Insulating basement walls

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kwithers
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Insulating basement walls

Post by kwithers » Sat Mar 11, 2017 7:06 am

I enjoyed your video on insulating basement walls. I have two questions:

1) I noticed you use Bluwood or PT on the bottom studs. I've also seen strips of plastic (usually garbage bag material) placed in between the floor and the stud. Is this overkill?

2) You suggest installing rigid board up against the concrete walls, then framing with batts, then putting up a poly vapour barrier overtop. Are two vapour barriers necessary? Just wondering if you could get away with no poly since you already have the rigid foam board acting as a vapour barrier,

Thanks,

Kevin

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Shannon
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Re: Insulating basement walls

Post by Shannon » Sat Mar 11, 2017 8:11 am

Welcome to the foruM.

1)Basically if you do not use a treated lumber against the concrete then you need a barrier between the untreated lumber and concrete ...like poly or foam gasket.

2)The 1" rigid foam I used is not thick enough to be considered a air/vapour barrier and does not have enough R value to change the dew point enough to delete the inside air/vapour barrier (poly). Some places will allow the deletion of the inside air/vapour barrier (poly) if you were to use 2" rigid foam instead of 1".
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zackstone
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Re: Insulating basement walls

Post by zackstone » Sun Nov 03, 2019 7:28 pm

Hi,

Thank you for making these videos and helping us DIY-ers.

I had a follow-up question to this thread. I noticed you also used BluWood studs in your "How To Attach Rigid Foam Insulation To Concrete" video. My question is whether the BluWood (or similar PT) studs are recommended when going up against rigid foam? I understand why they're required on the floor but wasn't sure whether there is a moisture risk when going against rigid foam hence the Blue/PT choice?

Thank you.
ZS

Tobal
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Re: Insulating basement walls

Post by Tobal » Mon Nov 04, 2019 4:22 am

I didn’t know all the options for moisture barrier until watching your video - thanks!

If using rigid foam board and fully sealing wouldn’t it be a problem if the moisture has nowhere to go?

Also, for the house wrap option does it need to be sealed since your video just mentions going to the floor?

Can I attach wrap to the sill above the concrete that was built with the house or is it necessary to add one to the concrete wall?

Thanks,
-Tobey

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Shannon
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Re: Insulating basement walls

Post by Shannon » Mon Nov 04, 2019 7:47 am

zackstone wrote:
Sun Nov 03, 2019 7:28 pm
Hi,

Thank you for making these videos and helping us DIY-ers.

I had a follow-up question to this thread. I noticed you also used BluWood studs in your "How To Attach Rigid Foam Insulation To Concrete" video. My question is whether the BluWood (or similar PT) studs are recommended when going up against rigid foam? I understand why they're required on the floor but wasn't sure whether there is a moisture risk when going against rigid foam hence the Blue/PT choice?

Thank you.
ZS
So the Bluwood product is treated with a product that protects it from mould and fungus and by taking away those two factors makes it less likely that insects will attach it later on. Bluwood can be used in your entire home. Is in necessary to use in your entire home ? IMO no. This client asked for the product to be used and that's what we did. It had nothing to do with being required by our code.
You are required to not have regular un treated wood in direct contact with concrete on or below ground level because the lumber will wick moisture through the damp concrete . So ways to prevent that are by using either a treated bottom plate or a barrier between the plate and concrete like poly sheeting or foam gasket.
I hope this answers your question.
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Shannon
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Re: Insulating basement walls

Post by Shannon » Mon Nov 04, 2019 8:07 am

Tobal wrote:
Mon Nov 04, 2019 4:22 am
I didn’t know all the options for moisture barrier until watching your video - thanks!

If using rigid foam board and fully sealing wouldn’t it be a problem if the moisture has nowhere to go?

Also, for the house wrap option does it need to be sealed since your video just mentions going to the floor?

Can I attach wrap to the sill above the concrete that was built with the house or is it necessary to add one to the concrete wall?

Thanks,
-Tobey

So the purpose of a moisture barrier inside face of the concrete walls of a basement is not to control leaks in the wall. That needs to be ideally be done right from the exterior of that wall.
Moisture barriers are intended to control surface moisture from condensation that forms inside where warm moist interior air gets to the cool/cold surface of the concrete wall. I would guess that 80% of basements are still only insulated with batt type insulation and these types of insulated walls are not great at keeping warm /moist air from leaking through the vapour barrier past the insulation and up against the cold surface. Also you get a lot of warm air inside the wall assembly because of heat convection through the building materials (wood studs). In these types of wall systems you would use house wrap or poly to keep that condensation or frost that forms on the concrete from getting against the batt insulation. In these cases that moisture can be a fair bit every year. The reason these types of barriers are suppose to not go all the way to the floor joists or be sealed is to allow some exposed concrete to still breath easier and help control and dryout the humidity in the wall.

In a wall system where rigid foam is used that wall cavity is much warmer and dryer because the foam keeps the cold from the concrete out of the wall cavity. Sealing the foam keeps that cold on the cold side where it belongs. So very little moisture should be created between the foam and the cold concrete wall. Any that does should be able to be controlled by wicking through the concrete if it builds up at all. Infact if you use 2" of closed cell sprayfoam you should not need to add a air/vapour barrier under the drywall. Some areas may even let you eliminate that air/vapour barrier if you use 2" or more of rigid foam if its well sealed. Always check your local codes.
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Tobal
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Re: Insulating basement walls

Post by Tobal » Mon Nov 11, 2019 6:00 pm

Thanks for that clarification. I noticed the home builders did not use moisture barrier at all for a small section they built, so I will take that down and do it right.

Since local code requires an R value higher than I can get with 2x4 and foam boards, I’ve decided on 2x6 walls with poly for both vapour and moisture barriers.

There is a wood sill/joist in place just above the concrete walls. Could I “cheat” and staple poly wrap to this, run it down to the floor, then wrap back around to the top front of the 2x6 frame using one continuous sheet?

This would save time and maybe the need for a “sill gasket” but I’m wondering if there still needs to be that separation of poly at the floor if it’s not sealed at the top/sides behind the insulation?

Also do you recommend “breathable” house wrap like Tyvek rather than straight poly?

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Shannon
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Re: Insulating basement walls

Post by Shannon » Mon Nov 11, 2019 7:07 pm

You really should not cover the entire concrete wall. So attaching at the wood plate is not advisable.
I would recommend House wrap as the moisture barrier if you are not using rigid foam against the wall.

You could achieve the R value you need and still use 2x4 framing. This would save the extra cost of the 2x6 framing. Just place your framing forward so that the insulation still can sit back in the cavity.
I am interested what R value your area requires for basements?
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