skip to main content

Basement insulation question

last year

Contractor is finishing a walk-out basement for me, so some exterior walls are partial concrete and then above ground exterior framing. The unfinished basement had the fiberglass insulation on the framed exterior walls covered with a white plastic, and insulation of the concrete part held up with some fabric.

Finished basement has furred walls floor to ceiling in front of those exterior walls. My question is; when re-insulating, should the plastic holding the insulation in the exterior framing be removed, or can it stay? does it matter? if left as is, there will be the insulation in the exterior wall, that plastic, possibly a small air gap, and the insulation in the furred wall.

Is the plastic any sort of vapor barrier or is it just there to hold the insulation in place?

The home is in the northwest if that matters, so relatively cold winters and dry, hot summers. It was built in 2020

Sort by:Oldest

Comments (3)

worthy
last year
last modified: last year
Hard to tell without photos. But double vapour barriers are to be avoided. (Of course, a moisture sensitive insulation is a Code legal mistake in any basement.)

If it were my house, I'd pull out the fg and install XPS or EPS boards, then frame and fill the space with the fg that came with the house.

At the least, make a few razor cuts in the interior vapor barrier so it doesn't trap moisture moving from inside to outside.
Frank Marsh thanked worthy
Frank Marsh
Original Author
last year

Above is the before picture. I'm discussing e.g. the wall in the background in the upper left. The top part of the wall is above grade, framed, insulated, and has plastic sheet over the insulation. The bottom part of the wall is below grade and concrete below the framed wall. The insulation is held up against the concrete with some sort of breatheable fabric, there is no furred wall in front of the concrete.

Above is an after picture taken of that same wall, but standing in that room (same angle as first picture would be blocked). A furred wall has been framed an inch or two in front of the concrete and goes floor-to ceiling. In some places of the basement the existing insulation that was hanging against the concrete was saved and reused. In this particular area new insulation was put between the studs. I think it is R-19. And it is the same thickness all the way up, so there also isn't any additional insulation for the inch or two lip where the concrete is wider than the above ground framing. I can't get a picture of it now without removing the new vapor barrier, but when I posted the question, the old vapor barrier was still on the top framed part of the wall and they just put the new insulation in front of it. They might have scored it, but I don't think so.
Related Discussions
when too much insulation is a bad thing..
Q
Comments (15)
If you're watching holmes on homes, are you in Canada? If so I think the rules about combustion air are specific, you do need a louvred door. I don't understand why you wanted to insulate the mechanical room, generally, even with HE units, they are going to warm the place, not cool it. Does the furnace have a combustion air draw in? There will be an outside pipe that runs into the cold air return, that's not it, but a separate pipe that brings air in for the furnace, some he vents are a funnel shape outside, the part close to the wall draws air in, the other blows the combustion air out. I've seen 'make up air' setups that appear to be a duct outside, with a cage over the end near the floor next to the furnace. That's probably the sort of thing you need.
...See More
Basement question
Q
Comments (1)
If you can't move the framing, fill all the space between the studs and behind them with fibrous insulation or more XPS or EPS. Air circulation in the insulation or behind it will just reduce its effectiveness. Here is a link that might be useful: Building Science on Basements
...See More
Insulation question: space between basement wall and stud wall.
Q
Comments (2)
Probably ok as long as the rim area is insulated. Space between the XPS and batts will permit air currents that reduce the effectiveness of the batt insulation. I would prefer to box and keep the wall as tight to the foundation as possible. Otherwise, fill the space with low-cost EPS insulation. Here is a link that might be useful: US Dept. of Energy R Value Recommendations
...See More
basement insulation question
Q
Comments (7)
Thanks again, But if I may, I would like a little more clarification on this. First of all, although I realize all basements are going to have some moisture issues, relatively speaking, I have a very dry basement. So, from your last statement, if I attached 1" of XPS to the lower half of the wall (the cement block part) and built my new stud wall on top of that, I could confidently fill the remaining gaps with batt insulation to the ceiling? But, from what I've read in the building science articles, I should NOT put a vapour barrier on top of the insulation to allow for inward drying, unless it is MemBrain? I have no problems with this, it just seems counterintuitive. What happens at the top half of the wall (wood stud) that is built on top of the block wall above grade? It is brick veneer on the exterior if that matters.
...See More
worthy
last year
last modified: last year
My answer is the same. There should only be one vapour barrier. Even if the plastic that you're covering with more insulation was not supposed to be a vapour barrier, it is a vapour barrier. Incidentally, I would replace that mouldy piece of insulation on the right upper corner in the second photo. That it was mouldy from being against the concrete wall is telling.

Code minimum isn't the best way to go.
Frank Marsh thanked worthy

Related Stories

GREEN BUILDING Insulation Basics: Heat, R-Value and the Building Envelope

By Mariana Pickering (Emu Building Science)

Learn how heat moves through a home and the materials that can stop it, to make sure your insulation is as effective as you think

REMODELING GUIDES Cool Your House (and Costs) With the Right Insulation

By Buckminster Green LLC

Insulation offers one of the best paybacks on your investment in your house. Here are some types to discuss with your contractor

WINDOW TREATMENTS Easy Green: 9 Low-Cost Ways to Insulate Windows and Doors

By Laura Gaskill

Block drafts to boost both warmth and energy savings with these inexpensive but effective insulating strategies

MATERIALS Insulation Basics: What to Know About Spray Foam

By Mariana Pickering (Emu Building Science)

Learn what exactly spray foam is, the pros and cons of using it and why you shouldn’t mess around with installation

REMODELING GUIDES Considering a Fixer-Upper? 15 Questions to Ask First

By Sarah Phipps Design

Learn about the hidden costs and treasures of older homes to avoid budget surprises and accidentally tossing valuable features

MOVING Hiring a Home Inspector? Ask These 10 Questions

By Christine Tusher

How to make sure the pro who performs your home inspection is properly qualified and insured, so you can protect your big investment

GREEN BUILDING Considering Concrete Floors? 3 Green-Minded Questions to Ask

By Melanie Loftus

Learn what’s in your concrete and about sustainability to make a healthy choice for your home and the earth

LIGHTING 5 Questions to Ask for the Best Room Lighting

By Karen LeBlanc Design TV Host & Writer/Blogger

Get your overhead, task and accent lighting right for decorative beauty, less eyestrain and a focus exactly where you want

REMODELING GUIDES Survive Your Home Remodel: 11 Must-Ask Questions

By Mitchell Parker

Plan ahead to keep minor hassles from turning into major headaches during an extensive renovation

Design Dilemmas: 4 Questions for Houzzers

By Annie Thornton

Share Your Advice for a Low-Water Garden, Wet Bar, Family Room and Basement Spa!

More Discussions

JBN Builders, LLC Review

1

Awkward Spot for Bar

3

Need your opinion about a basement water stain

2

Help with basement layout

2

Noise barrier for painted basement ceiling

2