Improving airtightness in dormer through soffits into mid floor

Zenith74

Hey guys,

I've read a few of the threads here about people having issues with air entering soffit vents then getting into the crawl spaces of ceiling of the ground floor, hoping for some thoughts/insights on how to remediate it in my scenario.

I've included a sketch of the setup in our 2000s dormer. We're about to get the soffit and fascia boards replaced so it would be a great time to make some improvements, though I accept we're not going to get to passive house standard :), just trying to make things a bit better as we go.

If you were building this from scratch I assume you'd ideally have wanted the insulation/air-barrier in the cavity wall to continue right up and be joined to the insulation boards that are fitted between the rafters, which would allow air coming in through the soffit vents to blow up under the slates/sarking but not into the crawl space or ground floor ceiling. So my question is should we try to achieve this now and what are the options to achieve it?

Cut pieces of insulation board and fit them between each rafter/joist (sealing with spray foam) to join up the top plate of the outer leaf and the insulation board under the slates? Pieces of timber instead of insulation board? Or what about getting a spray foam company to come out and spray in foam to create a continuous layer between the outer leaf and the insulation board under the slates, being careful not to block the cavity that allows air circulate under the slates?

I intend to get the cavity in the cavity wall filled with decent bonded beads later in the year.

Any thoughts greatly appreciated!

MicktheMan

So my question is should we try to achieve this now

No. Not only is what you're proposing not going to be effective but more importantly your risking doing serious structural damage to your roof.

Zenith74

https://www.boards.ie/discussion/comment/120292270#Comment_120292270

Thanks for responding Mick. Could you tell me a bit more and maybe what some of the options might be? To be clear I’m not in the building trade and would not be doing this myself, I am curious and eager to learn more about this stuff and ideally improve our home a little bit here and there as we go.

Cheers!

MicktheMan

1st step is to define and stick to your thermal envelope. In your sketch, it looks like the designed thermal envelope is up the external wall, across the flat ceiling, up the knee wall and along the sloped ceiling. In this scenario, doing anything to the sloped section above the crawl space is a wasted effort because it is outside the thermal envelope.

Next step is to know where the leakages are occurring and the relative importance of each type. What you have sketched is good in that it shows a likely leakage path, but there are likely others and these need to be understood too. How to quickly gather the information is to actually test the house (as part of a comprehensive heat loss survey) and then act on the findings (maybe as you make other improvements to the house).

Zenith74

https://www.boards.ie/discussion/comment/120292391#Comment_120292391

Agreed yeah, I’d say the house was originally built with that thermal envelope in-mind ~20 years ago. A new owner 5 years ago did some major renovation work and had the attic and these crawl spaces spray foamed at the time. Proper ventilation cards were installed, so it doesn’t look like a cowboy job, but not sure consideration was given to the thermal envelope at the time!

So I think we now probably have a choice of which thermal envelope to work to? There are quite a few unlagged heating pipes in the crawl spaces, so I suspect if we reduced the air penetration into them they’d be a somewhat heated/conditioned space.

We just had a heat loss survey done for a heatpump grant, but I presume you mean something with an airtightness test involved? Unfortunately I don’t think we have time for that as the soffits/fascias will be replaced in the next couple of weeks. We have just finished some renovation works on the house (including UFH, MVHR, removing passive vents, new windows/doors throughout) and this is the last piece other than having the cavity wall bonded bead filled later in the year.

If we were to confided the crawl spaces parts of the thermal envelope, would it then make sense to work on improving the airtightness by closing off the air path in my drawing? When you mention the risk to the structural integrity of the roof, do you mean if the ventilation path under the sarking/slates gets blocked so the rafters become unventilated?

Thanks very much for replying!

Yellow_Fern

Just wondering about thermal looping, if the wall plate was exposed, maybe check for this? Filling a gap, if present, between inner leaf and insulation would be a big deal.

Zenith74

https://www.boards.ie/discussion/comment/120294383#Comment_120294383

Thanks for replying. Given how casually insulation boards seem to be installed in Irish houses generally, especially 20 years ago, I have no doubt that the insulation board would not be carefully in contact with the inner leaf so I'm sure there is an element of looping. Although maybe the looping effect is reduced if the top of the cavity is simply open to the air🤣?

I think having the wall cavity pumped and dealing with the air infiltration into the ground floor ceiling would have a transformative impact on the comfort of the house to be honest, just need to figure out how to do the latter safely while the soffits are all off!

MicktheMan

If we were to confided the crawl spaces parts of the thermal envelope, would it then make sense to work on improving the airtightness by closing off the air path in my drawing?

I don't understand the question.

When you mention the risk to the structural integrity of the roof, do you mean if the ventilation path under the sarking/slates gets blocked so the rafters become unventilated?

Yes, anything that limits the ventilation of the structural roof timbers is never a good idea.

MicktheMan

dealing with the air infiltration into the ground floor ceiling ..... need to figure out how to do the latter safely while the soffits are all off!

I'm not sure what you have in mind but please be aware that for cold roof constructions you don't, or more accurately you can't, deal with this airtight weakness at the soffit vent

Zenith74

https://www.boards.ie/discussion/comment/120299397#Comment_120299397

Wow that question is so mangled I’m not even sure what I was getting at there!! I think what I was asking is if we included the crawl spaces within the thermal envelope, would it then make sense to close off the air infiltration path in my drawing? The crawl spaces are already insulated (rightly or wrongly), floored and mostly airtight on all sides so I think they could be considered either way, we’d just need to think about heating and ventilation.