About to fit MVHR external Supply/Exhaust through flat roof and how the ....?

[DeanAlan]

Hi All,

 

I'll be asking the MVHR supplier for suggestions but the wealth of knowledge here is so great thought I would reach out.

 

I have supply and exhaust vents to put up through flat through here. Flat roof installer says he will then fit a collar around them to provide waterproof seal.

 

However, the roof is flat & level and at the moment (winter) has constant few mm of water sitting on it (due for sedum installation soon) and I'm wondering how to hold that water back and counter any small rainfall whilst trying to install the vents and before the flat roofer gets to seal with correct collar.

 

Almost like some sandbags to create a temporary (and maybe 95%) waterproof wall. Any thoughts?

 

Also, any additional thoughts on other damp/water protection I should take? I was wondering if I should use DPM anywhere but I'm basically coming down to "the flat roof membrane is either waterproof or not" and anything else I add is just complication and actually too many layers create opportunities for installation issues (bubbles, rucks, tears etc).

 

I wish the MVHR had gone out the side wall but space is too limited the designer tells me.

 

cheers,

- Dean

 

 

[Redbeard]

Particularly if the roof covering is EPDM (or perhaps GRP if finished nice and smooth I'd have thought you could adhere a collar to the roof covering, and the pipes would be an interference fit. Anything less tight than that and you maybe have a Polo roof.

[Iceverge]

15 minutes ago, DeanAlan said:

wish the MVHR had gone out the side wall but space is too limited the designer tells me.

 

cheers,

 

Can we dig into this? Various trades onsite and designers can be all too happy to solve their problems by doing the easy thing and shoving the problem to the next guy. 

 

Do you have any plans + pics to post and maybe the collective here could come up with some clever ideas for routing the MVHR? Rectangular ducting, combined external terminals etc all offer flexibility that your MVHR designers may not be aware of. 

 

Avoid holes in the roof if possible!! 

 

 

 

 

Edited January 8, 2023 by Iceverge

[DeanAlan]

Thanks for asking @Iceverge

 

Original design from MVHR team had the supply / exhaust coming out into a very tight alleyway (600mm wide) between neighbour and I. As the design progressed they switched to through the roof which I didn't like but they said they didn't have enough clearance in the alleyway.

 

I wonder if better to punch through the side external wall and if the calcs show not enough air available then come up to the same height (above the flat roof parapet) to where they would have been if through flat roof (slight to the right). I would rather go through side wall than roof.

 

That said, the side wall now has its silicon rendered cement board on batten on SIPs but still easier than going up through flat and having collar fitted and any issues that is likely to cause. We haven't skimmed internally yet so easy to unscrewed the plasterboard, cut a hole and then airtight seal it before putting plasterboard back.

 

I have nightmares about the flat roof as it is - the thought of cutting holes in it scares me!

 

 

 

[Iceverge]

Unless you have an enormous volume of air or the alley is severely enclosed it should be fine. Hard to say without seeing it. 

 

Any chance you have a section through the build showing the alley and dimentions?

[DeanAlan]

The alley is about 600mm wide from neighbours extension wall to my own @Iceverge

[Nickfromwales]

8 hours ago, DeanAlan said:

The alley is about 600mm wide from neighbours extension wall to my own @Iceverge

When you say alley, is this just a maintenance space or a thoroughfare? 

[DeanAlan]

Maintenance space. the houses are semi detached in the original form but but houses now have 10m of extension that are detached. So access for maintenance of some drains and that is it.

 

cheers,

-Dean

[Nickfromwales]

50 minutes ago, DeanAlan said:

Maintenance space. the houses are semi detached in the original form but but houses now have 10m of extension that are detached. So access for maintenance of some drains and that is it.

 

cheers,

-Dean

Go back to the installers / designers and make a simple statement....

 

 

"Make the ducts go through the wall, please".

[DeanAlan]

Yep, that's what I'm doing this morning @Nickfromwales

 

Hopefully they will be ok with me going straight out and terminating on that wall but they may want me to the go up the outside of the wall to above the roof top (3m higher) but that would still be better than going through the flat roof and worrying about compromising that seal.

 

Thanks all.

 

cheers,

-Dean

[Iceverge]

How on earth are they justifying a 3m stack? 

[Andehh]

Hi Dean, you and I are obviously kindred spirits! 

 

I went through all of this last week, as the thought of cutting through the roof had me in cold sweats & sleepless nights, but we had no option of sideways venting. MVHR supplier/fitter let me down due to a miss communication between him & builder, and I saved £3k doing it myself vs going through builder's normal supplier...so I own the job/risk, having never done this before. Hence belt, braces & everything on top.

 

Space was also an absolute bastard, and slanting roof, and awkward joists/wall config meant I risked having a low point in my ducts for any water to settle into.

 

I used the below onto the joints I made, and then again on the roof whilst we wait for the roofers to come back to make the 'sleeve' as the final covering. A very wet weekend showed no leaks, and some close up investigation of it showed it to be very robust & confidence inspiring at preventing water ingress.  Ill smudge on a second coating for good measure later on down the line, but even as a ''temporary'' solution, its great stuff!

https://www.amazon.co.uk/Everbuild-EB25-Ultimate-Adhesive-Cartridge/dp/B07T76J481/ref=sr_1_1_sspa?crid=OQXA6AYBIWBC&keywords=Everbuild%2BEB25%2BGrey&qid=1673276197&s=industrial&sprefix=everbuild%2Beb25%2Bgrey%2Cindustrial%2C70&sr=1-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

 

Notes:

 

1) Yes I know they aren't dead straight, they look(ed) fine in person until I noticed them in that photo!! fecking spirit level broke on me (joys of working on the roof) and I should have ceased work until i got a new one, tired, stressed & fed up of worrying about it...oh well!

2) The duct lids are pushed far down, as they need trimming down & adjusting before commissioning in another couple of months. Ill let them sleeve them, then trim to 150mm off the roof and secure them properly once done.

3) ill be finishing off insulating it tomorrow & trying to tidy it up a bit.

 

 

 

 

 

 

(ignore what looks like a split, that's an odd hair/branch thingy making it look like its split..... Its all 100% smooth sealed)

 

 

 

 

Edited January 9, 2023 by Andehh

[Nickfromwales]

Sorry @Andehh, but that is not great. That duct should not be exposed externally, so please insulate it at the very least. And internally too! The cold bridge you've introduced there is "significant".

You should have bonded these down and then gone through with a pre-insulated duct.

https://flexibleflueliners.com/product/high-temp-177-330mm-multi-purpose-flat-roof-flashing-black/?utm_source=Google Shopping&utm_campaign=Main Feed&utm_medium=cpc&utm_term=633&campaignid=18820525657&adgroupid=140474702782&keyword=&device=c&gclid=CjwKCAiAk--dBhABEiwAchIwkSyJgYhRAyV1DIPmCmTWlkPxB28yFB7SYkzKJGPVnFJwlRbkKIqNPRoCpOQQAvD_BwE

 

I hope they last, but I would not accept that on any of my ventilation projects. ( Sorry again ).

[Andehh]

Thanks Nick, appreciate the thoughts & given me something to think about.

 

I was intending to double up on the insulation on the ducting internally anyway (and did so on the wall mounted ducting for our 2nd MVHR), however following your point out,  I will triple layer it to try and improve as best I can!

Edited January 9, 2023 by Andehh

[Nickfromwales]

2 minutes ago, Andehh said:

Thanks Nick, appreciate the thoughts & given me something to think about.

It's our thing....it's what we do lol.

 

3 minutes ago, Andehh said:

I cant help but think if the air being drawn in is at ambient temp, whatever you do to the top 15cm of exposed duct will be negated by the air passing down into the system below?

Not quite. As it is, eg metal exposed to the elements, you're in a world of hurt. Just imagine snow resting up against that and a bit of wind-chill to boot. The duct will conduct that like crazy. 

Much as you want to pull the door closed on this, I'd pull it out and do it again, properly. The issue is the spiral seam which naturally wants to track water through the point at which the sealant / or compression seal is trying to keep water out.

Random image grab;

https://www.google.com/search?q=ubbink+roof+terminal&rlz=1C1CHBF_en-GBGB910GB910&source=lnms&tbm=shop&sa=X&ved=2ahUKEwjI9Mu1ibv8AhXSQcAKHfkfAvMQ_AUoAnoECAEQBA&biw=1280&bih=609&dpr=2#spd=16728725204361805693

This is what it should look like, so imagine rain on that, and how the joint is lost up high within the cowl of the fitting.