Another question on crawlspace insulation


Old 09-26-01, 02:25 PM
Visiting Guest
Posts: n/a

My situation is bit different then the last thread on crawlspace insulation so I thought it better to start a new discussion.

Here is the problem:
During the cooler months, our ground floor is quite cool (wood and carpeting) to walk on. Although the thermostat reads 68 degrees, we find that we need to wear footwear around the house to keep yourself warm.

Now the situation:
1) The home is only 3 years old and is located in the foothills of the Cascade Mtns
2) There is R-19 fiberglass insulation (per the spec sheet) already between the floor joists. It is exposed fiberglass to the crawlspace and is suspended with twine (zig-zagged among the joists). The insulation appears to be lying on the twine.
3) There is heavy plastic covering the ground.
4) There are numerous vents on the South, West and North sides of the crawl space.
5) The crawlspace walls are bare concrete.
6) The crawlspace contains a sump pump.
7) All heating ducts in the crawlspace are wrapped w/ insulation (not certain of the rating).
8) During the cooler periods, the temperature in the crawlspace is within a couple of degrees of the outside temperature. (This is usually 45-49 degrees)
9) During the winter months, we have a prevailing wind from the South at 10-20 mph and the typical PNW drizzle. Half a dozen times a season we will have a wind storm where winds average 35-40 mph with gusts to 55+mph.

My assessment:
a) The house was designed w/ Seattle building codes. We are (on average) 7 degrees colder and have a prevailing wind factor to consider.
b) The winds hit the South facing vents and keep the crawlspace cold
c) The fiberglass insulation in the joists is ineffective (w/ the amount of air movement under the house.)

I see that I have a couple of options:
i) Put some Styrofoam insulation over the vents on the South side of the house to limit the wind. (I want an all-season solution, so I donít know if I will create problems by doing this.)
ii) I am contemplating getting some type of radiant barrier (e.g. to staple onto the joists. I do not want to lock moisture into the joists or the existing insulation.

So, I am looking for options Ė am I on the right track? Anything else I should consider? Any recommendations on good radiant barriers?

Thanks in advance!
Sponsored Links
Old 09-26-01, 02:53 PM
Visiting Guest
Posts: n/a
You have the right idea about the radiant barrier (RB) but not in combo with the fiber glass (FG).

Your primary source of heat lose thru the floor is radiant heat (exceeds 90%. See Machcanical Engineering Handbook).
By the way the greatest amount of heat gain in your atttic is also radiant heat. I have developed an improved method for both areas.

The FG is less than 10% efficient plus the increase heat flow due to moisture that is suspended in thr FG. Eventally the excess moisture will rot out the joists. I'm surprised that your not getting mold/mildew problems too.

If you go down the list on this page you'll see, Did I trap moist....... from Robert Smith. He's having crawl space problems too. The info I gave him applies to you also.

If you put a RB under the FG you will be putting in a very efficient vapor barrier and will trap even more moisture. The way I have out lined the RB method, via Smith, will give you no moisture problems and raise the floor temps.

If I can be of further help, please let me know.

Thank you for considering my opinion.
Old 10-05-01, 06:45 AM
Visiting Guest
Posts: n/a
After reviewing the other thread, it looks like you are recommending:

1) Add poly sheeting on the bare walls.
2) Install RB over the poly.
3) Remove the insulation under the floor.

Some follow-on questions:
a) Is the poly sheeting necessary?
b) I have been reading (on the web) about the limited utility of RB in cooler climates. Granted, a lot of this was for attic installation. Will this be effective for warming the crawlspace?
c) Ventilation: Do I need to seal the vents, or will this create other problems? I was contemplating sealing several on the windward side of the house on the windward side of the house. (There are still quite a few of them on two other sides of the house) I do not want to create moisture or Radon problems.
d) Can you recommend some specific RB for such and installation?
e) Is it necessary to remove the floor insulation?

Thanks again!
Old 10-05-01, 08:19 AM
Visiting Guest
Posts: n/a
Thank you for your questions about radiant barriers (RB).
The first thing to consider is where the claim comes from, that RBs are not as effective in winter conditions.
Considering that RBs are 97% efficent in any direction and our biggest challenge is the "others" at about 10 to 20%, I'll let you figure that one out.

More seriously and accurately, you have to look at the difference in operatig conitions winter vs summer and the difference in energy costs winter vs summer.

Winter, zero out side, 70 degs inside, pretty stright forward, 70 deg temp difference.

Summer, 95 deg out side air, 80 deg inside, up to 100 deg difference. Why? Because the roof is radiating large quanities of energy to the insulation below. Your temp can get up to 180 degs. on top of the insulation because some insulations such as fiber glass (FG) absorb over 90% of that energy and since it is over 98% air space, the energy radiates thru the material. This is true of the crawl space too. RBs reflect 97% of that energy. There is a formula that tells you how large of a % of heat energy radiates thru FG. RBs do not condensate and hold moisture which also increase the energy flow. Ask your FG salesmann to tell you about this, they just love to do so.

You want the poly because any moisture coming thru the wall will condensate on the poly and run down to the ground where you have two more poly's to prevent moisture from the ground.

An aside. You do not put a RB on the floor poly because your flooring radiates about 22 BTU's / hr/ sf to the ground, which helps to cool the house. It is only put on the walls because that is where the greatest temp difference is in winter. Your can RB the floor if your particular demands require it.

I developed this method about 10 yrs ago. The first time I used it, was in a very, very cold Feb. Sub zero nites, about 10 degs day. New house, no heat and the plumbers came in to finish some work in the crawl space. They got down there and turned on their 500 watt light and were removing clothes in about 5 mins.

Once the RB is in place. the FG is of very little, if any, value. It could continue to condensate moisture and hold that against the wood, which could cause dry rot, plus, you don't know if you have any mold/mildew in it. Why take chances. The code requires vents because of moisture. We already know that is a joke. IF your space is well drained, by gravity or sump pump, coverd with the 2 layers, crossed, vapor barrier, you should not have any moisture problems. Yes seal the vents.

You also want to remove any FG at the rim board and replace it with RB too. Dryrot can be particularly bad there due to FG hold'g moisture.

If there are no radon problems in your immediate area, the chances are excellent that you don't have any either. I was reading recently that radon probably doesn't cause cancer after all. What else is new.

Product type: Plastic film with foil laminated to both sides. Not bubble foil, that's too expensive for what you need it for and it isn't any more efficient. Remember it's the surface that reflects. I have some factory seconds at $170.00 / 1000 sf roll + tax and frt.. 48" wide. This is what I use.

Any more ?'s, just ask

Thank you for considering my opinion.

George H.
Old 10-23-01, 03:10 PM
Visiting Guest
Posts: n/a
I seem to be getting closer to a solution, I appreciate your help.

The only RB that I have been able to find locally (Seattle area) at Home Depot and Lowes is Reflectix ( Does anyone know where I can find some other RBs? This runs about $40 per 25 feet x 48".

Thanks again
Old 10-30-01, 03:56 AM
Visiting Guest
Posts: n/a

I apologise for the delayed response.

You're looking at bubble foil which is ok, but costs too much. I have a factory second that will suit yor requirements. The defect was in the lamination of one of the foils that can peel off in small bits if handled too roughly. For your use it will be fine, you'll probably not even notice the defect unless you drag it over rough concrete with a heasvy weight on it. It costs $180.00/ 1,000 sf roll x 48" wide,and is perforated, plus frieght.

This is the material I use on my jobs.

I can send it collect. Let me know your address and I'll get a qoute for frieght and other costs. Or, if you chose you can pay up front plus frieght.

Contact me at [email protected]


Geeorge H.


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
Display Modes