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Insulation of timber suspended floor in basement

Hello all,

I have a ~100 year old timber framed house that was moved onto a concrete block walk-in basement in the 1950's or early 1960's. This basement was screeded in early 1980's (unknown if insulated or vapor barrier). The one wall that is above grade is ext insulated above grade only (about half), and this wall has a modern garage door. The other three walls of the basement do have their upper 1-3 feet above grade, mostly against crawlspaces lying under wings of the house. The house is on fairly sandy substrate on a hill, but nevertheless the basement regularly floods in springs (as Vermont snowmelt is pretty severe). Otherwise, the basement is fairly dry. It is an unconditioned space, used only for laundry, storage, etc.

Above this basement is the main living space. This is a 50-100 year old maple strip floor laid over ~6 inch softwood planks, open to the basement (fairly regular 3x8 joists, not entirely regular spacing). The house is reasonably insulated for it's age, still far from air tight but not bad.

My questions are: I would like to insulate under the floor of the house (basement ceiling). I am not up for the expense of digging out the basement walls to insulate them externally. Question 1) any opinions about the worthiness of insulating the internal surfaces of the basement walls? The basement has some damp (including the walls which have no barrier), so I would assume this is not worthwhile and might invite trouble.

Question 2) It would seem to make more sense to insulate the basement ceiling (floor of house). What are the worries about condensation and vapor build-up? What material(s) would be advisable? I intuit that I am getting a fair amount of air movement through the floor so would like to restrict this and add r-value, but am hesitant to create a vapor tight space for free of moisture problems. This is a beautiful old house and I don't want to create mold/rot conditions due to condensation around the insulation (moist air moving from house onto colder surface of insulation). I have considered adding a radiant floor heating system to this while it's open. Advice? Would a space between insulation and underside of floor be advisable? What materials? What difference would radiant system make? How to finish the underside of insulation (drywall, netting, etc.)? I would like to keep the project green but above all else r-value, reducing air movement and not endangering mold, etc. is the most important.


Asked by T Vermont
Posted Dec 14, 2011 2:58 PM ET


5 Answers

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T. Vermont,
If you want to insulate the basement walls, you first have to address the moisture issues. The best way to do that is to excavate down to the footings on the exterior, install footing drains, and install a waterproofing system.

If you decide to install insulation on your basement ceiling, your best options are spray foam or a layer of rigid polyisocyanurate, installed under the joists, with taped joints.

Answered by Martin Holladay
Posted Dec 14, 2011 4:53 PM ET


Thanks Martin,

I would agree with the basement wall method you suggest, I suppose it's far more cost-effective to simply insulate the basement ceiling. I am interested to know your rationale for using spray foam or rigid foam. I am concerned about the eco-profile of these materials, and also as I referenced in my original post, concerns about breathability. Fixing foam to the underside of the joists would create a fairly vapor tight space where condensation coming from vapor in the living space above (see description of floor) might cause problems. Do you or anyone agree/disagree? Would venting the space be one alternative? I have considered using something with more permeability, but not sure what options would be readily available (cellulose is one)., and what pros/cons I would be looking at.

Answered by T Vermont
Posted Dec 14, 2011 5:46 PM ET


You are worrying unnecessarily about vapor and moisture flow between the house and the basement.

The basement is cool but damp. The space above is warm but dry. Vapor flow under these circumstances is variable, depending on the interior relative humidity and the temperature. Don't worry about it.

There is nothing wrong with creating a vapor barrier at your basement ceiling, as long as the vapor barrier includes R-value. That's why I suggest you use polyiso or spray foam. Either type of insulation won't be negatively affected by high relative humidity.

Answered by Martin Holladay
Posted Dec 14, 2011 5:51 PM ET


Thanks again. Is there any reason not to put rigid insulation between the joists rather than under? I know it isn't as tight this way, but headroom is limited as it stands so would like to avoid eating into available space. Not keen on spray foam, both because there are services in the joist bays, but also find it an unnecessarily permanent installation. Any other non-foam suggestions?

Answered by T Vermont
Posted Dec 14, 2011 6:46 PM ET


Installing rigid foam between joists is fussy and time-consuming, but possible.

Answered by Martin Holladay
Posted Dec 14, 2011 7:30 PM ET

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