). And I did mention adding the foam panels at the gap under the knee wall between the floor joist of the second floor. Is that acceptable. Vapor would be able to pass from the first floor into the cavity of the area below the second floor and I assume just enter the second floor space. It would have to pass through the ceiling, the subfloor and the pad and carpet on its way.

 

The foam panels will work provided you air seal them in place with either caulking or foam. The other areas of concern are penetrations. You can identify them from the first floor ceiling. Look for ceiling lighting fixtures in that area or any other electrical device, like a smoke detector, etc. You can either air seal them from the first floor or upstairs. To air seal them from the first floor, you have to remove he fixture and foam or caulk the hole in the ceiling. If it is a recessed lighting fixture and is an IC type, IC stands for insulated ceiling, remove the bulb and then the cover to expose the canister. You either caulk or foam between the canister and ceiling. Let dry and replace the cover and bulb.

If you do it from upstairs, you have to locate the fixtures by pulling back the loose fill and then either caulking or foaming the holes in the ceiling that allow heat to by pass the ceiling.

To determine if you should install a vapor barrier is by inspecting the loose fill. In an area where there are no penetrations, feel the loose fill to see if it is damp. Reach down and grab a handful of loose fill and squeeze it in your hand. If there is moisture it will be left on your hand when you release the loose fill. Do not confuse warmth with moisture. Loose fill near the penetration will naturally be moist. Do not use that as an indicator for installing a vapor barrier. There is a good chance that the paint on the ceiling is working as a vapor barrier.

 

Good - most of your recommendations are backing up my plan of action. I do have a foam spray product I intend to use to seal the foam board.

I don't believe I have a moisture problem with the loose fill but i will inspect to verify.

If I have the plastic barrier on the knee wall on the warm side and wanted to add foil faced foam board on the attic side with the foil to the attic side for heat reflection summer gain in the summer- would this be a fire hazard not having the foam board covered with drywall? I don't know if this is an acceptable practice.

I also have thought about the reflective bubble wrap material in this same area.

Would having the fiberglass insulated area protected from the warm inside moisture with the plastic barrier be sufficient to allow me to enclose the wall insulation this way?

 

The foil faced bubble material is usually used for garage doors and duct insulation and it does qualify as a vapor barrier. Radiant barriers in most cases are vapor barriers. I do understand your concern about heat in this area during the summer, unfortunately, applying anything in this area over the insulation will disappoint you. Both as far as performance and preventing possible moisture problems.

There is a simple rule that most people in the air sealing and insulation industries and similar industries overlook. That is it takes considerable longer to remove moisture or humidity in air or materials than it is to do the same with temperature. One must keep this in mind when considering the processes that occur within a structure. For example, ventilation uses convective heat transfer. The principle applied here is that air at a lower temperature and humidity level will extract heat and humidity from air or materials that have a higher temperature and humidity level. Free venting then extracts that air from the structure. However, based on that rule mentioned earlier, we need to slow down the humidity transfer, since we cannot stop the flow of heat or humidity. Hence the need for vapor barriers.

I know this is difficult to explain and understand, but there are 2 different processes here that are interacting effectively. But what happens if I introduce another product that will clearly interact with the 2 present processes? On top of that, this product has its own process, which will interact with the 2 other processes. From my own experience, the rule is overlooked. People assume that it will not have any or a negative effect.

I know I am not explaining this well, let's try it this way. The vapor barrier reduces the moisture content in heat by diffusion, primarily because of the rule stated above. The insulation prohibits heat flow, either out of the home or into the home. Free venting ventilation removes the unwanted humidity in the winter and some heat from the home. Yes, that is true, convective heat transfer removes heat from the home in the winter but is considered to be the lesser of 2 evils. In other words it is worth losing that heat considering the moisture problems that are avoided. These 2 separate processes work in conjunction with each other, in order to sustain the integrety of the structure and retain as much heat as possible.

If I apply a house wrap to this, what happens? The house wrap is specifically designed to address wind pressure. This in reality is convective heat transfer but only accelerated. Though this is a completely separate process, the house wrap does not adversely effect the 2 other processes. The reason for this is the house wrap has a 5 times greater perm rating than the vapor barrier and its thermal insulating properties are negligible. I cannot say the same for the other products you mentioned. In my opinion, it would create a moisture problem.

 

Greetings,

To begin with. The FG is probably increasing the a/c load to the up stairs, plus, the large amout of additional heat energy coming thru the rafters that are exposed to the room area. Increasing the air flow will do little or nothing for the hvac costs.

Since this type of construction can be difficult and costly to rehab, you might want to check Koolcoat.com. They have a paint additive that is very effective for this type of problem, and, it's not expensive. This a RB paint additive and will help that care of the rafter problem too.

If part of the ceiling that is over the down stairs is insulated and easy to get to, you could install a perforated RB dirctly over that part. You would also see a difference from doing that.

If you want to go the whole way, then let me know I will will detail the other way.

Thank you for considering my opinion.

 

rbisys-

I am confused by a couple of your reponses.

You mention increasing the air flow not helping with HVAC cost. What air flow would you be referring to? In what area?

Would the radiant paint be used on the underside of the roof area? Not sure exactly. The website for that product was not real detailed.

I follow you on the radiant barrier over the insulation except I have read that dust that settles will reduce the effectiveness. Have you had any experiance in that instance?

I might be interested in going the whole way but that depends on what the whole way means. If it means removal of drywall on the finished side then that would not be an interest due to the size of the project and cost.

Thanks

 

Greetings,

HVAC. Air infiltration into the room may or may not be improved with RB. It depends on the nature of the construction and what can actually be installed. Cracks and wall boxes may be more of a problem. If there is no sign of condensation on the roof sheathing I wouldn't worry too much about upgrading attic ventilation. The primary goal of air venting devices is to prevent condensation. Air infiltration can be a serious cause of excessive energy use. You will have to decide how much of a problem it is.

In my previous message I failed to mrntiopn that removing the FG from the side walls and replacing with RB would make a difinite improvement. You wanted to maintain as much "R" value as possible. Sorry there is not much to start with. You could also install a single layer of perforated RB over tyhe FG, but I'm concerned that that might cause condensation.

You could use the paint on the sheathing, but if you did I would still paint the inside (exterior ) surfaces for better winter control.

In your case a combo would be best depending on the accessibility to get to the various areas. It may wind up that the most efficient and cost effective way would be to us the RB paint. It would be worth the effort and cost.

The claim of dust is exaggerated, by guess who. If you notice they never tell you how much decrease or the fact that the underside is unaffected. I don't care if you paint the top side flat black, at 110 degs surface temp the RB is only radiating about 2 btu/hr/sf as compared to FG's 37 btu/hr/sf. I usually use a two layer method over the attic insulation. I do not install in /on the rafters unless there is no other way.

I hope this answers your ?'s. any more, just ask.

Thank you for considering my opinion.