SIPS but no MVHR?

[sips novice]

Hi everyone. Hope you don't mind me reaching out. I'm not a self-builder (yet -though I'd love to be sometime) but am hoping to buy a house built by a self-builder and would appreciate some thoughts. The house is built using SIPS - and it's the first time this builder has used them. It is 3000ft2 and on the EPC it has an air tightness reading of 2.9m3. But there is no MVHR - just trickle vents, so I'm worried about air quality, condensation and mould (though there is none there visible currently).

 

I asked the guy why he didn't put in an MVHR and he said that he didn't want  the smells of BBQs from neighbouring properties being drawn in, and that the bathroom extractor fans have a background ventilation setting. He also said he put more windows in to ensure enough trickle vents. But I'm struggling to get comfortable with this because everything online recommends a mechanical ventilation system when you build with SIPS. Any thoughts appreciated. Thank you.

[Nickfromwales]

Are there comprehensive pictures of the house being built? The VCL would be my biggest concern, as ventilating internal spaces can be done later tbh.

[ProDave]

SIPS is one of the easiest methods to get correct and air tight so to get an air test of 2.9 seems like a lot of poor detail, or deliberate leaks?

 

To be honest the difference between 2.9 (needs mvhr) and 3.1 (does not need mvhr) is not significant.  I doubt you are going to get problems, especially if the bathroom fans have a slow constant speed.  But the house would have been better if he had tried harder and done the job properly.

 

You will no more get BBQ smells via mvhr than any other ventilation method, and they do have an off switch so if there was a rare case of a bad smell outside you could turn it off for a while.

[JohnMo]

25 minutes ago, sips novice said:

just trickle vents, so I'm worried about air quality, condensation and mould (though there is none there visible currently).

I would replace all trickle vents with humidity activated, then no one can sneak the vents closed, as normally happens. These stay a little bit open for background ventilation and as room humidity increases they open more. Make sure there are no trickle vents in wet rooms or kitchen, these will make the fans just draw from that rooms ventilation, not the whole house.

 

Make sure all internal doors have at least 5 to 10mm clearance at the bottom. This allows building cross ventilation with doors closed.

The fans in wet rooms should be almost silent, they should be set to building regs flow rates, and ideally automatically boost based on humidity, not via a switch.

 

Next ensure building is signed off as complete before handing over any money - or YOU could end up having to install MVHR to satisfy building control.

[Nickfromwales]

If a certain volume of air is removed from the property, the exact and equal amount of outdoor air will be drawn in to replace it, via whatever gaps or engineered openings (such as trickle vents) there are.

 

Smells from outside get into my house, same as any house, but with MVHR you get to shut it off in extreme cases whereas I’m bloody well stuck with it.

 

Builders are great…….😌 

[sips novice]

Thanks so much everyone for your replies. To answer the queries:

 

• no, no pictures of VCL during construction unfortunately.
• sadly no room to retrofit an MVHR as there is no loft space. But we could always change the bathroom extractors for better ones. 
• windows are by ID systems. Don't think they are humidity activated vents currently but good idea -  I could change them if I can find them colour-matched. 
• I reckon he must have been cost saving - but odd, because he chose to spend £14k on a bespoke front door...Priorities, priorities...!

Thanks again

 

[RossMcO]

On 16/05/2025 at 10:38, JohnMo said:

I would replace all trickle vents with humidity activated, then no one can sneak the vents closed, as normally happens. These stay a little bit open for background ventilation and as room humidity increases they open more. Make sure there are no trickle vents in wet rooms or kitchen, these will make the fans just draw from that rooms ventilation, not the whole house.

 

Make sure all internal doors have at least 5 to 10mm clearance at the bottom. This allows building cross ventilation with doors closed.

The fans in wet rooms should be almost silent, they should be set to building regs flow rates, and ideally automatically boost based on humidity, not via a switch.

 

Next ensure building is signed off as complete before handing over any money - or YOU could end up having to install MVHR to satisfy building control.

On the topic of 5-10mm clearance at the bottom of internal doors.   One of our concerns in our build is good acoustic performance - we've always lived in old buildings with good soundproofing between rooms.   We have specified no trickle vent windows, extra acoustic insulation between rooms, acoustic plasterboard, etc, and will have MVHR, but I'm trying to understand why 'cross room' ventilation is required, and how you can manage the acoustic penalty of non-sealed doors.   Each room will be supplied with air from the MVHR, with extraction in the wet rooms, so is there no way to avoid skimming 10mm off of tightly fitted doors?

To further declare my perspective, trickle vents seem archaic to me as well...why spend all that money and effort to build high specification window frames and then drill holes in them?  

Ross.

[JohnMo]

4 hours ago, RossMcO said:

will have MVHR, but I'm trying to understand why 'cross room' ventilation is required

Because MVHR supplies air to dry rooms and extracts from wet rooms. Without the undercut air stops flowing when you close doors. Don't fret about acoustics the under cut makes zero difference.

 

4 hours ago, RossMcO said:

so is there no way to avoid skimming 10mm off of tightly fitted doors?

No

 

4 hours ago, RossMcO said:

further declare my perspective, trickle vents seem archaic to me as well...why spend all that money and effort to build high specification window frames and then drill holes in them

You don't with MVHR but you need to allow cross ventilation with other classes of ventilation such MEV. But then you add humidity activated trickle vents to minimise ventilation when it's not needed. All houses need ventilation just a matter of how it’s done.

[RossMcO]

Thanks John,

 

Humidity activated trickle vents make sense.

 

If we didn't skim the doors, then I guess the airflow from the MVHR supply would stall, as there is no escape for the added volume.   In most cases the bedroom doors are offset to the main room by a short corridor to allow cupboard access, so yes, maybe I'm overthinking the acoustics aspect.  

An alternative to the skim on bathroom/shower room doors would then be a vented plate or louver, I guess.  

Thanks.

Ross

[JohnMo]

36 minutes ago, RossMcO said:

An alternative to the skim on bathroom/shower room doors would then be a vented plate or louver, I guess.  

You could make holes in the wall and add plates, or just get the chippy to set door frame slightly higher in the wall. No skimming of doors, zero cost adder. 

[ProDave]

52 minutes ago, JohnMo said:

just get the chippy to set door frame slightly higher in the wall. No skimming of doors, zero cost adder. 

That's what I did.  I fitted my door frames to match the doors, not the other way around, so no trimming of doors whatsoever and the ventilation gap left at the bottom.

 

I also ommitted the door stop along the top of the door frame.  It adds nothing to actually stopping the door, and adds another small space for ventilation.

[saveasteading]

2 hours ago, JohnMo said:

Don't fret about acoustics the under cut makes zero difference.

It is amazing how much sound escapes through small gaps, but for some reason this doesn't seem the same with underneath doors.

Without further consideration, I'm thinking this is because  of the way sound bounces about: not much gets from your mouth to the ears next door by that long route.

So door gaps rather than grilles I say.

Plus furniture absorbs a lot, and carpets hugely.

[RossMcO]

Thanks everyone, omitting the doorstops at either the top or the bottom of the frame makes sense...and had often been mentioned in various discussions, old houses manage this naturally as nothing fits tightly anyway. 

 

Cheers.