All Spaces Containing Fiberglass Insulation Require Ventilation

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Last updated on December 10, 2022


Jake Schloegel, CR, President and Founder

Seven years ago I remodeled our screened in porch on the back of the house. I placed 4mill visqueen down on the concrete surface and installed 2×4’s flat over the visqueen 16″cc to receive 2×6 joists. These were cut on a skew as the slab is on a slope. After installing the joist I placed two runs of duct work for heat and a/c between the joist then insulated the deck with fiberglass (6″). Three quarter inch exterior plywood was installed as a subfloor. Wall Side windows were installed on the existing three walls. About five years we began to get a slight odor throughout the entire home. I still cannot ID the odor. It does not smell like mold. I have a feeling it may be coming from our septic system that is about 60 years old. I have had a company that installs and services these things out here on several occasions to check on the functioning of the system and pump it clean. They tell me that everything looks good. System is 60yrs. old. There is no ventilation in that joist space and I cannot say for sure if that cavity is totally free from air. If I was to remove the flooring and expose the joist, and line the open area with visqueen and fill the joist cavities with cellulose insulation and top with another layer of visqueen then replace the decking, would this help solve the problem if it were the problem? How else would I be able to vent that cavity? Thanks….Ed


Areas (crawl spaces, attics, joists spaces etc) that contain fiberglass insulation require some sort of ventilation to help remove the moisture that will occur when warm air meets cold air and the humidity is at a certain level.  If this joist space is unvented it is conceivable that you have a moisture problem which can lead to odor issues.  I don’t see how the septic odor could be confused with a mold odor.  These odors are usually discernible.

If you are willing to go the extent of removing the floor and re-insulate, I would suggest you used a closed-cell foam type insulation like  or   These products do not require ventilation and offer more R value per inch of insulation.  Visit their web sites for more detailed information.