Crawlspace Insulation


Old 10-27-03, 07:04 AM
Visiting Guest
Posts: n/a
Crawlspace Insulation

I've been doing some internet research on insulating my crawlspace and am finding somewhat different answers on the proper method to be used.

Situation: Home in Berkshire Mountains of Massachusetts. One wing of the house has a full basement, while one wing has crawlspace with no insulation and vapor barriers.

Problem: high relative humidity readings and musty smells in section of the house over the crawlspace. However, basement itself has lower humidity level than living area above it.

Other factors: Forced air oil furnace is located in full basement, with ductwork and water pipes extending through most of crawlspace areas. Crawlspace is not vented. Basement is dry and crawlspaces appear dry.

From what I have read it appears that I should do the following:

1. Install plastic vapor barrier over the dirt floors of crawlspace. Some people say vapor barrier should extend up the foundation walls up to the sill - others say it should be brought to 1 to 2 feet below the sill. Not sure which is the correct approach and would appreciate some comments on this.

2. Install fiberglass insulation against the foundation walls, including the rim joist. Questions: If the plastic vapor barrier mentioned above is installed up to the sill - does this mean I should use unfaced insulation on the walls? If it the insulation should have facing, I assume it faces the wall and not the interior of the crawlspace?

I have also read that someone recommends the insulation against the crawlspace walls not be brought down to the floor, but only 1 to 2 feet below grade. They say this insulates the coldest part of the wall but can prevent frost heave. Comments?

3.Unsure whether ductwork and waterpipes should be insulated. ??

Also, there seems to be 2 schools of thought on venting : some people say crawlspaces should be vented (preferably automatically and perhaps with a van) while others say it should not be vented. (mine is not). Any comments?
Sponsored Links
Old 10-27-03, 11:29 AM
Banned. Rule And/Or Policy Violation
Join Date: Nov 2001
Location: USA
Posts: 1,875
Received 0 Votes on 0 Posts
Thermal/Moisture Dynamic; RH%; ErH%

One characteristic of the Thermal/Moisture Dynamic is that warm air condenses against cooler surfaces and not vice-versa. The reason for this is Relative Humidity (RH%). What this means is when air is heated, it expands and this causes the air to have the ability to hold more moisture. So an air mass in a crawl space with a temperature of 50 degrees Fahrenheit with a RH% of 60%, when the air mass temperature rises the RH% goes down. The reverse is true as the temperature drops, the RH% in the air increases. The further the temperature drops, the greater increase of the RH% in the air. Until the RH% of the air in the crawl space reaches 100%, then condensation begins to form. This also explains why condensation forms on some surfaces in the crawl space while not on others. Those surfaces that have condensation on them are cooler than those surfaces that do not. eg. above grade crawl space masonry wall. This supports insulating the crawl space wall above grade.

Equilibrium Relative Humidity (ErH%) states that air or objects with a lower humidity than other air or objects it comes in contact with, will absorb humidity from the higher humidity air or objects until the humidity levels are equal in the air or objects. Now there are several applications in your situation, I am only explaining a single aspect concerning the plastic or moisture barrier application. The heating ducts in the crawl space rises the temperature of the air, this reduces the RH% in the air or lowers its humidity level. ErH% produces a pressure induce moisture transfer with the moisture in the dirt floor because of the lower humidity level in the air in the crawl space or if you prefer, capilliary action. This pressure inducement is so strong that if you covered the dirt floor with plastic, it would force the moisture in the ground up into your masonry walls so it could equalize with the air in the crawl space. This supports applying the plastic to the dirt floor and up the masonry walls one to two feet.

You do not apply the plastic up the masonry walls to the sills in a crawl space because of ErH%, which is another application in your situation. All materials have the ability to absorb and expel moisture, some more than others. When you apply insulation to a wall with a vapor barrier, it does not stop either heat or moisture flow through it, it slows it down. What this means is there is constant flow of heat that possesses a percentage of humidity in it through the insulation. So if you were to install plastic all the way up the wall and then insulate the wall, instead of preventing a moisture problem, you would have created one. This low amount of heat and humidity is easily absorbed by the masonry wall and expelled to the outside of the structure.

Insulate the entire masonry wall. Frost heaving is caused when water freezes, this is because water expands about 12% in volume when it freezes. I cannot see how insulating the inside walls of a crawl space can cause water to freeze, or lower the frost line below your foundation. If someone knows how this can be done, I am willing to listen.

To ventilate depends upon your situation. Your remarks indicate that ventilation is not needed but your perception can be deceiving. Without seeing the site it is difficult to determine. For example, if the crawl space is open to the basement and the furnace is a combustion system, then you have ventilation. The reason for this is the volume of air within the structure remains constant. Meaning to say you cannot let air into the basement without letting out the same volume of air and vice-versa. There is what is known as the combustion triangle. There are three parts to combustion, fuel, temperature and oxygen. The oxygen comes from the air in the basement and crawl space if open to the basement. All oil furnaces are forced draft, most high energy efficient gas furnaces are induced draft and then you have the natural draft furnaces which consume the most amount of air. On the average a combustion system today in similar situation like yours, the furnace consumes about 600 cubic feet of air per hour. That air must be replaced as it is consumed by the furnace for combustion and that is more air exchange than if you had vents in the crawl space. Again I am only guessing and I am deducing it from your remarks.

Yes, insulate your ductwork and pipes in both the basement and crawl space. It will reduce the probability of condensation and a possible moisture problem. With that I'll leave you with a couple of questions. Why does condensation form on the cold water pipes and not the heating ducts and hot water pipes? How does the heat from the ducts and hot water pipes produce condensation on cooler surfaces? I bet you can answer them.
Old 10-28-03, 06:19 AM
Visiting Guest
Posts: n/a
Crawlspace insulation

Thanks! That was very informative. Let me see if I understand the practical application of this;

1. I should first insulate above the sill with the paperside against the exterior wall.
2. I should insulate the crawlspace walls the same way with the paperside against the exterior walls.
3. I should put down a vapor barrier on the ground and extend it 1 or two feet above the ground.
4. I should insulate all ductwork and pipes in both the crawlspace and full basement.

Do I have this correct?

A few more questions regarding this...

1) i am a little confused about the area where the plastic sheet vapor barrier meets the insulation on the crawlspace walls. Do they overlap? If they do overlap, is it then correct that the vapor barrier that is part of the insulation batts is placed against the plastic sheet? Or are there some places were I should use unfaced insulation?

2) Regarding ductwork and pipes - what the best type of insulation to use for these? Also, wouldn't this have the effect of making my full basement colder, since it would stop some heat transfer from the ductwork to the basement?

3) back to the insulation on the crawlspace walls...what is the best method to seal the seams between the batts?

4) with regards the to plastic ground vapor barrier - what is the best way to secure this to the wall?

Thanks so much for your help...
Old 10-28-03, 11:59 AM
Banned. Rule And/Or Policy Violation
Join Date: Nov 2001
Location: USA
Posts: 1,875
Received 0 Votes on 0 Posts

This site gives an example of basement wall insulation and you can get instruction from that site too.

1. The vapor barrier faces the warm side of he structure which is inside the crawl space. So it does not go up against the wall as the site above will explain.

2. Same as 1.

3. The moisture barrier should go on the ground and extend up the wall one to two feet.

4. Yes, insulate all the pipes, cold and hot, ducts supplies and returns in both the crawl space and the basement.


1. Yes they will overlap at the bottom of the wall. This will not make a difference or create a moisture problem because most, if not all of the heat with a percentage of humidity absorption will be done on the masonry above grade.

2. No it will not make that much of a difference if you insulate the pipes and ducts in the basement. Truth is the vast majority of heat in the basement comes from your flue stack temperature which is usually above 200 degrees Fahrenheit. You get this heat from not only the furnace but also from your water heater. PLEASE do not touch the flue pipes to find out if I am right, because they are hot and you will get burned.

3. Since these walls are not framed out, you are better off using a simlar product shown on that site. Very easy to install, works better than trying to tape up all those seams and ask if their product comes with special tape for the seams, most of them do provide some with their product.

4. Liquid Nails, Foam in a can, I prefer the the former, liquid nails but you have to be careful with the temperature when applying. Read the instructions carefully with any product before use.
Old 10-28-03, 12:17 PM
Ed Imeduc's Avatar
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 18,386
Received 0 Votes on 0 Posts
crawl way

For what you have I would not insulate the duct there. I would cut a small register in the duct for and outlet in the crawl way. also one there in the basement with a small return this lets the crawl way work as a heat sink for you and help with warm floors.We use a 2" polystyrene in the walls in the crawl way ED
just my .02 cents been doing it this way for over 30 years now and it works
Old 10-28-03, 01:07 PM
Visiting Guest
Posts: n/a
If the paper side of the insulation goes toward the interior of the crawl space, how is that reconciled with the following advice from the web site you pointed out?

Never leave faced insulation exposed. The facings on kraft-and foil-faced insulation will burn and must be installed in substantial contact with an approved ceiling wall or floor construction material
Old 10-28-03, 02:25 PM
Banned. Rule And/Or Policy Violation
Join Date: Nov 2001
Location: USA
Posts: 1,875
Received 0 Votes on 0 Posts
In some towns they require vapor barriers either be fire resistant or be covered and not exposed in basements. This does not apply to crawl spaces. Furthermore, the product shown on that site the vapor barrier is fire resistant.
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: