Insulating basement after interior waterproofing has been completed(In Toronto)
Su_Toronto
| Posted in General Questions on
Hi everyone,
Read a few very helpful answers to questions around insulation and hoping to get some clarity.
I had to recently waterproof our basement after finding mould and water issues. Upon initial inspection, I was told that the basement had been underpinned but without the necessary water mitigation tactics i.e. water proofing. Looks likes the old owners did it themselves (old owner was a carpenter and neighbour confirmed that they were diy specialists) Anyhow, I decided to do the waterproofing on the inside because of exterior access issues. Ripped out all the exterior walls down to the studs and got it done. That is, installation of dimpled membrane, French drain system and sump pump. Now I am framing back up. When we did the demo I took out the base plates but left the top plates. There looked to be about 2.25 inches of room and I planned to put a 2inch foam board. I did not want to increase the thickness of the exterior wall because that would mean cutting out the ceiling drywall and adjusting all the partition walls further inward. In an ideal world, I would have just installed 2 inch foamboard and then framed against that but it would mean big changes to all the walls (meaning I would have adjust stairs, bathroom… etc). I’m on a time crunch as wife is almost due and this was all unexpected. The basement is fully finished. Pretty sure there is no insulation under they installed dimpled membrane under the plywood subfloor.
Hopefully, that is enough context. Attached some pics pre and post
My questions are:
1) Is it ok to install 1.5 inch EPS (not XPS) foamboard right against the waterproofing membrane? This is the most that the framing allows because I am following the top plate that I didn’t remove. The other issue is that the wall actually flares downwards. So for 3/4 of the wall I have about 1-1.75 inches of space which shrinks to .25 inches towards the bottom. I am thinking of just shaving some of the studs at the bottom to accommodate the 1.5 inches throughout since the walls are not structural and I will add some blocking which should provide additional stability. I am hoping this will be enough of a barrier to stop condensation on the studs?
2) for the top of the wall, I will cut out the rigid insulation and seal it to avoid air from entering the joist space above (sorry for mucking up the lingo)
2) I have started the framing and already put the base plates down (with ramset). I put standard 6mil vapor barrier under as you can see from the pics. BUT, I just read a post about putting 1inch rigid foam under the base plate… should I take all the base plates out and re do? It will be a pain but if it will cause major issues I’d rather do it now.
3)After installing the 1.5 inches on the wall, I would then put R-14 rockwool for the 2*4 studs and then drywall on top. I understand that I should not be putting a vapor barrier. Am I correct?
4) Lastly, as extra precaution, I am installing a whole house dehumidifier, would that help or is it utterly pointless?
Really hoping for some guidance here and hoping all the questions are not a problem- thank you kindly
Su
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Replies
Su,
Nice job! Everything you are suggesting sounds good.
1. Yes, that will work fine.
2(a). And getting behind the new foam on the wall. Air-sealing the perimeter and joints is important to stop warm, moist air getting to the dimple matt and condensing.
2(b). If you have a flood in the future, those base plates will be vulnerable, but that said it's how it is usually done. The foam underneath is a niche technique.
3. No interior vapour-barrier, so the wall can dry to the inside.
4. I would stick to just dehumidifying the basement as necessary, unless you have high RH upstairs.
Thank you so much Malcolm, appreciate the response. Question, I was at home depot and I saw 1 inch GPS insulation. Would it be an acceptable alternative to 1.5inch EPS? 1 inch will make working with the existing framing much easier but some sources online say that foam insulation only becomes a vapor retarder at 1.5 inches. Any thoughts? And point noted on perimeter sealing. I am not overly worried about flooding as we have a backwater valve as well. At this point, if I am getting flooded, I don't think an inch of foam on the floor will save me. Hope you had a nice Christmas
Su,
It's not so much the vapour permanence that is the worry, as that the layer of foam will be too thin to stop condensation occurring on the inside face. That is mainly a problem near the top of the wall where it is exposed to lower outside temperatures.
You can see the suggested R-values in Assembly #2 in this link: https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
Christmas as great. Hope yours was too.
Thank you Malcolm, editing my previous response just so I don't misunderstand because I wasn't clear initially I think.
If I go the 1.5 inches EPS then that will be enough thickness to prevent condensation? And would it matter if it is 1.5 inch EPS? or should it be XPS?
I looked at the climate zone map and Toronto looks like it is in 5 and up....
Really appreciate your time with this. Thanks!